Roger V Yelle

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Courses

Scholarly Contributions

Journals/Publications

• Braude, A., Montmessin, F., Schneider, N., Gupta, S., Jain, S., Lef{\`evre}, F., M{\"a\"att\"anen}, A., Verdier, L., Flimon, Z., Jiang, F., Yelle, R., Deighan, J., & Curry, S. (2023). Seasonal, Latitudinal, and Longitudinal Trends in Nighttime Ozone Vertical Structure on Mars From MAVEN/IUVS Stellar Occultations. Journal of Geophysical Research (Planets), 128(5), e2022JE007697.
• Cangi, E., Chaffin, M., Yelle, R., Gregory, B., & Deighan, J. (2023). Fully Coupled Photochemistry of the Deuterated Ionosphere of Mars and Its Effects on Escape of H and D. Journal of Geophysical Research (Planets), 128(7), e2022JE007713.
• Lian, Y., Richardson, M. I., Newman, C. E., Lee, C., Toigo, A., Guzewich, S., & Yelle, R. V. (2023). Dynamical Core Damping of Thermal Tides in the Martian Atmosphere. Journal of the Atmospheric Sciences, 80(2), 535-547.
• Chadney, J., Koskinen, T., Hu, X., Galand, M., Lavvas, P., Unruh, Y., Serigano, J., H{\"orst}, S., & Yelle, R. (2022). Energy deposition in Saturn's equatorial upper atmosphere. \icarus, 372, 114724.
• Gupta, S., Yelle, R. V., Schneider, N. M., Jain, S. K., Gonz{\'alez-Galindo}, F., Verdier, L., Braude, A. S., Montmessin, F., Mayyasi, M., Deighan, J., & Curry, S. (2022). Thermal Structure of the Martian Upper Mesosphere/Lower Thermosphere From MAVEN/IUVS Stellar Occultations. Journal of Geophysical Research (Planets), 127(11), e2022JE007534.
• Lo, D. Y., Yelle, R. V., Deighan, J. I., Jain, S. K., Evans, J. S., Stevens, M. H., Ajello, J. M., Mayyasi, M. A., & Schneider, N. M. (2022). MAVEN/IUVS observations of C I 156.1 nm and 165.7 nm dayglow: Direct detection of carbon and implications on photochemical escape. \icarus, 371, 114664.
• Serigano, J., H{\"orst}, S., He, C., Gautier, T., Yelle, R., Koskinen, T., Trainer, M., & Radke, M. (2022). Compositional Measurements of Saturn's Upper Atmosphere and Rings From Cassini INMS: An Extended Analysis of Measurements From Cassini's Grand Finale Orbits. Journal of Geophysical Research (Planets), 127(6), e07238.
• Stone, S. W., Yelle, R. V., Benna, M., Elrod, M. K., & Mahaffy, P. R. (2022). Neutral Composition and Horizontal Variations of the Martian Upper Atmosphere From MAVEN NGIMS. Journal of Geophysical Research (Planets), 127(6), e07085.
• Chaufray, J. -., Gonzalez-Galindo, F. .., Lopez-Valverde, M., Forget, F., Qu{\'emerais}, E., Bertaux, J. -., Montmessin, F., Chaffin, M., Schneider, N., Clarke, J., Leblanc, F., Modolo, R., & Yelle, R. (2021). Study of the hydrogen escape rate at Mars during martian years 28 and 29 from comparisons between SPICAM/Mars express observations and GCM-LMD simulations. \icarus, 353, 113498.
• Ergun, R., Andersson, L., Fowler, C., Thaller, S., & Yelle, R. (2021). In-Situ Measurements of Electron Temperature and Density in Mars' Dayside Ionosphere. \grl, 48(14), e93623.
• Hanley, K., McFadden, J., Mitchell, D., Fowler, C., Stone, S., Yelle, R., Mayyasi, M., Ergun, R., Andersson, L., Benna, M., Elrod, M., & Jakosky, B. (2021). In Situ Measurements of Thermal Ion Temperature in the Martian Ionosphere. Journal of Geophysical Research (Space Physics), 126(12), e29531.
• Lo, D. Y., Yelle, R. V., Lillis, R. J., & Deighan, J. I. (2021). Carbon photochemical escape rates from the modern Mars atmosphere. \icarus, 360, 114371.
• Mahieux, A., Yelle, R., Yoshida, N., Robert, S., Piccialli, A., Nakagawa, H., Kasaba, Y., Mills, F., & Vandaele, A. (2021). Determination of the Venus eddy diffusion profile from CO and CO$_2$ profiles using SOIR/Venus Express observations. \icarus, 361, 114388.
• Yelle, R. V., Koskinen, T., & Palmer, M. (2021). Titan occultations of Orion's belt observed with Cassini/UVIS. \icarus, 368, 114587.
• Bhattacharyya, D., Chaufray, J., Mayyasi, M., Clarke, J., Stone, S., Yelle, R., Pryor, W., Bertaux, J., Deighan, J., Jain, S., & Schneider, N. (2020). Two-dimensional model for the martian exosphere: Applications to hydrogen and deuterium Lyman \ensuremath{\alpha} observations. \icarus, 339, 113573.
• Koskinen, T., Sandel, B., Yelle, R., Holsclaw, G., & Quemerais, E. (2020). Corrigendum to ``Saturn in Lyman \ensuremath{\alpha}: A comparison of Cassini and Voyager observations'' [Icarus 339 (2020) 113594]. \icarus, 351, 113919.
• Koskinen, T., Sandel, B., Yelle, R., Holsclaw, G., & Quemerais, E. (2020). Saturn in Lyman \ensuremath{\alpha}: A comparison of Cassini and Voyager observations. \icarus, 339, 113594.
• Lo, D. Y., Yelle, R. V., & Lillis, R. J. (2020). Carbon photochemistry at Mars: Updates with recent data. \icarus, 352, 114001.
• Nakagawa, H., Jain, S. K., Schneider, N. M., Montmessin, F., Yelle, R. V., Jiang, F., Verdier, L., Kuroda, T., Yoshida, N., Fujiwara, H., Imamura, T., Terada, N., Terada, K., Seki, K., Gr{\"oller}, H., & Deighan, J. I. (2020). A Warm Layer in the Nightside Mesosphere of Mars. \grl, 47(4), e85646.
• Nakagawa, H., Terada, N., Jain, S. K., Schneider, N. M., Montmessin, F., Yelle, R. V., Jiang, F., Verdier, L., England, S. L., Seki, K., Fujiwara, H., Imamura, T., Yoshida, N., Kuroda, T., Terada, K., Gr{\"oller}, H., Deighan, J., & Jakosky, B. M. (2020). Vertical Propagation of Wave Perturbations in the Middle Atmosphere on Mars by MAVEN/IUVS. Journal of Geophysical Research (Planets), 125(9), e06481.
• Peterson, W., Andersson, L., Ergun, R., Thiemann, E. d., Pilinski, M., Thaller, S., Fowler, C., Mitchell, D., Benna, M., Yelle, R., & Stone, S. (2020). Subsolar Electron Temperatures in the Lower Martian Ionosphere. Journal of Geophysical Research (Space Physics), 125(2), e27597.
• Serigano, J., H{\"orst}, S., He, C., Gautier, T., Yelle, R., Koskinen, T., & Trainer, M. (2020). Compositional Measurements of Saturn's Upper Atmosphere and Rings from Cassini INMS. Journal of Geophysical Research (Planets), 125(8), e06427.
• Stone, S. W., Yelle, R. V., Benna, M., Lo, D. Y., Elrod, M. K., & Mahaffy, P. R. (2020). Hydrogen escape from Mars is driven by seasonal and dust storm transport of water. Science, 370(6518), 824-831.
• Vriesema, J., Koskinen, T., & Yelle, R. (2020). Electrodynamics in Saturn's thermosphere at low and middle latitudes. \icarus, 344, 113390.
• Wu, X. -., Cui, J., Yelle, R., Cao, Y. -., He, Z. -., He, F., & Wei, Y. (2020). Photoelectrons as a Tracer of Planetary Atmospheric Composition: Application to CO on Mars. Journal of Geophysical Research (Planets), 125(7), e06441.
• Cui, J., Cao, Y. -., Wu, X. -., Xu, S. -., Yelle, R., Stone, S., Vigren, E., Edberg, N., Shen, C. -., He, F., & Wei, Y. (2019). Evaluating Local Ionization Balance in the Nightside Martian Upper Atmosphere during MAVEN Deep Dip Campaigns. \apjl, 876(1), L12.
• Jiang, F., Yelle, R. V., Jain, S., Cui, J., Montmessin, F., Schneider, N., Deighan, J., Gr{\"oller}, H., & Verdier, L. (2019). Detection of Mesospheric CO$_2$ Ice Clouds on Mars in Southern Summer. \grl, 46(14), 7962-7971.
• Korablev, O., Avandaele, A. C., Montmessin, F., Fedorova, A. A., Trokhimovskiy, A., Forget, F., Lef{\`evre}, F., Daerden, F., Thomas, I. R., Trompet, L., Erwin, J. T., Aoki, S., Robert, S., Neary, L., Viscardy, S., Grigoriev, A. V., Ignatiev, N. I., Shakun, A., Patrakeev, A., , Belyaev, D. A., et al. (2019). No detection of methane on Mars from early ExoMars Trace Gas Orbiter observations. \nat, 568(7753), 517-520.
• Lian, Y., & Yelle, R. V. (2019). Damping of gravity waves by kinetic processes in Jupiter's thermosphere. \icarus, 329, 222-245.
• Lillis, R., Lo, D., Deighan, J., Fox, J., Yelle, R., Lee, Y., Leblanc, F., Chaufray, J. -., Cravens, T., Rahmati, A., Gacesa, M., Jakosky, B., & Chaffin, M. (2019). Photochemical Escape from Mars as a Climate Driver: How Important?. LPI Contributions, 2089, 6033.
• Mayyasi, M., Clarke, J., Bhattacharyya, D., Chaufray, J., Benna, M., Mahaffy, P., Stone, S., Yelle, R., Thiemann, E., Chaffin, M., Deighan, J., Jain, S., Schneider, N., & Jakosky, B. (2019). Seasonal Variability of Deuterium in the Upper Atmosphere of Mars. Journal of Geophysical Research (Space Physics), 124(3), 2152-2164.
• M{\"uller-Wodarg}, I., Koskinen, T., Moore, L., Serigano, J., Yelle, R., H{\"orst}, S., Waite, J., & Mendillo, M. (2019). Atmospheric Waves and Their Possible Effect on the Thermal Structure of Saturn's Thermosphere. \grl, 46(5), 2372-2380.
• Pilinski, M., Thiemann, E., Jain, S., Stone, S., Lo, D., Yelle, R., Girazian, Z., Eparvier, F., Schneider, N., Benna, M., & Bougher, S. (2019). The MAVEN Neutral Data Working Group: Combining all MAVEN Neutral Measurements to Provide a Global Picture of Structure and Variability in the Mars Thermosphere. LPI Contributions, 2089, 6286.
• Rymer, A., Mandt, K., Hurley, D., Lisse, C., Izenberg, N., Smith, H. T., Westlake, J., Bunce, E., Arridge, C., Masters, A., Hofstadter, M., Simon, A., {Brand, t. P., Clark, G., Cohen, I., Allen, R., Vine, S., Hansen, K., Hospodarsky, G., , Kurth, W., et al. (2019). Solar System Ice Giants: Exoplanets in our Backyard.. \baas, 51(3), 176.
• Siddle, A., Mueller-Wodarg, I., Stone, S., & Yelle, R. (2019). Global characteristics of gravity waves in the upper atmosphere of Mars as measured by MAVEN/NGIMS. \icarus, 333, 12-21.
• Stone, S., Yelle, R., Benna, M., Elrod, M., & Mahaffy, P. (2019). Transport of Water to the Martian Upper Atmosphere amid Regional and Global Dust Storms. LPI Contributions, 2089, 6363.
• Vandaele, A. C., Korablev, O., Daerden, F., Aoki, S., Thomas, I. R., Altieri, F., L{\'opez-Valverde}, M., Villanueva, G., Liuzzi, G., Smith, M. D., Erwin, J. T., Trompet, L., Fedorova, A. A., Montmessin, F., Trokhimovskiy, A., Belyaev, D. A., Ignatiev, N. I., Luginin, M., Olsen, K. S., , Baggio, L., et al. (2019). Martian dust storm impact on atmospheric H$_2$O and D/H observed by ExoMars Trace Gas Orbiter. \nat, 568(7753), 521-525.
• Vuitton, V., Yelle, R., Klippenstein, S., H{\"orst}, S., & Lavvas, P. (2019). Simulating the density of organic species in the atmosphere of Titan with a coupled ion-neutral photochemical model. \icarus, 324, 120-197.
• Wu, X. -., Cui, J., Xu, S., Lillis, R., Yelle, R., Edberg, N., Vigren, E., Rong, Z. -., Fan, K., Guo, J. -., Cao, Y. -., Jiang, F. -., Wei, Y., & Mitchell, D. (2019). The Morphology of the Topside Martian Ionosphere: Implications on Bulk Ion Flow. Journal of Geophysical Research (Planets), 124(3), 734-751.
• Chaufray, J., Yelle, R., Gonzalez-Galindo, F. .., Forget, F., Lopez-Valverde, M. .., Leblanc, F., & Modolo, R. (2018). Effect of the Lateral Exospheric Transport on the Horizontal Hydrogen Distribution Near the Exobase of Mars. Journal of Geophysical Research (Space Physics), 123, 2441-2454.
• Crismani, M., Schneider, N., Evans, J., Plane, J., Carrillo-S{\'anchez}, J., Jain, S., Deighan, J., & Yelle, R. (2018). The Impact of Comet Siding Spring's Meteors on the Martian Atmosphere and Ionosphere. Journal of Geophysical Research (Planets), 123, 2613-2627.
• Cui, J., Yelle, R., Zhao, L., Stone, S., Jiang, F., Cao, Y., Yao, M., Koskinen, T., & Wei, Y. (2018). The Impact of Crustal Magnetic Fields on the Thermal Structure of the Martian Upper Atmosphere. \apjl, 853, L33.
• Gr{\"oller}, H., Montmessin, F., Yelle, R., Lef{\`evre}, F., Forget, F., Schneider, N., Koskinen, T., Deighan, J., & Jain, S. (2018). MAVEN/IUVS Stellar Occultation Measurements of Mars Atmospheric Structure and Composition. Journal of Geophysical Research (Planets), 123, 1449-1483.
• Jakosky, B., Brain, D., Chaffin, M., Curry, S., Deighan, J., Grebowsky, J., Halekas, J., Leblanc, F., Lillis, R., Luhmann, J., Andersson, L., Andre, N., Andrews, D., Baird, D., Baker, D., Bell, J., Benna, M., Bhattacharyya, D., Bougher, S., , Bowers, C., et al. (2018). Loss of the Martian atmosphere to space: Present-day loss rates determined from MAVEN observations and integrated loss through time. \icarus, 315, 146-157.
• Montmessin, F., Schneider, N., Deighan, J., Jain, S., Evans, S., Crismani, M., Stevens, M., Lo, D., Clarke, J., Chaffin, M., Mayyasi, M., Lefevre, F., Stiepen, A., Royer, E., Milby, Z., Groeller, H., Yelle, R., & Nakagawa, H. (2018). Four years of upper atmospheric exploration at Mars with MAVEN and IUVS. European Planetary Science Congress, 12, EPSC2018-656.
• Nixon, C., Lorenz, R., Achterberg, R., Buch, A., Coll, P., Clark, R., Courtin, R., Hayes, A., Iess, L., Johnson, R., Lopes, R., Mastrogiuseppe, M., Mandt, K., Mitchell, D., Raulin, F., Rymer, A., Todd Smith, H., Solomonidou, A., Sotin, C., , Strobel, D., et al. (2018). Titan's cold case files - Outstanding questions after Cassini-Huygens. \planss, 155, 50-72.
• Perry, M., Waite, J., Mitchell, D., Miller, K., Cravens, T., Perryman, R., Moore, L., Yelle, R., Hsu, H., Hedman, M., Cuzzi, J., Strobel, D., Hamil, O., Glein, C., Paxton, L., Teolis, B., & McNutt, R. (2018). Material Flux From the Rings of Saturn Into Its Atmosphere. \grl, 45, 10.
• Peterson, W., Fowler, C., Andersson, L., Thiemann, E., Jain, S., Mayyasi, M., Esman, T., Yelle, R., Benna, M., & Espley, J. (2018). Martian Electron Temperatures in the Subsolar Region: MAVEN Observations Compared to a One-Dimensional Model. Journal of Geophysical Research (Space Physics), 123, 5960-5973.
• Schneider, N., Jain, S., Deighan, J., Nasr, C., Brain, D., Larson, D., Lillis, R., Rahmati, A., Halekas, J., Lee, C., Chaffin, M., Stiepen, A., Crismani, M., Evans, J., Stevens, M., Lo, D., McClintock, W., Stewart, A., Yelle, R., , Clarke, J., et al. (2018). Global Aurora on Mars During the September 2017 Space Weather Event. \grl, 45, 7391-7398.
• Siddle, A., Mueller-Wodarg, I. .., Yelle, R., & Stone, S. (2018). Global characteristics of gravity waves in the upper atmosphere of Mars as measured by MAVEN/NGIMS. European Planetary Science Congress, 12, EPSC2018-186.
• Slipski, M., Jakosky, B., Benna, M., Elrod, M., Mahaffy, P., Kass, D., Stone, S., & Yelle, R. (2018). Variability of Martian Turbopause Altitudes. Journal of Geophysical Research (Planets), 123, 2939-2957.
• Stone, S., Yelle, R., Benna, M., Elrod, M., & Mahaffy, P. (2018). Thermal Structure of the Martian Upper Atmosphere From MAVEN NGIMS. Journal of Geophysical Research (Planets), 123, 2842-2867.
• Waite, J., Perryman, R., Perry, M., Miller, K., Bell, J., Cravens, T., Glein, C., Grimes, J., Hedman, M., Cuzzi, J., Brockwell, T., Teolis, B., Moore, L., Mitchell, D., Persoon, A., Kurth, W., Wahlund, J., Morooka, M., Hadid, L., , Chocron, S., et al. (2018). Chemical interactions between Saturn's atmosphere and its rings. Science, 362, aat2382.
• Yelle, R., Serigano, J., Koskinen, T., H{\"orst}, S., Perry, M., Perryman, R., & Waite, J. (2018). Thermal Structure and Composition of Saturn's Upper Atmosphere From Cassini/Ion Neutral Mass Spectrometer Measurements. \grl, 45, 10.
• Benna, M., Grebowsky, J., Mahaffy, P., Plane, J., Yelle, R., & Jakosky, B. (2017). Signature of Metallic ion in the upper atmosphere of Mars following the passage of comet C/2013 A1 (Siding Spring). European Planetary Science Congress, 11, EPSC2017-162.
• Brain, D., Chaffin, M., Jakosky, B., Luhmann, J., Dong, C., Yelle, R., & Egan, H. (2017). Would Mars be Habitable If It Orbited an M Dwarf? Lessons from the MAVEN Mission. LPI Contributions, 2042, 4043.
• Chaufray, J., Yelle, R., Gonzalez-Galindo, F. .., Forget, F., Lopez-Valverde, M. .., Leblanc, F., & Modolo, R. (2017). Effect of the lateral exospheric transport on the horizontal hydrogen distribution at the exobase of Mars. European Planetary Science Congress, 11, EPSC2017-252.
• Crismani, M., Schneider, N., Plane, J., Evans, J., Jain, S., Chaffin, M., Carrillo-Sanchez, J., Deighan, J., Yelle, R., Stewart, A., McClintock, W., Clarke, J., Holsclaw, G., Stiepen, A., Montmessin, F., & Jakosky, B. (2017). Detection of a persistent meteoric metal layer in the Martian atmosphere. Nature Geoscience, 10, 401-404.
• Crismani, M., Schnider, N., Jain, S., Plane, J., Deighan, J., Evans, S., Yelle, R., Carrillo-Sanchez, J. .., & Chaffin, M. (2017). The Metals Delivered by Comet Siding Spring to Mars. European Planetary Science Congress, 11, EPSC2017-375.
• Elrod, M., Bougher, S., Bell, J., Mahaffy, P., Benna, M., Stone, S., Yelle, R., & Jakosky, B. (2017). He bulge revealed: He and CO$_2$ diurnal and seasonal variations in the upper atmosphere of Mars as detected by MAVEN NGIMS. Journal of Geophysical Research (Space Physics), 122, 2564-2573.
• Jakosky, B., Slipski, M., Benna, M., Mahaffy, P., Elrod, M., Yelle, R., Stone, S., & Alsaeed, N. (2017). Mars\rsquo atmospheric history derived from upper-atmosphere measurements of $^38$Ar/$^36$Ar. Science, 355, 1408-1410.
• Lillis, R., Deighan, J., Fox, J., Bougher, S., Lee, Y., Combi, M., Leblanc, F., Chaufray, J., Cravens, T., Rahmati, A., Groller, H., Yelle, R., & Jakosky, B. (2017). Photochemical Escape of Oxygen from Mars: Consequences for Climate History. LPI Contributions, 2014, 3023.
• Schneider, N., Crismani, M., Deighan, J., Plane, J., Evans, J., Jain, S., Stewart, A., Carrillo-Sanchez, J., McClintock, W., Chaffin, M., Stiepen, A., Stevens, M., Yelle, R., Clarke, J., Holsclaw, G., Montmessin, F., & Jakosky, B. (2017). MAVEN IUVS Observations of the Aftermath of the Comet Siding Spring Meteor Shower on Mars. European Planetary Science Congress, 11, EPSC2017-380.
• Yelle, R., Koskinen, T., & Lavvas, P. (2017). Thermal Structure of Pluto's Lower Atmospehre. European Planetary Science Congress, 11, EPSC2017-923.
• {Capalbo}, F., {B{\'e}nilan}, Y., {Fray}, N., {Schwell}, M., {Champion}, N., {Es-sebbar}, E., {Koskinen}, T., {Lehocki}, I., , R. (2016). New benzene absorption cross sections in the VUV, relevance for Titan's upper atmosphere. \icarus, 265, 95-109.
• {Chadney}, J., {Galand}, M., {Koskinen}, T., {Miller}, S., {Sanz-Forcada}, J., {Unruh}, Y., , R. (2016). EUV-driven ionospheres and electron transport on extrasolar giant planets orbiting active stars. \aap, 587, A87.
• {Andersson}, L., {Weber}, T., {Malaspina}, D., {Crary}, F., {Ergun}, R., {Delory}, G., {Fowler}, C., {Morooka}, M., {McEnulty}, T., {Eriksson}, A., {Andrews}, D., {Horanyi}, M., {Collette}, A., {Yelle}, R., , B. (2015). Dust observations at orbital altitudes surrounding Mars. Science, 350, 0398.
• {Benna}, M., {Mahaffy}, P., {Grebowsky}, J., {Fox}, J., {Yelle}, R., , B. (2015). First measurements of composition and dynamics of the Martian ionosphere by MAVEN's Neutral Gas and Ion Mass Spectrometer. \grl, 42, 8958-8965.
• {Benna}, M., {Mahaffy}, P., {Grebowsky}, J., {Plane}, J., {Yelle}, R., , B. (2015). Metallic ions in the upper atmosphere of Mars from the passage of comet C/2013 A1 (Siding Spring). \grl, 42, 4670-4675.
• {Bougher}, S., {Jakosky}, B., {Halekas}, J., {Grebowsky}, J., {Luhmann}, J., {Mahaffy}, P., {Connerney}, J., {Eparvier}, F., {Ergun}, R., {Larson}, D., {McFadden}, J., {Mitchell}, D., {Schneider}, N., {Zurek}, R., {Mazelle}, C., {Andersson}, L., {Andrews}, D., {Baird}, D., {Baker}, D., , {Bell}, J., et al. (2015). Early MAVEN Deep Dip campaign reveals thermosphere and ionosphere variability. Science, 350, 0459.
• {Capalbo}, F., {B{\'e}nilan}, Y., {Yelle}, R., , T. (2015). Titan's Upper Atmosphere from Cassini/UVIS Solar Occultations. \apj, 814, 86.
• {Crismani}, M., {Schneider}, N., {Deighan}, J., {Stewart}, A., {Combi}, M., {Chaffin}, M., {Fougere}, N., {Jain}, S., {Stiepen}, A., {Yelle}, R., {McClintock}, W., {Clarke}, J., {Holsclaw}, G., {Montmessin}, F., , B. (2015). Ultraviolet observations of the hydrogen coma of comet C/2013 A1 (Siding Spring) by MAVEN/IUVS. \grl, 42, 8803-8809.
• {Cui}, J., {Galand}, M., {Yelle}, R., {Wei}, Y., , S. (2015). Day-to-night transport in the Martian ionosphere: Implications from total electron content measurements. Journal of Geophysical Research (Space Physics), 120, 2333-2346.
• {Fossati}, L., {Bourrier}, V., {Ehrenreich}, D., {Haswell}, C., {Kislyakova}, K., {Lammer}, H., Etangs}, A., {Alibert}, Y., {Ayres}, T., {Ballester}, G., {Barnes}, J., {Bisikalo}, D., {Collier}, A., {Cameron}, ., {Czesla}, S., {Desert}, J., {France}, K., {Guedel}, M., {Guenther}, E., , {Helling}, C., et al. (2015). Characterising exoplanets and their environment with UV transmission spectroscopy. ArXiv e-prints.
• {Gr{\"o}ller}, H., {Yelle}, R., {Koskinen}, T., {Montmessin}, F., {Lacombe}, G., {Schneider}, N., {Deighan}, J., {Stewart}, A., {Jain}, S., {Chaffin}, M., {Crismani}, M., {Stiepen}, A., {Lef{\`e}vre}, F., {McClintock}, W., {Clarke}, J., {Holsclaw}, G., {Mahaffy}, P., {Bougher}, S., , B. (2015). Probing the Martian atmosphere with MAVEN/IUVS stellar occultations. \grl, 42, 9064-9070.
• {Gr{\"o}ller}, H., {Yelle}, R., {Montmessin}, F., {Lacombe}, G., {Schneider}, N., {Stewart}, I., {Deighan}, J., {McClintock}, W., {Clarke}, J., {Holsclaw}, G., , B. (2015). Martian CO2 and O2 abundances obtained from MAVEN/IUVS stellar occultations. European Planetary Science Congress 2015, held 27 September - 2 October, 2015 in Nantes, France, Online at http://meetingorganizer.copernicus.org/EPSC2015, id.EPSC2015-401, 10, EPSC2015-401.
• {Jakosky}, B., {Grebowsky}, J., {Luhmann}, J., {Connerney}, J., {Eparvier}, F., {Ergun}, R., {Halekas}, J., {Larson}, D., {Mahaffy}, P., {McFadden}, J., {Mitchell}, D., {Schneider}, N., {Zurek}, R., {Bougher}, S., {Brain}, D., {Ma}, Y., {Mazelle}, C., {Andersson}, L., {Andrews}, D., , {Baird}, D., et al. (2015). MAVEN observations of the response of Mars to an interplanetary coronal mass ejection. Science, 350, 0210.
• {Jakosky}, B., {Lin}, R., {Grebowsky}, J., {Luhmann}, J., {Mitchell}, D., {Beutelschies}, G., {Priser}, T., {Acuna}, M., {Andersson}, L., {Baird}, D., {Baker}, D., {Bartlett}, R., {Benna}, M., {Bougher}, S., {Brain}, D., {Carson}, D., {Cauffman}, S., {Chamberlin}, P., {Chaufray}, J., , {Cheatom}, O., et al. (2015). The Mars Atmosphere and Volatile Evolution ( MAVEN) Mission. \ssr, 195, 3-48.
• {Koskinen}, T., {Erwin}, J., , R. (2015). On the escape of CH$_{4}$ from Pluto's atmosphere. \grl, 42, 7200-7205.
• {Koskinen}, T., {Sandel}, B., {Yelle}, R., {Strobel}, D., {M{\"u}ller-Wodarg}, I., , J. (2015). Saturn's variable thermosphere from Cassini/UVIS occultations. \icarus, 260, 174-189.
• {Koskinen}, T., {Strobel}, D., {West}, R., , R. (2015). Variability in Saturn's upper atmosphere from Cassini/UVIS occultations. European Planetary Science Congress 2015, held 27 September - 2 October, 2015 in Nantes, France, Online at http://meetingorganizer.copernicus.org/EPSC2015, id.EPSC2015-294, 10, EPSC2015-294.
• {Lavvas}, P., {Yelle}, R., {Heays}, A., {Campbell}, L., {Brunger}, M., {Galand}, M., , V. (2015). Hot N2 in Titan's upper atmosphere. European Planetary Science Congress 2015, held 27 September - 2 October, 2015 in Nantes, France, Online at http://meetingorganizer.copernicus.org/EPSC2015, id.EPSC2015-124, 10, EPSC2015-124.
• {Lavvas}, P., {Yelle}, R., {Heays}, A., {Campbell}, L., {Brunger}, M., {Galand}, M., , V. (2015). N$_{2}$ state population in Titan's atmosphere. \icarus, 260, 29-59.
• {Lillis}, R., {Brain}, D., {Bougher}, S., {Leblanc}, F., {Luhmann}, J., {Jakosky}, B., {Modolo}, R., {Fox}, J., {Deighan}, J., {Fang}, X., {Wang}, Y., {Lee}, Y., {Dong}, C., {Ma}, Y., {Cravens}, T., {Andersson}, L., {Curry}, S., {Schneider}, N., {Combi}, M., , {Stewart}, I., et al. (2015). Characterizing Atmospheric Escape from Mars Today and Through Time, with MAVEN. \ssr, 195, 357-422.
• {Lo}, D., {Yelle}, R., {Schneider}, N., {Jain}, S., {Stewart}, A., {England}, S., {Deighan}, J., {Stiepen}, A., {Evans}, J., {Stevens}, M., {Chaffin}, M., {Crismani}, M., {McClintock}, W., {Clarke}, J., {Holsclaw}, G., {Lef{\`e}vre}, F., , B. (2015). Nonmigrating tides in the Martian atmosphere as observed by MAVEN IUVS. \grl, 42, 9057-9063.
• {Mahaffy}, P., {Benna}, M., {Elrod}, M., {Yelle}, R., {Bougher}, S., {Stone}, S., , B. (2015). Structure and composition of the neutral upper atmosphere of Mars from the MAVEN NGIMS investigation. \grl, 42, 8951-8957.
• {Mahieux}, A., {Erwin}, J., {Chamberlain}, S., {Robert}, S., {Thomas}, I., {Vandaele}, A., {Trompet}, L., {Wilquet}, V., , R. (2015). Studying the Venus terminator thermal structure observed by SOIR/VEx with a 1D radiative transfer model. European Planetary Science Congress 2015, held 27 September - 2 October, 2015 in Nantes, France, Online at http://meetingorganizer.copernicus.org/EPSC2015, id.EPSC2015-807, 10, EPSC2015-807.
• {McClintock}, W., {Schneider}, N., {Holsclaw}, G., {Clarke}, J., {Hoskins}, A., {Stewart}, I., {Montmessin}, F., {Yelle}, R., , J. (2015). The Imaging Ultraviolet Spectrograph (IUVS) for the MAVEN Mission. \ssr, 195, 75-124.
• {Sagni{\`e}res}, L., {Galand}, M., {Cui}, J., {Lavvas}, P., {Vigren}, E., {Vuitton}, V., {Yelle}, R., {Wellbrock}, A., , A. (2015). Influence of local ionization on ionospheric densities in Titan's upper atmosphere. Journal of Geophysical Research (Space Physics), 120, 5899-5921.
• {Sandel}, B., {Gr{\"o}ller}, H., {Yelle}, R., {Koskinen}, T., {Lewis}, N., {Bertaux}, J., {Montmessin}, F., merais}, E. (2015). Altitude profiles of O$_{2}$ on Mars from SPICAM stellar occultations. \icarus, 252, 154-160.
• {Schneider}, N., {Deighan}, J., {Stewart}, A., {McClintock}, W., {Jain}, S., {Chaffin}, M., {Stiepen}, A., {Crismani}, M., {Plane}, J., {Carrillo-S{\'a}nchez}, J., {Evans}, J., {Stevens}, M., {Yelle}, R., {Clarke}, J., {Holsclaw}, G., {Montmessin}, F., , B. (2015). MAVEN IUVS observations of the aftermath of the Comet Siding Spring meteor shower on Mars. \grl, 42, 4755-4761.
• {Schneider}, N., {McClintok}, W., {Stewart}, A., {Deighan}, J., {Clarke}, J., {Holsclaw}, G., {Montmessin}, F., {Lefevre}, F., {Chaufray}, J., {Jain}, S., {Stiepen}, A., {Chaffin}, M., {Crismani}, M., {Matta}, M., {Evans}, J., {Stevens}, M., {Yelle}, R., , B. (2015). First Results From MAVEN's Imaging UV Spectrograph. European Planetary Science Congress 2015, held 27 September - 2 October, 2015 in Nantes, France, Online at http://meetingorganizer.copernicus.org/EPSC2015, id.EPSC2015-410, 10, EPSC2015-410.
• {Teolis}, B., {Niemann}, H., {Waite}, J., {Gell}, D., {Perryman}, R., {Kasprzak}, W., {Mandt}, K., {Yelle}, R., {Lee}, A., {Pelletier}, F., {Miller}, G., {Young}, D., {Bell}, J., {Magee}, B., {Patrick}, E., {Grimes}, J., {Fletcher}, G., , V. (2015). A Revised Sensitivity Model for Cassini INMS: Results at Titan. \ssr, 190, 47-84.
• {Tinetti}, G., {Drossart}, P., {Eccleston}, P., {Hartogh}, P., {Isaak}, K., {Linder}, M., {Lovis}, C., {Micela}, G., {Ollivier}, M., {Puig}, L., & al., e. (2015). The EChO science case. Experimental Astronomy, 40, 329-391.
• {Vigren}, E., {Galand}, M., {Yelle}, R., {Wellbrock}, A., {Coates}, A., {Snowden}, D., {Cui}, J., {Lavvas}, P., {Edberg}, N., {Shebanits}, O., {Wahlund}, J., {Vuitton}, V., , K. (2015). Ionization balance in Titan's nightside ionosphere. \icarus, 248, 539-546.
• {Vuitton}, V., {Yelle}, R., {Klippenstein}, S., {Lavvas}, P., rst}, S. (2015). Simulating the density of HC15N in the Titan atmosphere with a coupled ion-neutral photochemical model. European Planetary Science Congress 2015, held 27 September - 2 October, 2015 in Nantes, France, Online at http://meetingorganizer.copernicus.org/EPSC2015, id.EPSC2015-478, 10, EPSC2015-478.
• Cui, J., Yelle, R. V., Li, T., Snowden, D. S., & Müller-Wodarg, I. F. (2014). Density waves in Titan's upper atmosphere. Journal of Geophysical Research A: Space Physics, 119(1), 490-518.
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  Abstract: Analysis of the Cassini Ion Neutral Mass Spectrometer data reveals the omnipresence of density waves in various constituents of Titan's upper atmosphere, with quasi-periodical structures visible for N2, CH 4,29N2, and some of the minor constituents. The N2 amplitude lies in the range of ≈4%-16%with a mean of ≈8%. Compositional variation is clearly seen as a sequence of decreasing amplitude with increasing scale height. The observed vertical variation of amplitude implies significant wave dissipation in different constituents, possibly contributed by molecular viscosity for N2but by both molecular viscosity and binary diffusion for the others. A wave train with near horizontally propagating wave energy and characterized by a wavelength of ≈730 km and a wave period of ≈10 h is found to best reproduce various aspects of the observations in a globally averaged sense. Some horizontal and seasonal trends in wave activity are identified, suggesting a connection between the mechanism driving the overall variability in the background atmosphere and the mechanism driving the waves. No clear association of wave activity with magnetospheric particle precipitation can be identified from the data. Key Points Density waves are an omnipresent feature of Titan's upper atmosphere Compositional variation and wave dissipation are clearly seen Possible ranges of wavelength and wave period are estimated ©2013. American Geophysical Union. All Rights Reserved.
• Koskinen, T. T., Lavvas, P., Harris, M. J., & Yelle, R. V. (2014). Thermal escape from extrasolar giant planets. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 372(2014).
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  Abstract: The detection of hot atomic hydrogen and heavy atoms and ions at high altitudes around close-in extrasolar giant planets (EGPs) such as HD209458b implies that these planets have hot and rapidly escaping atmospheres that extend to several planetary radii. These characteristics, however, cannot be generalized to all close-in EGPs. The thermal escape mechanism and mass loss rate from EGPs depend on a complex interplay between photochemistry and radiative transfer driven by the stellar UV radiation. In this study, we explore how these processes change under different levels of irradiation on giant planets with different characteristics. We confirm that there are two distinct regimes of thermal escape from EGPs, and that the transition between these regimes is relatively sharp. Our results have implications for thermal mass loss rates from different EGPs that we discuss in the context of currently known planets and the detectability of their upper atmospheres. © 2014 The Author(s) Published by the Royal Society.
• Snowden, D., & Yelle, R. V. (2014). The thermal structure of Titan's upper atmosphere, II: Energetics. Icarus, 228, 64-77.
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  Abstract: Temperature profiles derived from Cassini Ion Neutral Mass Spectrometer data in Paper I show that the thermal structure of Titan's upper atmosphere is extremely variable. The median temperature of each vertical profile, which is approximately equal to the temperature derived by fitting the barometric equation to the N2 density profile, varied between 112 and 175K. Here we attempt to understand the cause of the 60K variation in temperature, as well as large local perturbations in temperature, by estimating the strength of potentially important energy sources and sinks in Titan's thermosphere including ion and electron precipitation from Saturn's magnetosphere, Joule heating, and wave dissipation. The apparent correlation between the temperature of Titan's thermosphere and Titan's plasma environment suggest that particle precipitation from Saturn's magnetosphere may be the most significant heat source, but we find that the energy deposited by magnetospheric sources is less than solar EUV and results from a thermal structure model indicate that magnetospheric particle precipitation only increases the temperature of Titan's thermosphere by ~7K; therefore, heating due to magnetospheric particle precipitation is too small to explain the largest temperature variations observed. We also estimate the energy deposited by waves in Titan's thermosphere and show that wave dissipation may be a significant source of heating or cooling in Titan's upper atmosphere. © 2013 Elsevier Inc.
• {Cui}, J., {Yelle}, R., {Li}, T., {Snowden}, D., , I. (2014). Density waves in Titan's upper atmosphere. European Planetary Science Congress 2014, EPSC Abstracts, Vol.~9, id.~EPSC2014-688, 9, EPSC2014-688.
• {Erwin}, J., {Yelle}, R., , T. (2014). Escape of Hydrogen from HD209458b. European Planetary Science Congress 2014, EPSC Abstracts, Vol.~9, id.~EPSC2014-594, 9, EPSC2014-594.
• {Gr{\"o}ller}, H., {Sandel}, B., {Yelle}, R., {Koskinen}, T., {Lewis}, N., {Bertaux}, J., {Montmessin}, F., , E. (2014). O2 abundance on Mars observed by Mars Express / SPICAM. European Planetary Science Congress 2014, EPSC Abstracts, Vol.~9, id.~EPSC2014-536, 9, EPSC2014-536.
• {Koskinen}, T., {Yelle}, R., {Lavvas}, P., , J. (2014). Electrodynamics in Giant Planet Atmospheres. AGU Fall Meeting Abstracts.
• {Koskinen}, T., {Yelle}, R., {Lavvas}, P., , J. (2014). Electrodynamics on Extrasolar Giant Planets. \apj, 796, 16.
• {Lavvas}, P., {Koskinen}, T., , R. (2014). Electron Densities and Alkali Atoms in Exoplanet Atmospheres. \apj, 796, 15.
• {Lavvas}, P., {Koskinen}, T., , R. (2014). Electron densities and alkali atoms in exoplanet atmospheres. ArXiv e-prints.
• {Lavvas}, P., {Yelle}, R., , T. (2014). Na, K, and electrons in exoplanet atmospheres. European Planetary Science Congress 2014, EPSC Abstracts, Vol.~9, id.~EPSC2014-132, 9, EPSC2014-132.
• {Ma}, Y., {Jia}, Y., {Russell}, C., {Nagy}, A., {Toth}, G., {Combi}, M., {Yelle}, R., {Dong}, C., , S. (2014). The Mars Magnetosphere in the Tail of Comet C/2013 A1(Siding Spring). AGU Fall Meeting Abstracts.
• {Sagni{\`e}res}, L., {Galand}, M., {Cui}, J., {Lavvas}, P., {Vigren}, E., {Vuitton}, V., {Yelle}, R., {Wellbrock}, A., , A. (2014). Influence of the local ionization sources on ionospheric densities in Titan's upper atmosphere. European Planetary Science Congress 2014, EPSC Abstracts, Vol.~9, id.~EPSC2014-469, 9, EPSC2014-469.
• {Snowden}, D., , R. (2014). The global precipitation of magnetospheric electrons into Titan's upper atmosphere. AGU Fall Meeting Abstracts.
• {Snowden}, D., , R. (2014). The global precipitation of magnetospheric electrons into Titan's upper atmosphere. \icarus, 243, 1-15.
• {Vigren}, E., {Galand}, M., {Shebanits}, O., {Wahlund}, J., {Geppert}, W., {Lavvas}, P., {Vuitton}, V., , R. (2014). Increasing Positive Ion Number Densities below the Peak of Ion-Electron Pair Production in Titan's Ionosphere. \apj, 786, 69.
• {Yelle}, R., , B. (2014). MAVEN In Situ Measurements of Siding Spring and its Effect on Mars. AGU Fall Meeting Abstracts.
• {Yelle}, R., {Mahieux}, A., {Morrison}, S., {Vuitton}, V., rst}, S. (2014). Perturbation of the Mars atmosphere by the near-collision with Comet C/2013 A1 (Siding Spring). \icarus, 237, 202-210.
• Bonnet, J., Thissen, R., Frisari, M., Vuitton, V., Quirico, É., Orthous-Daunay, F., Dutuit, O., Roy, L. L., Fray, N., Cottin, H., Hörst, S. M., & Yelle, R. V. (2013). Compositional and structural investigation of HCN polymer through high resolution mass spectrometry. International Journal of Mass Spectrometry, 354-355, 193-203.
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  Abstract: Nitrogen rich compounds are found in numerous planetary environments such as planetary atmospheres, meteorites and comets. To better understand the structure and composition of this natural organic material, laboratory analogs have been studied. Though HCN polymers have been studied since the beginning of the 19th century, their structure and composition are still poorly understood. In this work we report the first extended high resolution mass spectrometry study of HCN polymers. The mass spectra of the polymer contain hundreds of peaks to which we try to assign an elemental composition. Elemental analysis has been used to constrain the molecular formulae and isotopic signatures have also been used to confirm them. The large quantity of amine functions observed with both infrared (IR) spectroscopy and mass spectrometry indicates that amine groups are present in most chains found in HCN polymer. Collision induced dissociation (CID) tandem (MSn) measurements were also performed on eight molecular ions and aromatic rings have been identified. © 2013 Elsevier B.V.
• Capalbo, F. J., Bénilan, Y., Yelle, R. V., Koskinen, T. T., Sandel, B. R., Holsclaw, G. M., & McClintock, W. E. (2013). Solar occultation by titan measured by cassini/UVIS. Astrophysical Journal Letters, 766(2).
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  Abstract: We present the first published analysis of a solar occultation by Titan's atmosphere measured by the Ultraviolet Imaging Spectrograph on board Cassini. The data were measured during flyby T53 in 2009 April and correspond to latitudes between 21° and 28° south. The analysis utilizes the absorption of two solar emission lines (584 Å and 630 Å) in the ionization continuum of the N2 absorption cross section and solar emission lines around 1085 Å where absorption is due to CH4. The measured transmission at these wavelengths provides a direct estimate of the N2 and CH4 column densities along the line of sight from the spacecraft to the Sun, which we inverted to obtain the number densities. The high signal-to-noise ratio of the data allowed us to retrieve density profiles in the altitude range 1120-1400 km for nitrogen and 850-1300 km for methane. We find an N2 scale height of 76 km and a temperature of 153 K. Our results are in general agreement with those from previous work, although there are some differences. Particularly, our profiles agree, considering uncertainties, with the density profiles derived from the Voyager 1 Ultraviolet Spectrograph data, and with in situ measurements by the Ion Neutral Mass Spectrometer with revised calibration. © 2013. The American Astronomical Society. All rights reserved..
• Koskinen, T. T., Harris, M. J., Yelle, R. V., & Lavvas, P. (2013). The escape of heavy atoms from the ionosphere of HD209458b. I. A photochemical-dynamical model of the thermosphere. Icarus, 226(2), 1678-1694.
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  Abstract: The detections of atomic hydrogen, heavy atoms and ions surrounding the extrasolar giant planet (EGP) HD209458b constrain the composition, temperature and density profiles in its upper atmosphere. Thus the observations provide guidance for models that have so far predicted a range of possible conditions. We present the first hydrodynamic escape model for the upper atmosphere that includes all of the detected species in order to explain their presence at high altitudes, and to further constrain the temperature and velocity profiles. This model calculates the stellar heating rates based on recent estimates of photoelectron heating efficiencies, and includes the photochemistry of heavy atoms and ions in addition to hydrogen and helium. The composition at the lower boundary of the escape model is constrained by a full photochemical model of the lower atmosphere. We confirm that molecules dissociate near the 1μbar level, and find that complex molecular chemistry does not need to be included above this level. We also confirm that diffusive separation of the detected species does not occur because the heavy atoms and ions collide frequently with the rapidly escaping H and H+. This means that the abundance of the heavy atoms and ions in the thermosphere simply depends on the elemental abundances and ionization rates. We show that, as expected, H and O remain mostly neutral up to at least 3Rp, whereas both C and Si are mostly ionized at significantly lower altitudes. We also explore the temperature and velocity profiles, and find that the outflow speed and the temperature gradients depend strongly on the assumed heating efficiencies. Our models predict an upper limit of 8000K for the mean (pressure averaged) temperature below 3Rp, with a typical value of 7000K based on the average solar XUV flux at 0.047AU. We use these temperature limits and the observations to evaluate the role of stellar energy in heating the upper atmosphere. © 2012 Elsevier Inc.
• Koskinen, T. T., Sandel, B. R., Yelle, R. V., Capalbo, F. J., Holsclaw, G. M., McClintock, W. E., & Edgington, S. (2013). The density and temperature structure near the exobase of Saturn from Cassini UVIS solar occultations. Icarus, 226(2), 1318-1330.
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  Abstract: We analyzed 15 solar occultations observed by the Cassini UVIS instrument to constrain the density and temperature structure near the exobase of Saturn. We retrieved the density of H2 and thus the temperature at altitudes higher than 1900km above the 1bar level by analyzing the ionization continuum of H2 at wavelengths shorter than 804Å. We find that the exospheric temperature ranges from 370K to 540K, with a typical uncertainty of less than 20K. According to our data the temperature increases with latitude from the equator to the poles by 100-150K. At similar latitudes, the temperature varies by 20-50K at different times with no evidence for any systematic diurnal trend so far. Based on our data, the exobase of Saturn is 2700-3000km above the 1bar level and the thermal escape parameter near the exobase ranges from 260 to 340, implying that thermal escape from Saturn is firmly in the Jeans regime. The mixing ratio of H2 is close to unity at all altitudes below the exobase. We find that the pressure levels in the thermosphere deviate significantly from a simple spheroid predicted by potential theory. This is consistent with significant meridional temperature variations in the lower thermosphere. A global analysis of the temperature structure at different depths in the atmosphere is required to constrain both the shape and the deposition and redistribution of energy in the upper atmosphere further. © 2013 Elsevier Inc.
• Koskinen, T. T., Yelle, R. V., Harris, M. J., & Lavvas, P. (2013). The escape of heavy atoms from the ionosphere of HD209458b. II. Interpretation of the observations. Icarus, 226(2), 1695-1708.
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  Abstract: Transits in the H I 1216Å (Lyman α), O I 1334Å, C II 1335Å, and Si III 1206.5Å lines constrain the properties of the upper atmosphere of HD209458b. In addition to probing the temperature and density profiles in the thermosphere, they have implications for the properties of the lower atmosphere. Fits to the observations with a simple empirical model and a direct comparison with a more complex hydrodynamic model constrain the mean temperature and ionization state of the atmosphere, and imply that the optical depth of the extended thermosphere of the planet in the atomic resonance lines is significant. In particular, it is sufficient to explain the observed transit depths in the H I 1216Å line. The detection of O at high altitudes implies that the minimum mass loss rate from the planet is approximately 6×106kgs-1. The mass loss rate based on our hydrodynamic model is higher than this and implies that diffusive separation is prevented for neutral species with a mass lower than about 130amu by the escape of H. Heavy ions are transported to the upper atmosphere by Coulomb collisions with H+ and their presence does not provide as strong constraints on the mass loss rate as the detection of heavy neutral atoms. Models of the upper atmosphere with solar composition and heating based on the average solar X-ray and EUV flux agree broadly with the observations but tend to underestimate the transit depths in the O I, C II, and Si III lines. This suggests that the temperature and/or elemental abundances in the thermosphere may be higher than expected from such models. Observations of the escaping atmosphere can potentially be used to constrain the strength of the planetary magnetic field. We find that a magnetic moment of m≲0.04mJ, where mJ is the jovian magnetic moment, allows the ions to escape globally rather than only along open field lines. The detection of Si2+ in the thermosphere indicates that clouds of forsterite and enstatite do not form in the lower atmosphere. This has implications for the temperature and dynamics of the atmosphere that also affect the interpretation of transit and secondary eclipse observations in the visible and infrared wavelengths. © 2012 Elsevier Inc.
• Lavvas, P., Yelle, R. V., Koskinen, T., Bazin, A., Vuitton, V., Vigren, E., Galand, M., Wellbrock, A., Coates, A. J., Wahlund, J., Crary, F. J., & Snowden, D. (2013). Aerosol growth in Titan's ionosphere. Proceedings of the National Academy of Sciences of the United States of America, 110(8), 2729-2734.
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  PMID: 23382231;PMCID: PMC3581881;Abstract: Photochemically produced aerosols are common among the atmospheres of our solar system and beyond. Observations and models have shown that photochemical aerosols have direct consequences on atmospheric properties as well as important astrobiological ramifications, but the mechanisms involved in their formation remain unclear. Here we show that the formation of aerosols in Titan's upper atmosphere is directly related to ion processes, and we provide a complete interpretation of observed mass spectra by the Cassini instruments from small to large masses. Because all planetary atmospheres possess ionospheres, we anticipate that the mechanisms identified here will be efficient in other environments as well, modulated by the chemical complexity of each atmosphere.
• Snowden, D., Snowden, D., Yelle, R. V., Yelle, R. V., Cui, J., Cui, J., Wahlund, J. -., Wahlund, J. -., Edberg, N. J., Edberg, N. J., Ågren, K., & Ågren, K. (2013). The thermal structure of titan's upper atmosphere, I: Temperature profiles from Cassini INMS observations. Icarus, 226(1), 552-582.
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  Abstract: We derive vertical temperature profiles from Ion Neutral Mass Spectrometer (INMS) N2 density measurements from 32 Cassini passes. We find that the average temperature of Titan's thermosphere varies significantly from pass-to-pass between 112 and 175K. The temperatures from individual temperature profiles also varies considerably, with many passes exhibiting wave-like temperature perturbations and large temperature gradients. Wave-like temperature perturbations have wavelengths between 150 and 420km and amplitudes between 3% and 22% and vertical wave power spectra of the INMS data and HASI data have a slope between -2 and -3, which is consistent with vertically propagating atmospheric waves. The lack of a strong correlation between temperature and latitude, longitude, solar zenith angle, or local solar time indicates that the thermal structure of Titan's thermosphere is not primarily determined by the absorption of solar EUV flux. At N2 densities greater than 108cm-3, Titan's thermosphere is colder when Titan is observed in Saturn's magnetospheric lobes compared to Saturn's plasma sheet as proposed by Westlake et al. (Westlake, J.H. et al. [2011]. J. Geophys. Res. 116, A03318. http://dx.doi.org/10.1029/2010JA016251). This apparent correlation suggests that magnetospheric particle precipitation causes the temperature variability in Titan's thermosphere; however, at densities smaller than 108cm-3 the lobe passes are hotter than the plasma sheet passes and we find no correlation between the temperature of Titan's thermosphere and ionospheric signatures of enhanced particle precipitation, which suggests that the correlation is not indicative of a physical connection. The temperature of Titan's thermosphere also may have decreased by ~10K around mid-2007. Finally, we classify the vertical temperature profiles to show which passes are hot and cold and which passes have the largest temperature variations. In a companion paper (Part II), we estimate the strength of energy sources and sinks in Titan's thermosphere. © 2013 Elsevier Inc.
• Snowden, D., Yelle, R. V., Galand, M., Coates, A. J., Wellbrock, A., Jones, G. H., & Lavvas, P. (2013). Auroral electron precipitation and flux tube erosion in Titan's upper atmosphere. Icarus, 226(1), 186-204.
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  Abstract: Cassini dasta shows that Titan's atmosphere strongly depletes the electron content in Saturn's flux tubes, producing features known as electron bite-outs, which indicate that the flux of auroral electrons decreases over time. To understand this process we have developed a time-dependent two-stream model, which uses field line geometries and drift paths calculated by a three-dimensional multi-fluid model of Titan's plasma interaction. The boundary conditions of the model account for the time-dependent reduction or increase in electron flux along Saturn's magnetic field lines because of the loss or production of electrons in Titan's atmosphere. The modification of the auroral electron flux depends on the electron bounce period in Saturn's outer magnetosphere; therefore, we also calculate electron bounce periods along several Kronian field lines accounting for both the magnetic mirroring force and the field-aligned electric potential in Saturn's plasma sheet. We use the time-dependent two-stream model to calculate how the reduction in the auroral electron flux affects electron impact ionization and energy deposition rates in Titan's upper atmosphere. We find that the flux of higher energy (>50. eV) electrons entering Titan's atmosphere is strongly reduced over time, resulting in smaller ionization and energy deposition rates below ~1300. km altitude. Finally, we show that sample spectrograms produced from our calculations are consistent with CAPS-ELS data. © 2013 Elsevier Inc.
• Vigren, E., Galand, M., Yelle, R. V., Cui, J., Wahlund, J. -., Ågren, K., Lavvas, P. P., Mueller-Wodarg, I., Strobel, D. F., Vuitton, V., & Bazin, A. (2013). On the thermal electron balance in Titan's sunlit upper atmosphere. Icarus, 223(1), 234-251.
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  Abstract: The Cassini mission has investigated Titan's upper atmosphere in detail and found that, under solar irradiation, it has a well-developed ionosphere, which peaks between 1000 and 1200km. In this paper we focus on the T40, T41, T42 and T48 Titan flybys by the Cassini spacecraft and use in situ measurements of N2 and CH4 densities by the Ion Neutral Mass Spectrometer (INMS) as input into a solar energy deposition model to determine electron production rates. We combine these electron production rates with estimates of the effective recombination coefficient based on available laboratory data for Titan ions' dissociative recombination rates and electron temperatures derived from the Langmuir probe (LP) to predict electron number densities in Titan's upper atmosphere, assuming photochemical equilibrium and loss of electrons exclusively through dissociative recombination with molecular ions. We then compare these predicted electron number densities with those observed in Titan's upper atmosphere by the LP. The assumption of photochemical equilibrium is supported by a reasonable agreement between the altitudes where the electron densities are observed to peak and where the electron production rates are calculated to peak (roughly corresponding to the unit optical depth for HeII photons at 30.38nm). We find, however, that the predicted electron number densities are nearly a factor of two higher than those observed throughout the altitude range between 1050 and 1200km (where we have made estimates of the effective recombination coefficient). There are different possible reasons for this discrepancy; one possibility is that there may be important loss processes of free electrons other than dissociative recombination in Titan's upper atmosphere. © 2012 Elsevier Inc.
• Zhang, X., Nixon, C. A., Shia, R. L., West, R. A., Irwin, P. G., Yelle, R. V., Allen, M. A., & Yung, Y. L. (2013). Radiative forcing of the stratosphere of Jupiter, Part I: Atmospheric cooling rates from Voyager to Cassini. Planetary and Space Science.
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  Abstract: We developed a line-by-line heating and cooling rate model for the stratosphere of Jupiter, based on two complete sets of global maps of temperature, C2H2 and C2H6, retrieved from the Cassini and Voyager observations in the latitude and vertical plane, with a careful error analysis. The non-LTE effect is found unimportant on the thermal cooling rate below the 0.01 mbar pressure level. The most important coolants are molecular hydrogen between 10 and 100 mbar, and hydrocarbons, including ethane (C2H6), acetylene (C2H2) and methane (CH4), in the region above. The two-dimensional cooling rate maps are influenced primarily by the temperature structure, and also by the meridional distributions of C2H2 and C2H6. The temperature anomalies at the 1 mbar pressure level in the Cassini data and the strong C2H6 latitudinal contrast in the Voyager epoch are the two most prominent features influencing the cooling rate patterns, with the effect from the 'quasi-quadrennial oscillation (QQO)' thermal structures at ~20 mbar. The globally averaged CH4 heating and cooling rates are not balanced, clearly in the lower stratosphere under 10 mbar, and possibly in the upper stratosphere above the 1 mbar pressure level. Possible heating sources from the gravity wave breaking and aerosols are discussed. The radiative relaxation timescale in the lower stratosphere implies that the temperature profile might not be purely radiatively controlled. © 2013 Elsevier Ltd.
• Cui, J., Yelle, R. V., Strobel, D. F., Mller-Wodarg, I., Snowden, D. S., Koskinen, T. T., & Galand, M. (2012). The CH₄ structure in Titan's upper atmosphere revisited. Journal of Geophysical Research E: Planets, 117(11).
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  Abstract: In this study, we reanalyze the CH4 structure in Titan's upper atmosphere combining the Cassini Ion Neutral Mass Spectrometer (INMS) data from 32 flybys and incorporating several updates in the data reduction algorithms. We argue that based on our current knowledge of eddy mixing and neutral temperature, strong CH4 escape must occur on Titan. Ignoring ionospheric chemistry, the optimal CH4 loss rate is ∼3×1027s-1 or 80kgs-1 in a globally averaged sense, consistent with the early result of Yelle et al. (2008). The considerable variability in CH4 structure among different flybys implies that CH4 escape on Titan is more likely a sporadic rather than a steady process, with the CH4 profiles from about half of the flybys showing evidence for strong escape and most of the other flybys consistent with diffusive equilibrium. CH4 inflow is also occasionally required to interpret the data. Our analysis further reveals that strong CH4 escape preferentially occurs on the nightside of Titan, in conflict with the expectations of any solar-driven model. In addition, there is an apparent tendency of elevated CH4 escape with enhanced electron precipitation from the ambient plasma, but this is likely to be a coincidence as the time response of the CH4 structure may not be fast enough to leave an observable effect during a Titan encounter. © 2012. American Geophysical Union. All Rights Reserved.
• Hörst, S., Yelle, R. V., Buch, A., Carrasco, N., Cernogora, G., Dutuit, O., Quirico, E., Sciamma-O'Brien, E., Smith, M. A., Somogyi, A., Szopa, C., Thissen, R., & Vuitton, V. (2012). Formation of amino acids and nucleotide bases in a titan atmosphere simulation experiment. Astrobiology, 12(9), 809-817.
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  PMID: 22917035;PMCID: PMC3444770;Abstract: The discovery of large (> 100 u) molecules in Titan's upper atmosphere has heightened astrobiological interest in this unique satellite. In particular, complex organic aerosols produced in atmospheres containing C, N, O, and H, like that of Titan, could be a source of prebiotic molecules. In this work, aerosols produced in a Titan atmosphere simulation experiment with enhanced CO (N2/CH4/CO gas mixtures of 96.2%/2.0%/1.8% and 93.2%/5.0%/ 1.8%) were found to contain 18 molecules with molecular formulae that correspond to biological amino acids and nucleotide bases. Very high-resolution mass spectrometry of isotopically labeled samples confirmed that C4H5N3O, C4H4N2O2, C5H6N2O2, C5H5N5, and C6H9N3O2 are produced by chemistry in the simulation chamber. Gas chromatography-mass spectrometry (GC-MS) analyses of the non-isotopic samples confirmed the presence of cytosine (C4H5N3O), uracil (C5H4N2O2), thymine (C5H6N2O2), guanine (C5H5N5O), glycine (C2H5NO2), and alanine (C3H7NO2). Adenine (C5H5N5) was detected by GC-MS in isotopically labeled samples. The remaining prebiotic molecules were detected in unlabeled samples only and may have been affected by contamination in the chamber. These results demonstrate that prebiotic molecules can be formed by the highenergy chemistry similar to that which occurs in planetary upper atmospheres and therefore identifies a new source of prebiotic material, potentially increasing the range of planets where life could begin. © Mary Ann Liebert, Inc.
• Tinetti, G., Beaulieu, J. P., Henning, T., Meyer, M., Micela, G., Ribas, I., Stam, D., Swain, M., Krause, O., Ollivier, M., Pace, E., Swinyard, B., Aylward, A., Boekel, R. v., Coradini, A., Encrenaz, T., Snellen, I., Zapatero-Osorio, M., Bouwman, J., , Cho, J. Y., et al. (2012). EChO: Exoplanet characterisation observatory. Experimental Astronomy, 34(2), 311-353.
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  Abstract: A dedicated mission to investigate exoplanetary atmospheres represents a major milestone in our quest to understand our place in the universe by placing our Solar System in context and by addressing the suitability of planets for the presence of life. EChO-the Exoplanet Characterisation Observatory-is a mission concept specifically geared for this purpose. EChO will provide simultaneous, multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. The use of passive cooling, few moving parts and well established technology gives a low-risk and potentially long-lived mission. EChO will build on observations by Hubble, Spitzer and ground-based telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. However, EChO's configuration and specifications are designed to study a number of systems in a consistent manner that will eliminate the ambiguities affecting prior observations. EChO will simultaneously observe a broad enough spectral region-from the visible to the mid-infrared-to constrain from one single spectrum the temperature structure of the atmosphere, the abundances of the major carbon and oxygen bearing species, the expected photochemically-produced species and magnetospheric signatures. The spectral range and resolution are tailored to separate bands belonging to up to 30 molecules and retrieve the composition and temperature structure of planetary atmospheres. The target list for EChO includes planets ranging from Jupiter-sized with equilibrium temperatures Teq up to 2,000 K, to those of a few Earth masses, with Teq \u223c 300 K. The list will include planets with no Solar System analog, such as the recently discovered planets GJ1214b, whose density lies between that of terrestrial and gaseous planets, or the rocky-iron planet 55 Cnc e, with day-side temperature close to 3,000 K. As the number of detected exoplanets is growing rapidly each year, and the mass and radius of those detected steadily decreases, the target list will be constantly adjusted to include the most interesting systems. We have baselined a dispersive spectrograph design covering continuously the 0. 4-16 μm spectral range in 6 channels (1 in the visible, 5 in the InfraRed), which allows the spectral resolution to be adapted from several tens to several hundreds, depending on the target brightness. The instrument will be mounted behind a 1. 5 m class telescope, passively cooled to 50 K, with the instrument structure and optics passively cooled to \u223c45 K. EChO will be placed in a grand halo orbit around L2. This orbit, in combination with an optimised thermal shield design, provides a highly stable thermal environment and a high degree of visibility of the sky to observe repeatedly several tens of targets over the year. Both the baseline and alternative designs have been evaluated and no critical items with Technology Readiness Level (TRL) less than 4-5 have been identified. We have also undertaken a first-order cost and development plan analysis and find that EChO is easily compatible with the ESA M-class mission framework. © 2012 Springer Science+Business Media B.V.
• Vuitton, V., Yelle, R. V., Lavvas, P., & Klippenstein, S. J. (2012). Rapid association reactions at low pressure: Impact on the formation of hydrocarbons on titan. Astrophysical Journal, 744(1).
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  Abstract: Photochemical models of Titan's atmosphere predict that three-body association reactions are the main production route for several major hydrocarbons. The kinetic rate constants of these reactions strongly depend on density and are therefore only important in Titan's lower atmosphere. However, radiative association reactions do not depend on pressure. The possible existence of large rates at low density suggests that association reactions could significantly affect the chemistry of Titan's upper atmosphere and better constraints for them are required. The kinetic parameters of these reactions are extremely difficult to constrain by experimental measurements as the low pressure of Titan's upper atmosphere cannot be reproduced in the laboratory. However, in the recent years, theoretical calculations of kinetics parameters have become more and more reliable. We therefore calculated several radical-radical and radical-molecule association reaction rates using transition state theory. The calculations indicate that association reactions are fast even at low pressure for adducts having as few as four C atoms. These drastic changes have however only moderate consequences for Titan's composition. Locally, mole fractions can vary by as much as one order of magnitude but the column-integrated production and condensation rates of hydrocarbons change only by a factor of a few. We discuss the impact of these results for the organic chemistry. It would be very interesting to check the impact of these new rate constants on other environments, such as giant and extrasolar planets as well as the interstellar medium.
• Cui, J., Yelle, R. V., Müller-Wodarg, I., Lavvas, P. P., & Galand, M. (2011). The implications of the H₂ variability in Titan's exosphere. Journal of Geophysical Research A: Space Physics, 116(11).
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  Abstract: We present in this paper an investigation of the distribution of H 2 in Titan's exosphere, based on the measurements made with the Ion Neutral Mass Spectrometer (INMS) onboard Cassini during 32 encounters with the satellite. The observed H2 density in Titan's exosphere shows significant variance from flyby to flyby. However, no appreciable trend with geophysical or solar conditions can be identified. A data-model comparison is made in the framework of the Chamberlain approach, taking into account two ideal cases. First, we assume that the observed variability is spatial. In this case, the damping of exobase perturbations when propagating into the exosphere is a diagnostic of the spatial scale of the perturbations. We find that for all reasonable choices of this spatial scale, the model predicts significantly more damping than implied by the INMS data. Second, we assume that at any given time, the physical conditions in Titan's upper atmosphere and exosphere are globally uniform, but these conditions evolve with time, indicating that the observed variability is temporal. In such a case, the observations can be interpreted as a result of exobase perturbations on timescales in the range of ∼10 3-106 s. The time-varying H2 exosphere of Titan essentially reflects the varying structure and energy deposition in the upper atmosphere of the satellite, which are ultimately determined by the variations in either the solar EUV/UV radiation or the level of magnetospheric particle precipitation. However, we do not expect the considerable variability observed for Titan's H2 exosphere to be induced by the varying solar inputs into Titan's atmosphere. Instead, we postulate that such a variability is more likely to be associated with Titan's varying plasma environment. Comparisons between different categories of Titan flybys tentatively reveal that the H 2 exosphere tends to be more energetic and more expanded, and H 2 molecules tend to escape more rapidly, with increasing levels of electron precipitation from the ambient plasma environment. © 2011 by the American Geophysical Union.
• Koskinen, T. T., Yelle, R. V., Snowden, D. S., Lavvas, P., Sandel, B. R., Capalbo, F. J., Benilan, Y., & West, R. A. (2011). The mesosphere and lower thermosphere of Titan revealed by Cassini/UVIS stellar occultations. Icarus, 216(2), 507-534.
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  Abstract: Stellar occultations observed by the Cassini/UVIS instrument provide unique data that probe the mesosphere and lower thermosphere of Titan at altitudes between 400 and 1400km. This region is a site of complex photochemistry that forms hydrocarbon and nitrile species, and plays a crucial role in the formation of the organic hazes observed in the stratosphere, but has yet to be adequately characterized. We analyzed publicly available data obtained between flybys Tb in December 2004 and T58 in July 2009, with an emphasis on two stable occultations obtained during flybys T41 and T53. We derived detailed density profiles for CH4, C2H2, C2H4, C4H2, HCN, HC3N and C6H6 between ∼400 and 1200km and extinction coefficients for aerosols between 400 and 900km. Our analysis reveals the presence of extinction layers in the occultation data that are associated with large perturbations in the density profiles of the gaseous species and extinction profiles of the aerosols. These relatively stable features vary in appearance with location and change slowly over time. In particular, we identify a sharp extinction layer between 450 and 550km that coincides with the detached haze layer. In line with recent images obtained by Cassini/ISS, the altitude of this layer changes rapidly around the equinox in 2009. Our results point to unexpected complexity that may have significant consequences for the dynamics and physical processes taking place in the upper atmosphere of Titan. © 2011 Elsevier Inc.
• Lavvas, P., Galand, M., Yelle, R. V., Heays, A. N., Lewis, B. R., Lewis, G. R., & Coates, A. J. (2011). Energy deposition and primary chemical products in Titan's upper atmosphere. Icarus, 213(1), 233-251.
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  Abstract: Cassini results indicate that solar photons dominate energy deposition in Titan's upper atmosphere. These dissociate and ionize nitrogen and methane and drive the subsequent complex organic chemistry. The improved constraints on the atmospheric composition from Cassini measurements demand greater precision in the photochemical modeling. Therefore, in order to quantify the role of solar radiation in the primary chemical production, we have performed detailed calculations for the energy deposition of photons and photoelectrons in the atmosphere of Titan and we validate our results with the Cassini measurements for the electron fluxes and the EUV/FUV emissions. We use high-resolution cross sections for the neutral photodissociation of N2, which we present here, and show that they provide a different picture of energy deposition compared to results based on low-resolution cross sections. Furthermore, we introduce a simple model for the energy degradation of photoelectrons based on the local deposition approximation and show that our results are in agreement with detailed calculations including transport, in the altitude region below 1200km, where the effects of transport are negligible. Our calculated, daytime, electron fluxes are in good agreement with the measured fluxes by the Cassini Plasma Spectrometer (CAPS), and the same holds for the measured FUV emissions by the Ultraviolet Imaging Spectrometer (UVIS). Finally, we present the vertical production profiles of radicals and ions originating from the interaction of photons and electrons with the main components of Titan's atmosphere, along with the column integrated production rates at different solar zenith angles. These can be used as basis for any further photochemical calculations. © 2011 Elsevier Inc.
• Lavvas, P., Griffith, C. A., & Yelle, R. V. (2011). Condensation in Titan's atmosphere at the Huygens landing site. Icarus, 215(2), 732-750.
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  Abstract: We present a self-consistent description of Titan's aerosols-clouds-gases system and compare our results with the optical properties retrieved from measurements made by the Descent Imager/Spectral Radiometer (DISR) experiment on the Huygens probe (Tomasko, M.G. et al. [2008]. Planet. Space Sci. 56, 669-707). Our calculations include the condensation of methane, ethane and hydrogen cyanide on photochemical aerosols produced in the thermosphere. Our results suggest that the two distinct extinction layers observed by DISR below 80. km are produced by HCN and methane condensation, while for the Huygens' equatorial conditions simulated here, the contribution of ethane clouds to the total opacity is negligible. The HCN mass flux is comparable to the mass flux of aerosols, thus the majority of the HCN cloud particles have a similar size with the aerosol particles, they are HCN-coated aerosols. Ethane cloud particles have sizes between 2 and 10μm depending on altitude, and methane clouds grow to an average size of ~100μm before starting to evaporate below 10. km. The reproduction by the simulation of the main features observed by DISR suggests that the atmospheric snapshot acquired by the Huygens instruments corresponds to a condition very close to the steady state simulated here. This points to the stability of the equatorial atmospheric conditions at the time of the descent. Moreover, we investigate the resulting abundance of ethane in the lower part of the atmosphere and its impact on the flux of condensates to the surface. Our results suggest that under the steady state conditions investigated, ethane condensates evaporate before reaching the surface and that the ethane gas abundance close to the surface is well below its saturation limit. © 2011 Elsevier Inc.
• Cravens, T. E., Yelle, R. V., Wahlund, J. -., Shemansky, D. E., & Nagy, A. F. (2010). Composition and structure of the ionosphere and thermosphere. Titan from Cassini-Huygens, 259-295.
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  Abstract: Airglow emissions, radio and solar occultation data from the Voyager mission over a quarter of a century ago provided the main source of information on the composition and structure of Titan's upper atmosphere and ionosphere until October 2004, when the Cassini Orbiter first encountered Titan during the Ta fly-by. During this encounter, in situ measurements were made by many instruments onboard the Orbiter, including the Ion and Neutral Mass Spectrometer (INMS), the Radio Wave and Plasma Wave Spectrometer (RPWS), the Magnetometer (MAG), and the Cassini Plasma Spectrometer (CAPS). For example, INMS measurements confirmed that the major neutral species were molecular nitrogen and methane. Other species detected included mole cular hydrogen, acetylene, ethylene, benzene, and propane. The Langmuir probe part of the RPWS experiment observed substantial ionospheric electron densities and measured electron temperatures significantly exceeding the neutral temperature. A large set of data on the upper atmosphere and ionosphere has been collected during the many Titan encounters following Ta. The first composition measurements for the ionosphere were made by INMS during the outbound leg of the T5 pass in April 2005. A rich and complex ion-neutral chemistry scheme was predicted prior to the Cassini mission and the INMS composition data indeed revealed the presence of a very large number of ion species, both predicted and unpre-dicted. Stellar occultation measurements made by the Cassini Ultraviolet Spectrometer (UVIS) provided important information on the structure and composition of Titan's upper atmosphere, and radio occultation measurements made by the Radio Science Subsystem (RSS) revealed the existence of a substantial ionosphere even for altitudes below 1000 km. The discovery of negative ions in the ionosphere was also very exciting. A vigorous modeling effort aimed at explaining the structure and composition of the upper atmosphere and ionosphere is helping to put the data into a broader theoretical context. For example, solar extreme ultraviolet and x-ray radiation and energetic electrons from Saturn's magnetosphere interact with the upper atmosphere producing the ionosphere and initiating a complex neutral and ion chemistry that has important effects extending deep into the atmosphere. © 2010 Springer Science+Business Media B.V.
• Cui, J., Galand, M., Yelle, R. V., Wahlund, J. -., Agren, K., Waite Jr., J. H., & Dougherty, M. K. (2010). Ion transport in Titan's upper atmosphere. Journal of Geophysical Research A: Space Physics, 115(6).
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  Abstract: Based on a combined Cassini data set including Ion Neutral Mass Spectrometer, Radio Plasma Wave Science, and Magnetometer measurements made during nine close encounters of the Cassini spacecraft with Titan, we investigate the electron (or total ion) distribution in the upper ionosphere of the satellite between 1250 and 1600 km. A comparison of the measured electron distribution with that in diffusive equilibrium suggests global ion escape from Titan with a total ion loss rate of ∼(1.7 ± 0.4) × 10 25 s-1. Significant diurnal variation in ion transport is implied by the data, characterized by ion outflow at the dayside and ion inflow at the nightside, especially below ∼1400 km. This is interpreted as a result of day-to-night ion transport, with a horizontal transport rate estimated to be ∼(1.4 ± 0.5) × 1024 s-1. Such an ion flow is likely to be an important source for Titan's nightside ionosphere, as proposed in Cui et al. [2009a]. Copyright © 2010 by the American Geophysical Union.
• Galand, M., Yelle, R., Cui, J., Wahlund, J., Vuitton, V., Wellbrock, A., & Coates, A. (2010). Ionization sources in Titan's deep ionosphere. Journal of Geophysical Research A: Space Physics, 115(7).
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  Abstract: We analyze a multi-instrumental data set from four Titan encounters by the Cassini spacecraft to investigate in detail the formation of the ionosphere. The data set includes observations of thermospheric and ionospheric species and suprathermal electrons. A model describing the solar and electron energy deposition is used as an organizing element of the Cassini data set. We first compare the calculated secondary electron production rates with the rates inferred from suprathermal electron intensity measurements. We then calculate an effective electron dissociative recombination coefficient, applying three different approaches to the Cassini data set. Our findings are threefold: (1) The effective recombination coefficient derived under sunlit conditions in the deep ionosphere (
• Koskinen, T. T., Yelle, R. V., Lavvas, P., & Lewis, N. K. (2010). Characterizing the thermosphere of HD209458b with UV transit observations. Astrophysical Journal Letters, 723(1), 116-128.
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  Abstract: Transmission spectroscopy at UV wavelengths is a rich and largely unexplored source of information about the upper atmospheres of extrasolar planets. So far, UV transit observations have led to the detection of atomic hydrogen, oxygen, and ionized carbon in the upper atmosphere of HD209458b. The interpretation of these observations is controversial-it is not clear if the absorption arises from an escaping atmosphere interacting with the stellar radiation and stellar wind, or from the thermosphere inside the Roche lobe. In this paper, we introduce an empirical model that can be used to analyze UV transit depths of extrasolar planets.We use this model to interpret the transits of HD209458b in the HI 1216 and the OI 1304 triplet emission lines. The results indicate that the mean temperature of the thermosphere is T = 8000-11,000K and that the H2/H dissociation front is located at pressures between p = 0.1 and 1 μbar, which correspond to a distance r ≈ 1.1 Rp from the center of the planet. The upper boundary of the model thermosphere is located at r = 2.7-3 Rp, above which the atmosphere is mostly ionized. We find that the HI transit depth in the wings of the H Lyα line reflects the optical depth of the thermosphere, but that the atmosphere also overflows the Roche lobe. By assuming a solar mixing ratio of oxygen, we obtain an OI transit depth that is statistically consistent with the observations. An OI transit depth comparable to the HI transit depth is possible if the atmosphere is undergoing fast hydrodynamic escape, the O/H ratio is supersolar, or if a significant quantity of neutral oxygen is found outside the Roche lobe. We find that the observations can be explained solely by absorption in the upper atmosphere and extended clouds of energetic neutral atoms or atoms strongly perturbed by radiation pressure are not required. Due to the large uncertainty in the data, repeated observations are necessary to better constrain the Oi transit depths and thus the composition of the thermosphere. © 2010. The American Astronomical Society.
• Lavvas, P., Yelle, R. V., & Griffith, C. A. (2010). Titan's vertical aerosol structure at the Huygens landing site: Constraints on particle size, density, charge, and refractive index. Icarus, 210(2), 832-842.
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  Abstract: We present a one dimension simulation of Titan's aerosol distribution and compare our results with the haze optical properties retrieved by the DISR observations (Tomasko, M.G., Doose, L., Engel, S., Dafoe, L.E., West, R., Lemmon, M., Karkoschka, E., See, C. [2008]. Planet. Space Sci. 56, 669-707). We set the mass production of aerosols in the thermosphere to 3×10-14gcm-2 s-1 and follow the evolution of the particles due to coagulation, sedimentation and atmospheric mixing. We use the observed aerosol phase functions at 100km to constrain the particle's charge density to 15e/μm. With this charge density, the microphysical model predictions for the number of monomers, and the particle size and density, are in good agreement with the DISR measurements. In addition, we derive a new refractive index for the aerosols based on the single scattering albedo inferred from DISR measurements. The imaginary index is larger than previous estimates based on laboratory analogs, with an increasing absorption toward the near-IR. Our simulation provides a good description of the particle properties above 80km, but we find that condensation effects must be included in order to interpret aerosol characteristics at lower altitudes. © 2010 Elsevier Inc.
• Tinetti, G., Cho, J. Y., Griffith, C. A., Grasset, O., Grenfell, L., Guillot, T., Koskinen, T. T., Moses, J. I., Pinfield, D., Tennyson, J., Tessenyi, M., Wordsworth, R., Aylward, A., Boekel, R. V., Coradini, A., Encrenaz, T., Snellen, I., Zapatero-Osorio, M. R., Bouwman, J., , Coude, V., et al. (2010). The science of EChO. Proceedings of the International Astronomical Union, 6(S276), 359-370.
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  Abstract: The science of extra-solar planets is one of the most rapidly changing areas of astrophysics and since 1995 the number of planets known has increased by almost two orders of magnitude. A combination of ground-based surveys and dedicated space missions has resulted in 560-plus planets being detected, and over 1200 that await confirmation. NASA's Kepler mission has opened up the possibility of discovering Earth-like planets in the habitable zone around some of the 100,000 stars it is surveying during its 3 to 4-year lifetime. The new ESA's Gaia mission is expected to discover thousands of new planets around stars within 200 parsecs of the Sun. The key challenge now is moving on from discovery, important though that remains, to characterisation: what are these planets actually like, and why are they as they are In the past ten years, we have learned how to obtain the first spectra of exoplanets using transit transmission and emission spectroscopy. With the high stability of Spitzer, Hubble, and large ground-based telescopes the spectra of bright close-in massive planets can be obtained and species like water vapour, methane, carbon monoxide and dioxide have been detected. With transit science came the first tangible remote sensing of these planetary bodies and so one can start to extrapolate from what has been learnt from Solar System probes to what one might plan to learn about their faraway siblings. As we learn more about the atmospheres, surfaces and near-surfaces of these remote bodies, we will begin to build up a clearer picture of their construction, history and suitability for life. The Exoplanet Characterisation Observatory, EChO, will be the first dedicated mission to investigate the physics and chemistry of Exoplanetary Atmospheres. By characterising spectroscopically more bodies in different environments we will take detailed planetology out of the Solar System and into the Galaxy as a whole. EChO has now been selected by the European Space Agency to be assessed as one of four M3 mission candidates. © International Astronomical Union 2011.
• Yelle, R. V., Vuitton, V., Lavvas, P., Klippenstein, S. J., Smith, M. A., Hörst, S., & Cui, J. (2010). Formation of NH₃ and CH₂NH in Titan's upper atmosphere. Faraday Discussions, 147, 31-49.
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  Abstract: The large abundance of NH3 in Titan's upper atmosphere is a consequence of coupled ion and neutral chemistry. The density of NH3 is inferred from the measured abundance of NH4+. NH 3 is produced primarily through reaction of NH2 with H2CN, a process neglected in previous models. NH2 is produced by several reactions including electron recombination of CH 2NH2+. The density of CH2NH 2+ is closely linked to the density of CH2NH through proton exchange reactions and recombination. CH2NH is produced by reaction of N(2D) and NH with ambient hydrocarbons. Thus, production of NH3 is the result of a chain of reactions involving non-nitrile functional groups and the large density of NH3 implies large densities for these associated molecules. This suggests that amine and imine functional groups may be incorporated as well in other, more complex organic molecules. © 2010 The Royal Society of Chemistry.
• Coustenis, A., Atreya, S. K., Balint, T., Brown, R. H., Dougherty, M. K., Ferri, F., Fulchignoni, M., Gautier, D., Gowen, R. A., Griffith, C. A., Gurvits, L. I., Jaumann, R., Langevin, Y., Leese, M. R., Lunine, J. I., McKay, C. P., Moussas, X., Müller-Wodarg, I., Neubauer, F., , Owen, T. C., et al. (2009). TandEM: Titan and Enceladus mission. Experimental Astronomy, 23(3), 893-946.
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  Abstract: TandEM was proposed as an L-class (large) mission in response to ESA's Cosmic Vision 2015-2025 Call, and accepted for further studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini-Huygens mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is designed to build on but exceed the scientific and technological accomplishments of the Cassini-Huygens mission, exploring Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time). In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in situ investigation components, i.e. a hot-air balloon (Montgolfière) and possibly several landing probes to be delivered through the atmosphere. © Springer Science + Business Media B.V. 2008.
• Cravens, T. E., Robertson, I. P., Waite Jr., J. H., Yelle, R. V., Vuitton, V., Coates, A. J., Wahlund, J. -., Agren, K., Richard, M. S., De, V., Wellbrock, A., & Neubauer, F. M. (2009). Model-data comparisons for Titan's nightside ionosphere. Icarus, 199(1), 174-188.
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  Abstract: Solar and X-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment. A model of Titan's nightside ionosphere will be described and the results compared with data from the Ion and Neutral Mass Spectrometer (INMS) and the Langmuir probe (LP) part of the Radio and Plasma Wave (RPWS) experiment for the T5 and T21 nightside encounters of the Cassini Orbiter with Titan. Electron impact ionization associated with the precipitation of magnetospheric electrons into the upper atmosphere is assumed to be the source of the nightside ionosphere, at least for altitudes above 1000 km. Magnetospheric electron fluxes measured by the Cassini electron spectrometer (CAPS ELS) are used as an input for the model. The model is used to interpret the observed composition and structure of the T5 and T21 ionospheres. The densities of many ion species (e.g., CH+5 and C2H+5) measured during T5 exhibit temporal and/or spatial variations apparently associated with variations in the fluxes of energetic electrons that precipitate into the atmosphere from Saturn's magnetosphere. © 2008 Elsevier Inc. All rights reserved.
• Cui, J., Galand, M., Yelle, R. V., Vuitton, V., Wahlund, J. -., Lawas, P. P., Müller-Wodarg, I., Cravens, T. E., Kasprzak, W. T., & Waite Jr., J. H. (2009). Diurnal variations of Titan's ionosphere. Journal of Geophysical Research A: Space Physics, 114(6).
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  Abstract: We present our analysis of the diurnal variations of Titan's ionosphere (between 1000 and 1300 km) based on a sample of Ion Neutral Mass Spectrometer (INMS) measurements in the Open Source Ion (OSI) mode obtained from eight close encounters of the Cassini spacecraft with Titan. Although there is an overall ion depletion well beyond the terminator, the ion content on Titan's nightside is still appreciable, with a density plateau of ∼700 cm-3 below ∼1300 km. Such a plateau is a combined result of significant depletion of light ions and modest depletion of heavy ones on Titan's nightside. We propose that the distinctions between the diurnal variations of light and heavy ions are associated with their different chemical loss pathways, with the former primarily through "fast" ion-neutral chemistry and the latter through "slow" electron dissociative recombination. The strong correlation between the observed night-to-day ion density ratios and the associated ion lifetimes suggests a scenario in which the ions created on Titan's dayside may survive well to the nightside. The observed asymmetry between the dawn and dusk ion density profiles also supports such an interpretation. We construct a time-dependent ion chemistry model to investigate the effect of ion survival associated with solid body rotation alone as well as superrotating horizontal winds. For long-lived ions, the predicted diurnal variations have similar general characteristics to those observed. However, for short-lived ions, the model densities on the nightside are significantly lower than the observed values. This implies that electron precipitation from Saturn's magnetosphere may be an additional and important contributor to the densities of the short-lived ions observed on Titan's nightside. Copyright 2009 by the American Geophysical Union.
• Cui, J., Yelle, R. V., Vuitton, V., Waite Jr., J. H., Kasprzak, W. T., Gell, D. A., Niemann, H. B., Müller-Wodarg, I., Borggren, N., Fletcher, G. G., Patrick, E. L., Raaen, E., & Magee, B. A. (2009). Analysis of Titan's neutral upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements. Icarus, 200(2), 581-615.
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  Abstract: In this paper we present an in-depth study of the distributions of various neutral species in Titan's upper atmosphere, between 950 and 1500 km for abundant species (N2, CH4, H2) and between 950 and 1200 km for other minor species. Our analysis is based on a large sample of Cassini/INMS (Ion Neutral Mass Spectrometer) measurements in the CSN (Closed Source Neutral) mode, obtained during 15 close flybys of Titan. To untangle the overlapping cracking patterns, we adopt Singular Value Decomposition (SVD) to determine simultaneously the densities of different species. Except for N2, CH4, H2 and 40Ar (as well as their isotopes), all species present density enhancements measured during the outbound legs. This can be interpreted as a result of wall effects, which could be either adsorption/desorption of these molecules or heterogeneous surface chemistry of the associated radicals on the chamber walls. In this paper, we provide both direct inbound measurements assuming ram pressure enhancement only and abundances corrected for wall adsorption/desorption based on a simple model to reproduce the observed time behavior. Among all minor species of photochemical interest, we have firm detections of C2H2, C2H4, C2H6, CH3C2H, C4H2, C6H6, CH3CN, HC3N, C2N2 and NH3 in Titan's upper atmosphere. Upper limits are given for other minor species. The globally averaged distributions of N2, CH4 and H2 are each modeled with the diffusion approximation. The N2 profile suggests an average thermospheric temperature of 151 K. The CH4 and H2 profiles constrain their fluxes to be 2.6 × 109   cm- 2 s- 1 and 1.1 × 1010   cm- 2 s- 1, referred to Titan's surface. Both fluxes are significantly higher than the Jeans escape values. The INMS data also suggest horizontal/diurnal variations of temperature and neutral gas distribution in Titan's thermosphere. The equatorial region, the ramside, as well as the nightside hemisphere of Titan appear to be warmer and present some evidence for the depletion of light species such as CH4. Meridional variations of some heavy species are also observed, with a trend of depletion toward the north pole. Though some of the above variations might be interpreted by either the solar-driven models or auroral-driven models, a physical scenario that reconciles all the observed horizontal/diurnal variations in a consistent way is still missing. With a careful evaluation of the effect of restricted sampling, some of the features shown in the INMS data are more likely to be observational biases. © 2008 Elsevier Inc.
• Lavvas, P., Yelle, R. V., & Vuitton, V. (2009). The detached haze layer in Titan's mesosphere. Icarus, 201(2), 626-633.
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  Abstract: By comparing observations from the Cassini imaging system, UV spectrometer, and Huygens atmospheric structure instrument, we determine an apparent radius of ∼40 nm, an imaginary index
• Robertson, I. P., Cravens, T. E., Waite Jr., J. H., Yelle, R. V., Vuitton, V., Coates, A. J., Wahlund, J. E., Ågren, K., Mandt, K., Magee, B., Richard, M. S., & Fattig, E. (2009). Structure of Titan's ionosphere: Model comparisons with Cassini data. Planetary and Space Science, 57(14-15), 1834-1846.
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  Abstract: Solar extreme ultraviolet and X-ray radiation and energetic plasma from Saturn's magnetosphere interact with the upper atmosphere producing an ionosphere at Titan. The highly coupled ionosphere and upper atmosphere system mediates the interaction between Titan and the external environment. New insights into Titan's ionosphere are being facilitated by data from several instruments onboard the Cassini Orbiter, although the Ion and Neutral Mass Spectrometer (INMS) measurements will be emphasized here. We present dayside ionosphere models and compare the results with both Radio and Plasma Wave-Langmuir Probe (RPWS/LP) and INMS data, exploring the sensitivity of models to ionospheric chemistry schemes and solar flux variations. Modeled electron densities for the dayside leg of T18 and all of T17 (dayside) had reasonable agreement with the measured RPWS electron densities and INMS total ion densities. Magnetospheric inputs make at best minor contributions to the ionosphere for these flybys, at least for altitudes above about 1000 km. At lower (100 daltons) ions, which the INMS is not able to detect. Qualitatively, INMS spectra exhibit the same ion species and 12 amu family separations for the dayside ionospheres of T17 and T18 as were seen in the mass spectra measured during T5 (nightside). However, the relative abundance of high-mass (m>50) ion species is about 10 times less for the dayside T17 and T18 passes than it was for the polar nightside T5 flyby, which can perhaps be explained in several ways including differences in neutral composition, less dissociative recombination on the nightside than on the dayside (due to lower electron densities and affecting heavier ion species more than lighter ones), and transport of longer-lived high-mass species from day-to-night. © 2009 Elsevier Ltd. All rights reserved.
• Thissen, R., Vuitton, V., Lavvas, P., Lemaire, J., Dehon, C., Dutuit, O., Smith, M. A., Turchini, S., Catone, D., Yelle, R. V., Pernot, P., Somogyi, A., & Coreno, M. (2009). Laboratory studies of molecular growth in the titan ionosphere. Journal of Physical Chemistry A, 113(42), 11211-11220.
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  PMID: 19769328;Abstract: Experimental simulations of the initial steps of the ion-molecule reactions occurring in the ionosphere of Titan were performed at the synchrotron source Elettra in Italy. The measurements consisted of irradiating gas mixtures with a monochromatic photon beam, from the methane ionization threshold at 12.6 eV, up to and beyond the molecular nitrogen dissociative ionization threshold at 24.3 eV. Three gas mixtures of increasing complexity were used: N2/CH 4 (0.96/0.04), N2ZCH4/C2H 2 (0.96/0.04/0.001), and N2/CH4/C 2H2/C2H4 (0.96/ 0.04/0.001/0.001). The resulting ions were detected with a high-resolution (1 T) FT-ICR mass spectrometer as a function of time and VUV photon energy. In order to interpret the experimental results, a Titan ionospheric model was adapted to the laboratory conditions. This model had previously allowed the identification of the ions detected in the Titan upper atmosphere by the ion neutral mass spectrometer (INMS) onboard the Cassini spacecraft. Comparison between observed and modeled ion densities validates the kinetic model (reactions, rate constants, product branching ratios) for the primary steps of molecular growth. It also reveals differences that we attribute to an intense surface chemistry. This result implies that heterogeneous chemistry on aerosols might efficiently produce HCN and NH3 in the Titan upper atmosphere. © 2009 American Chemical Society.
• Vuitton, V., Lavvas, P., Yelle, R. V., Galand, M., Wellbrock, A., Lewis, G. R., Coates, A. J., & Wahlund, J. -. (2009). Negative ion chemistry in Titan's upper atmosphere. Planetary and Space Science, 57(13), 1558-1572.
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  Abstract: The Electron Spectrometer (ELS), one of the sensors making up the Cassini Plasma Spectrometer (CAPS) revealed the existence of numerous negative ions in Titan's upper atmosphere. The observations at closest approach (∼1000 km) show evidence for negatively charged ions up to ∼10,000 amu/q, as well as two distinct peaks at 22±4 and 44±8 amu/q, and maybe a third one at 82±14 amu/q. We present the first ionospheric model of Titan including negative ion chemistry. We find that dissociative electron attachment to neutral molecules (mostly HCN) initiates the formation of negative ions. The negative charge is then transferred to more acidic molecules such as HC3N, HC5N or C4H2. Loss occurs through associative detachment with radicals (H and CH3). We attribute the three low mass peaks observed by ELS to CN-, C3N-/C4H- and C5N-. These species are the first intermediates in the formation of the even larger negative ions observed by ELS, which are most likely the precursors to the aerosols observed at lower altitudes. © 2009 Elsevier Ltd. All rights reserved.
• Wahlund, J. -., Galand, M., Müller-Wodarg, I., Cui, J., Yelle, R. V., Crary, F. J., Mandt, K., Magee, B., Waite Jr., J. H., Young, D. T., Coates, A. J., Garnier, P., Ågren, K., André, M., Eriksson, A. I., Cravens, T. E., Vuitton, V., Gurnett, D. A., & Kurth, W. S. (2009). On the amount of heavy molecular ions in Titan's ionosphere. Planetary and Space Science, 57(14-15), 1857-1865.
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  Abstract: We present observational evidence that the ionosphere of Titan below an altitude of 1150 km is a significant source of heavy (>100 amu) molecular organic species. This study is based on measurements by five instruments (RPWS/LP, RPWS/E, INMS, CAPS/ELS, CAPS/IBS) onboard the Cassini spacecraft during three flybys (T17, T18, T32) of Titan. The ionospheric peaks encountered at altitudes of 950-1300 km had densities in the range 900-3000 cm-3. Below these peaks the number densities of heavy positively charged ions reached 100-2000 cm-3 and approached 50-70% of the total ionospheric density with an increasing trend toward lowest measured altitudes. Simultaneously measured negatively charged ion densities were in the range 50-150 cm-3. These results imply that ~105-106 heavy positively charged ions/m3/s are continuously recombining into heavy neutrals and supply the atmosphere of Titan. The ionosphere may in this way produce 0.1-1 Mt/yr of heavy organic compounds and is therefore a sizable source for aerosol formation. We also predict that Titan's ionosphere is dominated by heavy (>100 amu) molecular ions below 950 km. © 2009 Elsevier Ltd.
• Carrasco, N., Alcaraz, C., Dutuit, O., Plessis, S., Thissen, R., Vuitton, V., Yelle, R., & Pernot, P. (2008). Sensitivity of a Titan ionospheric model to the ion-molecule reaction parameters. Planetary and Space Science, 56(12), 1644-1657.
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  Abstract: Various aspects of ion-molecule reactions for Titan ionospheric chemistry modeling are reviewed in this work: temperature/collision energy effects on reaction rates and, more importantly, on products distributions; differential reactivity of isomers of ions; reactivity of excited states of ions; pathways to the building of complex ions. We evaluate here the extent to which these points affect model predictions. We find that the present limiting factors to model predictivity are the model incompleteness for heavy ion production pathways; the differential reactivity of isomers; and, to a lesser degree, the temperature effects on the branching ratios of ion-molecule reactions. Extensive experimental studies are required to fill these gaps in ion-molecule reactivity knowledge. © 2008 Elsevier Ltd. All rights reserved.
• Cui, J., Yelle, R. V., & Volk, K. (2008). Distribution and escape of molecular hydrogen in Titan's thermosphere and exosphere. Journal of Geophysical Research E: Planets, 113(10).
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  Abstract: We present an in-depth study of the distribution and escape of molecular hydrogen (H2) on Titan, based on the global average H2 distribution at altitudes between 1000 and 6000 km, extracted from a large sample of Cassini/Ion and Neutral Mass Spectrometer (INMS) measurements. Below Titan's exobase, the observed H2 distribution can be described by an isothermal diffusion model, with a most probable flux of (1.37 ± 0.01) × 1010 cm-2 s-1, referred to the surface. This is a factor of ∼3 higher than the Jeans flux of 4.5 × 109 cm-2 s-1, corresponding to a temperature of 152.5 ± 1.7 K, derived from the background N2 distribution. The H2 distribution in Titan's exosphere is modeled with a collisionless approach, with a most probable exobase temperature of 151.2 ± 2.2 K. Kinetic model calculations in the 13-moment approximation indicate a modest temperature decrement of several kelvin for H2, as a consequence of the local energy balance between heating/cooling through thermal conduction, viscosity, neutral collision, and adiabatic outflow. The variation of the total energy flux defines an exobase level of ∼1600 km, where the perturbation of the Maxwellian velocity distribution function, driven primarily by the heat flow, becomes strong enough to raise the H2 escape flux considerably higher than the Jeans value. Nonthermal processes may not be required to interpret the H2 escape on Titan. In a more general context, we suggest that the widely used Jeans formula may significantly underestimate the actual thermal escape flux and that a gas kinetic model in the 13-moment approximation provides a better description of thermal escape in planetary atmospheres. Copyright 2008 by the American Geophysical Union.
• Hörst, S., Vuitton, V., & Yelle, R. V. (2008). Origin of oxygen species in Titan's atmosphere. Journal of Geophysical Research E: Planets, 113(10).
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  Abstract: The detection of O+ precipitating into Titan's atmosphere by the Cassini Plasma Spectrometer (CAPS) represents the discovery of a previously unknown source of oxygen in Titan's atmosphere. The photochemical model presented here shows that those oxygen ions are incorporated into CO and CO2. We show that the observed abundances of CO, CO2 and H2O can be simultaneously reproduced using an oxygen flux consistent with the CAPS observations and an OH flux consistent with predicted production from micrometeorite ablation. It is therefore unnecessary to assume that the observed CO abundance is the remnant of a larger primordial CO abundance or to invoke outgassing of CO from Titan's interior as a source of CO. Copyright 2008 by the American Geophysical Union.
• Moore, L., Galand, M., Mueller-Wodarg, I., Yelle, R., & Mendillo, M. (2008). Plasma temperatures in Saturn's ionosphere. Journal of Geophysical Research A: Space Physics, 113(10).
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  Abstract: We have calculated self-consistent electron and ion temperatures in Saturn's ionosphere using a series of coupled fluid and kinetic models developed to help interpret Cassini observations and to examine the energy budget of Saturn's upper atmosphere. Electron temperatures in the midlatitude topside ionosphere during solar maximum are calculated to range between 500 and 560 K during the Saturn day, approximately 80-140 K above the neutral temperature. Ion temperatures, calculated for only the major ions H+ and H 3+, are nearly equal to the neutral temperature at altitudes near and below the height of peak electron density, while they can reach 500 K during the day at the topside. Plasma scale heights of the dusk electron density profile from radio occultation measurements of the Voyager 2 flyby of Saturn have been used to estimate plasma temperature as a comparison. Such an estimate agrees well with the temperatures calculated here, although there is a topside enhancement in electron density that remains unexplained by ionospheric calculations that include photochemistry and plasma diffusion. Finally, parameterizations of the heating rate from photoelectrons and secondary electrons to thermal, ambient electrons have been developed. They may apply for other conditions at Saturn and possibly at other giant planets and exoplanets as well. Copyright 2008 by the American Geophysical Union.
• Mueller-Wodarg, I., Strobel, D. F., Moses, J. I., Waite, J. H., Crovisier, J., Yelle, R. V., Bougher, S. W., & Roble, R. G. (2008). Neutral atmospheres. Space Science Reviews, 139(1-4), 191-234.
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  Abstract: This paper summarizes the understanding of aeronomy of neutral atmospheres in the solar system, discussing most planets as well as Saturn's moon Titan and comets. The thermal structure and energy balance is compared, highlighting the principal reasons for discrepancies amongst the atmospheres, a combination of atmospheric composition, heliocentric distance and other external energy sources not common to all. The composition of atmospheres is discussed in terms of vertical structure, chemistry and evolution. The final section compares dynamics in the upper atmospheres of most planets and highlights the importance of vertical dynamical coupling as well as magnetospheric forcing in auroral regions, where present. It is shown that a first order understanding of neutral atmospheres has emerged over the past decades, thanks to the combined effects of spacecraft and Earth-based observations as well as advances in theoretical modeling capabilities. Key gaps in our understanding are highlighted which ultimately call for a more comprehensive programme of observation and laboratory measurements. © 2008 Springer Science+Business Media B.V.
• Müller-Wodarg, I., Yelle, R. V., Cui, J., & Waite, J. H. (2008). Horizontal structures and dynamics of Titan's thermosphere. Journal of Geophysical Research E: Planets, 113(10).
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  Abstract: The Cassini Ion Neutral Mass Spectrometer (INMS) measures densities of gases including N2 and CH4 in situ in Titan's upper atmosphere. We have used data from 13 targeted flybys of Titan (T5-T32) to construct an empirical model that describes the mean state of the thermosphere in the northern hemisphere, giving the N2 and CH4 densities between 1000 and 1600 km as a function of latitude and height. The principal features in the INMS data are well reproduced by this simple model. We find a pronounced oblateness in the thermosphere, with densities above 1100 km altitude increasing by around 70% from the northern (winter) pole to the equator, resulting in isobaric surfaces being ≃45 km higher over the equator than at the northern pole. Thermospheric temperatures derived from the densities tend to decrease with height from 149 ± 10 K to 140 ± 13 K near 1600 km. Considerable latitude differences are present in the temperatures below 1200 km. Near 1000 km altitude, temperatures reach 164 ± 6 K at 20°N and 131 ± 6 K near 80°N. Using our Thermosphere General Circulation Model with this thermal structure imposed, we derive thermospheric horizontal wind speeds reaching ∼150 m s-1, with primarily poleward flow at equatorial latitudes which, northward of around 60°N, is accompanied by a band of prograde zonal winds of up to 50 m s-1. At high latitudes, converging horizontal winds generate regions of strong subsistence. We find thermospheric dynamics to be sensitive to coupling from below. CH4 abundances are enhanced in the northern polar region, which may result from transport by thermospheric winds. Copyright 2008 by the American Geophysical Union.
• Vuitton, V., Yelle, R. V., & Cui, J. (2008). Formation and distribution of benzene on Titan. Journal of Geophysical Research E: Planets, 113(5).
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  Abstract: We present a study of the formation and distribution of benzene (C6H6) on Titan. Analysis of the Cassini Mass Spectrometer (INMS) measurements of benzene densities on 12 Titan passes shows that the benzene signal exhibits an unusual time dependence, peaking ∼20 s after closest approach, rather than at closest approach. We show that this behavior can be explained by recombination of phenyl radicals (C6H5) with H atoms on the walls of the instrument and that the measured signal is a combination of (1) C6H6 from the atmosphere and (2) C6H6 formed within the instrument. In parallel, we investigate Titan benzene chemistry with a set of photochemical models. A model for the ionosphere predicts that the globally averaged production rate of benzene by ion-molecule reactions is ∼107 cm-7 s-1, of the same order of magnitude as the production rate by neutral reactions of ∼4 × 106 cm-2 s-1. We show that benzene is quickly photolyzed in the thermosphere, and that C6H5 radicals, the main photodissociation products, are ∼3 times as abundant as benzene. This result is consistent with the phenyl/benzene ratio required to match the INMS observations. Loss of benzene occurs primarily through reaction of phenyl with other radicals, leading to the formation of complex aromatic species. These species, along with benzene, diffuse downward, eventually condensing near the tropopause. We find a total production rate of solid aromatics of∼10-15 g cm-2 s-1, corresponding to an accumulated surface layer of ∼3 m. Copyright 2008 by the American Geophysical Union.
• Yelle, R. V., Cui, J., & Müller-Wodarg, I. (2008). Methance escape from Titan's atmosphere. Journal of Geophysical Research E: Planets, 113(10).
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  Abstract: Measurements of the mole fractions of CH4 and 40Ar by the Ion Neutral Mass Spectrometer on the Cassini orbiter are analyzed to determine the rate of vertical mixing in Titan's atmosphere and the escape flux of CH4. Analysis of the 40Ar data indicates an eddy mixing rate of 2-5 × 107 cm2 s-1, an order of magnitude smaller than previously determined from analysis of the CH4 distribution. The eddy profile determined from the 40Ar data implies that CH4 distribution is best explained by postulating that it is escaping the atmosphere at the diffusion limited rate of 2.5-3.0 × 109 cm-2 S-1, referred to the surface. This represents a significant loss of atmospheric CH4, smaller than but comparable to the photochemical destruction rate. The escape rate is much larger than predicted by the Jeans escape formula and vigorous nonthermal mechanisms are not apparent. Copyright 2008 by the American Geophysical Union.
• Yelle, R., Lammer, H., & Ip, W. -. (2008). Aeronomy of extra-solar giant planets. Space Science Reviews, 139(1-4), 437-451.
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  Abstract: The intense stellar UV radiation field incident upon extra-solar giant planets causes profound changes to their upper atmospheres. Upper atmospheric temperatures can be tens of thousands of kelvins, causing thermal dissociation of H2 to H. The stellar ionizing flux converts H to H+. The high temperatures also drive large escape rates of H, but for all but the planets with the smallest orbits, this flux is not large enough to affect planet evolution. The escape rate is large enough to drag off heavier atoms such as C and O. For very small orbits, when the hill sphere is inside the atmosphere, escape is unfettered and can affect planet evolution. © 2008 Springer Science+Business Media B.V.
• Agren, K., Wahlund, J. -., Modolo, R., Lummerzheim, D., Galand, M., Müller-Wodarg, I., Canu, P., Kurth, W. S., Cravens, T. E., Yelle, R. V., Waite Jr., J. H., Coates, A. J., Lewis, G. R., Young, D. T., Bertucci, C., & Dougherty, M. K. (2007). On magnetospheric electron impact ionisation and dynamics in Titan's ram-side and polar ionosphere – A Cassini case study. Annales Geophysicae, 25(11), 2359-2369.
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  Abstract: We present data from the sixth Cassini flyby of Titan (T5), showing that the magnetosphere of Saturn strongly interacts with the moon's ionosphere and exo-ionosphere. A simple electron ionisation model provides a reasonable agreement with the altitude structure of the ionosphere. Furthermore, we suggest that the dense and cold exo-ionosphere (from the exobase at 1430 km and outward to several Titan radii from the surface) can be explained by magnetospheric forcing and other transport processes whereas exospheric ionisation by impacting low energy electrons seems to play a minor role.
• De, V., Waite Jr., J. H., Johnson, R. E., Yelle, R. V., Cravens, T. E., Luhmann, J. G., Kasprzak, W. T., Gell, D. A., Magee, B., Leblanc, F., Michael, M., Jurac, S., & Robertson, I. P. (2007). Cassini Ion and Neutral Mass Spectrometer data in Titan's upper atmosphere and exosphere: Observation of a suprathermal corona. Journal of Geophysical Research A: Space Physics, 112(7).
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  Abstract: The neutral nitrogen and methane measurements made by Ion and Neutral Mass Spectrometer during Cassini flybys TA, TB, and T 5 in Titan's upper atmosphere and exosphere are presented. Large horizontal variations are observed in the total density, recorded to be twice as large during TA as during T5. Comparison between the atmospheric and exospheric data show evidence for the presence of a significant population of suprathermal molecules. Using a diffusion model to simultaneously fit the N2 and CH4 density profiles below 1500 km, the atmospheric structure parameters are determined, taking into account recent changes in the calibration parameters. The best fits are obtained for isothermal profiles with values 152.8 ± 4.6 K for TA, 149.0 ± 9.2 K for TB, and 157.4 ± 4.9 K for T5, suggesting a temperature ≃5 K warmer at night than at dusk, a trend opposite to that determined by solar-driven models. Using standard exospheric theory and a Maxwellian exobase distribution, a temperature of 20 to 70 K higher would be necessary to fit the TA, TB, and egress-T5 data above 1500 km. The suprathermal component of the corona was fit with various exobase energy distributions, using a method based on the Liouville theorem. This gave a density of suprathermals at the exobase of 4.4 ± 5.1 × 105 cm-3 and 1.1 ± 0.9 × 105 cm-3, and an energy deposition rate at the exobase of 1.1 ± 0.9 × 102 eV cm-3 s_1 and 3.9 ± 3.5 × 101 eV cm-3 s-1 for the hot N 2 and CH4 populations, respectively. The energy deposition rate allowed us to roughly estimate escape rates for nitrogen of ≃7.7 ± 7.1 × 107 N cm-2 s_1 and for methane of ≃ 2.8 ± 2.1 × 107 CH4 cm -2 s-1. Interestingly, no suprathermal component was observed in the ingress-T5 data. Copyright 2007 by the American Geophysical Union.
• Jessup, K. L., Spencer, J., & Yelle, R. (2007). Sulfur volcanism on Io. Icarus, 192(1), 24-40.
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  Abstract: In February 2003, March 2003 and January 2004 Pele plume transmission spectra were obtained during Jupiter transit with Hubble's Space Telescope Imaging Spectrograph (STIS), using the 0.1″ wide slit and the G230LB grating. The STIS spectra covered the 2100-3100 Å wavelength regions and extended spatially along Io's limb encompassing the region directly above and northward of the vent of the Pele volcano. The S2 and SO2 absorption signatures evident in these data indicate that the gas signature at Pele was temporally variable, and that an S2 absorption signature was present ∼12° from the Pele vent near 6 ± 5  S and 264 ± 15  W, suggesting the presence of another S2 bearing plume on Io. Contemporaneous with the spectral data, UV and visible-wavelength images of the plume were obtained in reflected sunlight with the Advanced Camera for Surveys (ACS) prior to Jupiter transit. The dust scattering recorded in these data provide an additional qualitative measure of plume activity on Io, indicating that the degree of dust scattering over Pele varied as a function of the date of observation, and that there were several other dust bearing plumes active during the observations. We present constraints on the composition and variability of the gas abundances of the Pele plume as well as the plumes detected by ACS and recorded within the STIS data, as a function of time. © 2007 Elsevier Inc. All rights reserved.
• Rodgers, D. H., Beauchamp, P. M., Soderblom, L. A., Brown, R. H., Chen, G., Lee, M., Sandel, B. R., Thomas, D. A., Benoit, R. T., & Yelle, R. V. (2007). Advanced technologies demonstrated by the miniature integrated camera and spectrometer (MICAS) aboard deep space 1. Space Science Reviews, 129(4), 309-326.
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  Abstract: MICAS is an integrated multi-channel instrument that includes an ultraviolet imaging spectrometer (80-185 nm), two high-resolution visible imagers (10-20 μrad/pixel, 400-900 nm), and a short-wavelength infrared imaging spectrometer (1250-2600 nm). The wavelength ranges were chosen to maximize the science data that could be collected using existing semiconductor technologies and avoiding the need for multi-octave spectrometers. It was flown on DS1 to validate technologies derived from the development of PICS (Planetary Imaging Camera Spectrometer). These technologies provided a novel systems approach enabling the miniaturization and integration of four instruments into one entity, spanning a wavelength range from the UV to IR, and from ambient to cryogenic temperatures with optical performance at a fraction of a wavelength. The specific technologies incorporated were: a built-in fly-by sequence; lightweight and ultra-stable, monolithic silicon-carbide construction, which enabled room-temperature alignment for cryogenic (85-140 K) performance, and provided superb optical performance and immunity to thermal distortion; diffraction-limited, shared optics operating from 80 to 2600 nm; advanced detector technologies for the UV, visible and short-wavelength IR; high-performance thermal radiators coupled directly to the short-wave infrared (SWIR) detector optical bench, providing an instrument with a mass less than 10 kg, instrument power less than 10 W, and total instrument cost of less than ten million dollars. The design allows the wavelength range to be extended by at least an octave at the short wavelength end and to 50 microns at the long wavelength end. Testing of the completed instrument demonstrated excellent optical performance down to 77 K, which would enable a greatly reduced background for longer wavelength detectors. During the Deep Space 1 Mission, MICAS successfully collected images and spectra for asteroid 9969 Braille, Mars, and comet 19/P Borrelly. The Borrelly encounter was a scientific hallmark providing the first clear, high resolution images and excellent, short-wavelength infrared spectra of the surface of an active comet's nucleus. © 2007 Springer Science+Business Media, Inc.
• Vuitton, V., Yelle, R. V., & McEwan, M. J. (2007). Ion chemistry and N-containing molecules in Titan's upper atmosphere. Icarus, 191(2), 722-742.
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  Abstract: High-energy photons, electrons, and ions initiate ion-neutral chemistry in Titan's upper atmosphere by ionizing the major neutral species (nitrogen and methane). The Ion and Neutral Mass Spectrometer (INMS) onboard the Cassini spacecraft performed the first composition measurements of Titan's ionosphere. INMS revealed that Titan has the most compositionally complex ionosphere in the Solar System, with roughly 50 ions at or above the detection threshold. Modeling of the ionospheric composition constrains the density of minor neutral constituents, most of which cannot be measured with any other technique. The species identified with this approach include the most complex molecules identified so far on Titan. This confirms the long-thought idea that a very rich chemistry is actually taking place in this atmosphere. However, it appears that much of the interesting chemistry occurs in the upper atmosphere rather than at lower altitudes. The species observed by INMS are probably the first intermediates in the formation of even larger molecules. As a consequence, they affect the composition of the bulk atmosphere, the composition and optical properties of the aerosols and the flux of condensable material to the surface. In this paper, we discuss the production and loss reactions for the ions and how this affects the neutral densities. We compare our results to neutral densities measured in the stratosphere by other instruments, to production yields obtained in laboratory experiments simulating Titan's chemistry and to predictions of photochemical models. We suggest neutral formation mechanisms and highlight needs for new experimental and theoretical data. © 2007 Elsevier Inc. All rights reserved.
• Cravens, T. E., Robertson, I. P., Waite, J., Yelle, R. V., Kasprzak, W. T., Keller, C. N., Ledvina, S. A., Niemann, H. B., Luhmann, J. G., McNutt, R. L., Ip, W. -., De, V., Mueller-Wodarg, I., Wahlund, J. -., Anicich, V. G., & Vuitton, V. (2006). Composition of Titan's ionosphere. Geophysical Research Letters, 33(7).
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  Abstract: We present Cassini Ion and Neutral Mass Spectrometer (INMS) measurements of ion densities on the nightside of Titan from April 16, 2005, and show that a substantial ionosphere exists on the nightside and that complex ion chemistry is operating there. The total ionospheric densities measured both by the INMS and the Cassini Radio and Plasma Wave (RPWS) experiments on Cassini suggest that precipitation from the magnetosphere into the atmosphere of electrons with energies ranging from 25 eV up to about 2 keV is taking place. The absence of ionospheric composition measurements has been a major obstacle to understanding the ionosphere. Seven "families" of ion species, separated in mass-to-charge ratio by 12 Daltons (i.e., the mass of carbon), were observed and establish the importance of hydrocarbon and nitrile chains in the upper atmosphere. Several of the ion species measured by the INMS were predicted by models (e.g., HCNH+ and C2H5+). But the INMS also saw high densities at mass numbers not predicted by models, including mass 18, which we suggest will be ammonium ions (NH4+) produced by reaction of other ion species with neutral ammonia. Copyright 2006 by the American Geophysical Union.
• Galand, M., Yelle, R. V., Coates, A. J., Backes, H., & Wahlund, J. -. (2006). Electron temperature of Titan's sunlit ionosphere. Geophysical Research Letters, 33(21).
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  Abstract: Titan's upper atmosphere is ionized by solar radiation and particle bombardment from Saturn's magnetosphere. The induced ionosphere plays a key role in the coupling of Titan's atmosphere with the Kronian environment. It also provides unique signatures for identifying energy sources upon Titan's upper atmosphere. Here we focus on observations from the first, close flyby by the Cassini spacecraft and assess the ionization and electron heating sources in Titan's sunlit ionosphere. We compare CAPS electron spectra with spectra produced by an electron transport model based on the INMS neutral densities and a MHD interaction model. In addition, we compare RPWS electron temperature against the models. The important terms in the electron energy equation include loss through excitation of vibrational states of N2 and CH 4, Coulomb collisions with suprathermal electrons, and thermal conduction. Our analysis highlights the important role of the magnetic field line configuration for aeronomic studies at Titan. Copyright 2006 by the American Geophysical Union.
• Müller-Wodarg, I., Mendillo, M., Yelle, R. V., & Aylward, A. D. (2006). A global circulation model of Saturn's thermosphere. Icarus, 180(1), 147-160.
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  Abstract: We present the first 3-dimensional self-consistent calculations of the response of Saturn's global thermosphere to different sources of external heating, giving local time and latitudinal changes of temperatures, winds and composition at equinox and solstice. Our calculations confirm the well-known finding that solar EUV heating alone is insufficient to produce Saturn's observed low latitude thermospheric temperatures of 420 K. We therefore carry out a sensitivity study to investigate the thermosphere's response to two additional external sources of energy, (1) auroral Joule heating and (2) empirical wave heating in the lower thermosphere. Solar EUV heating alone produces horizontal temperature variations of below 20 K, which drive horizontal winds of less than 20 m/s and negligible horizontal changes in composition. In contrast, Joule heating produces a strong dynamical response with westward winds comparable to the sound speed on Saturn. Joule heating alone, at a total rate of 9.8 TW, raises polar temperatures to around 1200 K, but values equatorward of 30° latitude, where observations were made, remain below 200 K due to inefficient meridional energy transport in a fast rotating atmosphere. The primarily zonal wind flow driven by strong Coriolis forces implies that energy from high latitudes is transported equatorward mainly by vertical winds through adiabatic processes, and an additional 0.29-0.44 mW/m2 thermal energy are needed at low latitudes to obtain the observed temperature values. Strong upwelling increases the H2 abundances at high latitudes, which in turn affects the H+3 densities. Downwelling at low latitudes helps increase atomic hydrogen abundances there. © 2005 Elsevier Inc. All rights reserved.
• Müller-Wodarg, I., Yelle, R. V., Borggren, N., & Waite Jr., J. H. (2006). Waves and horizontal structures in Titan's thermosphere. Journal of Geophysical Research A: Space Physics, 111(12).
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  Abstract: The Ion Neutral Mass Spectrometer (INMS) on board the Cassini spacecraft carried out in situ measurements of neutral gas composition above 1025 km altitude in Titan's atmosphere during its flybys in October 2004 (TA) and April 2005 (T5). Strong perturbations are present in the N2 and CH4 densities which we interpret as vertically propagating waves. Typical vertical wavelengths range from 170 to 360 km with density and pressure amplitudes reaching 4-12% of the background values and temperature amplitudes of 5-10 K. Amplitudes over our sampled height range, 1025 (T5) or 1176 (TA) to 1600 km, remain roughly constant, implying that the exponential increase in wave amplitudes with height due to the decrease of density is offset by damping. This finding allows us to constrain the wave periods to values in the order of hours. Estimates of wave-induced acceleration of the background thermosphere suggest that the waves we observe could deposit considerable momentum in Titan's thermosphere, thereby coupling the dynamics of the upper atmosphere with that of the middle atmosphere. In addition, we infer latitudinal structures in Titan's thermosphere with a factor of 3-4 increase of mass densities from pole to equator in the northern hemisphere. A preliminary evaluation of local time variations suggests densities and thermospheric temperatures to be largest near dusk, contradicting expectations for a thermosphere driven energetically and dynamically primarily by solar EUV. From the latitudinal density gradients we derived zonal wind speeds of around 245 ± 50 ms-1, implying that Titan's thermosphere, like its stratosphere, could be superrotating. Our analyses were based on the TA and TS flybys only, and future Cassini Titan flybys could either support or invalidate our findings. Copyright 2006 by the American Geophysical Union.
• Vuitton, V., Yelle, R. V., & Anicich, V. G. (2006). The nitrogen chemistry of titan's upper atmosphere revealed. Astrophysical Journal Letters, 647(2 II), L175-L178.
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  Abstract: Titan's atmosphere is unique because dissociation of N2 and CH4, the primary atmospheric constituents, provides the H, C, and N atoms necessary for the synthesis of complex organic molecules. The first steps in the synthesis of organic molecules occur in the upper atmosphere where energetic photons and electrons dissociate N2 and CH4. We determine the abundance of a suite of nitrogen-bearing molecules in Titan's upper atmosphere through analysis of measurements of the ionospheric composition made by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft. We show that the density of ions in Titan's upper atmosphere depends closely on the composition of the neutral atmosphere and that, for many species, measurement of associated ions coupled with simple chemical models provides the most sensitive determination of their abundance. With this technique we determine the densities of C2H4, C4H2, HCN, HC3N, CH3CN, NH3, C2H3CN, C2H5CN, and CH2NH. The latter four species have not previously been detected in the gas phase on Titan, and none of these species have been accurately measured in the upper atmosphere. The presence of these species implies that nitrogen chemistry on Titan is more extensive than previously realized. © 2006. The American Astronomical Society. All rights reserved.
• Waite Jr., J. H., Combi, M. R., Ip, W., Cravens, T. E., McNutt Jr., R. L., Kasprzak, W., Yelle, R., Luhmann, J., Niemann, H., Gell, D., Magee, B., Fletcher, G., Lunine, J., & Tseng, W. (2006). Cassini ion and neutral mass spectrometer: Enceladus plume composition and structure. Science, 311(5766), 1419-1422.
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  PMID: 16527970;Abstract: The Cassini spacecraft passed within 168.2 kilometers of the surface above the southern hemisphere at 19:55:22 universal time coordinated on 14 July 2005 during its closest approach to Enceladus. Before and after this time, a substantial atmospheric plume and coma were observed, detectable in the Ion and Neutral Mass Spectrometer (INMS) data set out to a distance of over 4000 kilometers from Enceladus. INMS data indicate that the atmospheric plume and coma are dominated by water, with significant amounts of carbon dioxide, an unidentified species with a mass-to-charge ratio of 28 daltons (either carbon monoxide or molecular nitrogen), and methane. Trace quantities (
• Yelle, R. V. (2006). Corrigendum to "Aeronomy of extra-solar giant planets at small orbital distances" [Icarus 170 (2004) 167-179] (DOI:10.1016/j.icarus.2004.02.008). Icarus, 183(2), 508-.
• Yelle, R. V., Borggren, N., de, V., Kasprzak, W. T., Niemann, H. B., Müller-Wodarg, I., & Waite Jr., J. H. (2006). The vertical structure of Titan's upper atmosphere from Cassini Ion Neutral Mass Spectrometer measurements. Icarus, 182(2), 567-576.
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  Abstract: Data acquired by the Ion Neutral Mass Spectrometer (INMS) on the Cassini spacecraft during its close encounter with Titan on 26 October 2004 reveal the structure of its upper atmosphere. Altitude profiles of N2, CH4, and H2, inferred from INMS measurements, determine the temperature, vertical mixing rate, and escape flux from the upper atmosphere. The mean atmospheric temperature in the region sampled by the INMS is 149 ± 3   K, where the variance is a consequence of local time variations in temperature. The CH4 mole fraction at 1174 km is 2.71 ± 0.1 %. The effects of diffusive separation are clearly seen in the data that we interpret as an eddy diffusion coefficient of 4-3+4 × 109   cm2 s-1, that, along with the measured CH4 mole fraction, implies a mole fraction in the stratosphere of 2.2 ± 0.2 %. The H2 distribution is affected primarily by upward flow and atmospheric escape. The H2 mole fraction at 1200 km is 4 ± 1 × 10-3 and analysis of the altitude profile indicates an upward flux of 1.2 ± 0.2 × 1010   cm-2 s-1, referred to the surface. If horizontal variations in temperature and H2 density are small, this upward flux also represents the escape flux from the atmosphere. The CH4 density exhibits significant horizontal variations that are likely an indication of dynamical processes in the upper atmosphere. © 2005 Elsevier Inc. All rights reserved.
• Cravens, T. E., Robertson, I. P., Clark, J., Wahlund, J. -., Waite, J., Ledvina, S. A., Niemann, H. B., Yelle, R. V., Kasprzak, W. T., Luhmann, J. G., McNutt, R. L., Ip, W. -., De, V., Müller-Wodarg, I., Young, D. T., & Coates, A. J. (2005). Titan's ionosphere: Model comparisons with Cassini Ta data. Geophysical Research Letters, 32(12), 1-5.
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  Abstract: On October 26, 2004, during its first encounter with Titan (Ta), the Cassini Orbiter moved from the dayside to the nightside with a closest approach altitude of 1174 km. In situ measurements of the main part of Titan's ionosphere were made by the Langmuir probe on the Cassini Radio and Plasma Wave Experiment (RPWS), while the Ion and Neutral Mass Spectrometer (INMS) measured the main constituents of the neutral atmosphere. The results of model calculations of Titan's ionosphere for Ta encounter conditions (e.g., near the terminator) are presented in this paper. The paper includes comparisons of calculated and measured electron densities along the spacecraft track. Ionization both by solar radiation and by incoming energetic electrons from Saturn's magnetosphere are needed to obtain good agreement between the measured and calculated electron densities. Copyright 2005 by the American Geophysical Union.
• Griffith, C. A., Penteado, P., Greathouse, T. K., Roe, H. G., & Yelle, R. V. (2005). Observations of Titan's mesosphere. Astrophysical Journal Letters, 629(1 II), L57-L60.
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  Abstract: We have recorded spectra of Titan's ν4 band of CH 4 at a higher resolving power (R = 70,000) than prior measurements to better constrain the thermal structure of Titan's atmosphere from 100 to 600 km altitude. Radiative transfer analyses of the spectra indicate a temperature profile below 300 km that is consistent with past measurements. The high resolving power of our observations provides the first infrared measurement of Titan's thermal structure between 300 and 600 km. We detect the presence of a mesosphere, with a drop of temperature above 380-100+50 km altitude of at least 15 K, consistent with radiative cooling of the atmosphere by emission from hydrocarbons. © 2005. The American Astronomical Society. All rights reserved.
• Waite Jr., J. H., Cravens, T. E., Ip, W. -., Kasprzak, W. T., Luhmann, J. G., McNutt, R. L., Niemann, H. B., Yelle, R. V., Mueller-Wodarg, I., Ledvina, S. A., & Scherer, S. (2005). Oxygen ions observed near Saturn's A ring. Science, 307(5713), 1260-1262.
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  PMID: 15731442;Abstract: Ions were detected in the vicinity of Saturn's A ring by the Ion and Neutral Mass Spectrometer (INMS) instrument onboard the Cassini Orbiter during the spacecraft's passage over the rings. The INMS saw signatures of molecular and atomic oxygen ions and of protons, thus demonstrating the existence of an ionosphere associated with the A ring. A likely explanation for these ions is photoionization by solar ultraviolet radiation of neutral O2 molecules associated with a tenuous ring atmosphere. INMS neutral measurements made during the ring encounter are dominated by a background signal.
• Waite Jr., J. H., Niemann, H., Yelle, R. V., Kasprzak, W. T., Cravens, T. E., Luhmann, J. G., McNutt, R. L., Ip, W., Gell, D., De, V., Müller-Wordag, I., Magee, B., Borggren, N., Ledvina, S., Fletcher, G., Walter, E., Miller, R., Scherer, S., Thorpe, R., , Jing, X. u., et al. (2005). Ion Neutral Mass Spectrometer results from the first flyby of Titan. Science, 308(5724), 982-986.
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  PMID: 15890873;Abstract: The Cassini Ion Neutral Mass Spectrometer (INMS) has obtained the first in situ composition measurements of the neutral densities of molecular nitrogen, methane, molecular hydrogen, argon, and a host of stable carbon-nitrite compounds in Titan's upper atmosphere. INMS in situ mass spectrometry has also provided evidence for atmospheric waves in the upper atmosphere and the first direct measurements of isotopes of nitrogen, carbon, and argon, which reveal interesting clues about the evolution of the atmosphere. The bulk composition and thermal structure of the moon's upper atmosphere do not appear to have changed considerably since the Voyager 1 flyby.
• Young, L. A., Yelle, R. V., Young, R., Seiff, A., & Kirk, D. B. (2005). Gravity waves in Jupiter's stratosphere, as measured by the Galileo ASI experiment. Icarus, 173(1), 185-199.
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  Abstract: The temperatures in Jupiter's stratosphere, as measured by the Galileo Atmosphere Structure Instrument (ASI), show fluctuations that have been interpreted as gravity waves. We present a detailed description of these fluctuations, showing that they are not likely to be due to either measurement error or isotropic turbulence. These fluctuations share features with gravity waves observed in the terrestrial middle atmosphere, including the shape and amplitude of the power spectrum of temperature with respect to vertical wavenumber. Under the gravity wave interpretation, we find that wave heating or cooling is likely to be important in Jupiter's upper stratosphere and unimportant in the lower stratosphere. © 2004 Elsevier Inc. All rights reserved.
• Buratti, B. J., Britt, D. T., Soderblom, L. A., Hicks, M. D., Boice, D. C., Brown, R. H., Meier, R., Nelson, R. M., Oberst, J., Owen, T. C., Rivkin, A. S., Sandel, B. R., Stern, S. A., Thomas, N., & Yelle, R. V. (2004). 9969 Braille: Deep Space 1 infrared spectroscopy, geometric albedo, and classification. Icarus, 167(1), 129-135.
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  Abstract: Spectra of Asteroid 9969 Braille in the 1.25-2.6 μm region returned by the Deep Space 1 (DS1) Mission show a ∼10% absorption band centered at 2 μm, and a reflectance peak at 1.6 μm. Analysis of these features suggest that the composition of Braille is roughly equal parts pyroxene and olivine. Its spectrum between 0.4 and 2.5 μm suggests that it is most closely related to the Q taxonomic type of asteroid. The spectrum also closely matches that of the ordinary chondrites, the most common type of terrestrial meteorite. The geometric albedo of Braille is unusually high (pv = 0.34), which is also consistent with its placement within the rarer classes of stony asteroids, and which suggests it has a relatively fresh, unweathered surface, perhaps due to a recent collision. © 2003 Elsevier Inc. All rights reserved.
• Jessup, K. L., Spencer, J. R., Ballester, G. E., Howell, R. R., Roesler, F., Vigel, M., & Yelle, R. (2004). The atmospheric signature of Io's Prometheus plume and anti-jovian hemisphere: Evidence for a sublimation atmosphere. Icarus, 169(1), 197-215.
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  Abstract: Using the Hubble Space Telescope's Space Telescope Imaging Spectrograph we have obtained for the first time spatially resolved 2000-3000 Å spectra of Io's Prometheus plume and adjoining regions on Io's anti-jovian hemisphere in the latitude range 60° N-60° S, using a 0.1″ slit centered on Prometheus and tilted roughly 45° to the spin axis. The SO2 column density peaked at 1.25 × 1017, cm-2 near the equator, with an additional 5 × 1016 cm-2 enhancement over Prometheus corresponding to a model volcanic SO2 output of 105 kg s-1. Apart from the Prometheus peak, the SO2 column density dropped fairly smoothly away from the subsolar point, even over regions that included potential volcanic sources. At latitudes less than ±30°, the dropoff rate was consistent with control by vapor pressure equilibrium with surface frost with subsolar temperature 117.3 ± 0.6 K, though SO2 abundance was higher than predicted by vapor pressure control at mid-latitudes, especially in the northern hemisphere. We conclude that, at least at low latitudes on the anti-jovian hemisphere where there are extensive deposits of optically-thick SO2 frost, the atmosphere is probably primarily supported by sublimation of surface frost. Although the 45° tilt of our slit prevents us from separating the dependence of atmospheric density on solar zenith angle from its dependence on latitude, the pattern is consistent with a sublimation atmosphere regardless of which parameter is the dominant control. The observed drop in gas abundance towards higher latitudes is consistent with the interpretation of previous Lyman alpha images of Io as indicating an atmosphere concentrated at low latitudes. Comparison with previous disk-resolved UV spectroscopy, Lyman-alpha images, and mid-infrared spectroscopy suggests that Io's atmosphere is denser and more widespread on the anti-jovian hemisphere than at other longitudes. SO2 gas temperatures were in the range of 150-250 K over the majority of the anti-jovian hemisphere, consistent with previous observations. SO was not definitively detected in our spectra, with upper limits to the SO/SO2 ratio in the range 1-10%, roughly consistent with previous observations. S2 gas was not seen anywhere, with an upper limit of 7.5 × 1014 cm-2 for the Prometheus plume, confirming that this plume is significantly poorer in S2 than the Pele plume (S2/SO2 < 0.005, compared to 0.08-0.3 at Pele). In addition to the gas absorption signatures, we have observed continuum emission in the near ultraviolet (near 2800 Å) for the first time. The brightness of the observed emission was directly correlated with the SO2 abundance, strongly peaking in the equatorial region over Prometheus. Emission brightness was modestly anti-correlated with the jovian magnetic latitude, decreasing when Io intersected the torus centrifugal equator. © 2003 Elsevier Inc. All rights reserved.
• Majeed, T., Waite Jr., J. H., Bougher, S. W., Yelle, R. V., Gladstone, G. R., McConnell, z., & Bhardwaj, A. (2004). The ionospheres-thermospheres of the giant planets. Advances in Space Research, 33(2), 197-211.
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  Abstract: The ionospheres of the giant planets Jupiter, Saturn, Uranus, and Neptune are reviewed in light of Pioneer, Voyager, Galileo, and groundbased infrared observations. A major focus of recent research has been on the interpretation of Jovian auroral emissions at infrared, ultraviolet, and X-ray wavelengths. Analysis of these emissions has provided valuable clues to the still poorly understood nature of the magnetosphere-ionosphere interaction at Jupiter. Models of the ionospheres of the outer planets incorporating a neutral thermal structure derived from spacecraft measurements and the effects of vibrationally excited H2 molecules, vertical plasma drift, and particle precipitation have been developed and reproduce reasonably well the ionospheric structures determined from the radio occultation experiments. Other recently reported research include the effect of upward propagating gravity waves on the Jovian ionosphere, a one-dimensional, time-dependent model of Saturn's ionosphere, and a photochemical model of Neptune's upper atmosphere. These studies suggest that the introduction of meteoric ions and gravity waves may impact the hydrocarbon ion chemistry of the lower ionospheric regions of the outer planets. Finally, we report the development of three-dimensional general circulation models, which are now able to investigate the physical processes of the outer planets' thermospheres self-consistently with the global thermal structure and global distribution of the neutral and ions. © 2003 COSPAR. Published by Elsevier Ltd. All rights reserved.
• Soderblom, L. A., Boice, D. C., Britt, D. T., Brown, R. H., Buratti, B. J., Kirk, R. L., Lee, M., Nelson, R. M., Oberst, J., Sandel, B. R., Stern, S. A., Thomas, N., & Yelle, R. V. (2004). Imaging Borrelly. Icarus, 167(1), 4-15.
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  Abstract: The nucleus, coma, and dust jets of short-period Comet 19P/Borrelly were imaged from the Deep Space 1 spacecraft during its close flyby in September 2001. A prominent jet dominated the near-nucleus coma and emanated roughly normal to the long axis of nucleus from a broad central cavity. We show it to have remained fixed in position for more than 34 hr, much longer than the 26-hr rotation period. This confirms earlier suggestions that it is co-aligned with the rotation axis. From a combination of fitting the nucleus light curve from approach images and the nucleus' orientation from stereo images at encounter, we conclude that the sense of rotation is right-handed around the main jet vector. The inferred rotation pole is approximately perpendicular to the long axis of the nucleus, consistent with a simple rotational state. Lacking an existing IAU comet-specific convention but applying a convention provisionally adopted for asteroids, we label this the north pole. This places the sub-solar latitude at ∼60° N at the time of the perihelion with the north pole in constant sunlight and thus receiving maximum average insolation. © 2003 Elsevier Inc. All rights reserved.
• Soderblom, L. A., Britt, D. T., Brown, R. H., Buratti, B. J., Kirk, R. L., Owen, T. C., & Yelle, R. V. (2004). Short-wavelength infrared (1.3-2.6 μm) observations of the nucleus of Comet 19P/Borrelly. Icarus, 167(1), 100-112.
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  Abstract: During the last two minutes before closest approach of Deep Space 1 to Comet 19P/Borrelly, a long exposure was made with the short-wavelength infrared (SWIR) imaging spectrometer. The observation yielded 46 spectra covering 1.3-2.6 μm; the footprint of each spectrum was ∼160 m × width of the nucleus. Borrelly's highly variegated and extremely dark 8-km-long nucleus exhibits a strong red slope in its short-wavelength infrared reflection spectrum. This slope is equivalent to J-K and H-K colors of ∼0.82 and ∼0.43, respectively. Between 2.3-2.6 μm thermal emission is clearly detectable in most of the spectra. These data show the nucleus surface to be hot and dry; no trace of H2O ice was detected. The surface temperature ranged continuously across the nucleus from ≤300 K near the terminator to a maximum of ∼340 K, the expected sub-solar equilibrium temperature for a slowly rotating body. A single absorption band at ∼2.39 μm is quite evident in all of the spectra and resembles features seen in nitrogen-bearing organic molecules that are reasonable candidates for compositional components of cometary nuclei. However as of yet the source of this band is unknown. © 2003 Elsevier Inc. All rights reserved.
• Waite Jr., J. H., Lewis, W. S., Kasprzak, W. T., Anicich, V. G., Block, B. P., Cravens, T. E., Fletcher, G. G., Ip, W. -., Luhmann, J. G., Mcnutt, R. L., Niemann, H. B., Parejko, J. K., Richards, J. E., Thorpe, R. L., Walter, E. M., & Yelle, R. V. (2004). The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation. Space Science Reviews, 114(1-4), 113-231.
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  Abstract: The Cassini Ion and Neutral Mass Spectrometer (INMS) investigation will determine the mass composition and number densities of neutral species and low-energy ions in key regions of the Saturn system. The primary focus of the INMS investigation is on the composition and structure of Titan's upper atmosphere and its interaction with Saturn's magnetospheric plasma. Of particular interest is the high-altitude region, between 900 and 1000 km, where the methane and nitrogen photochemistry is initiated that leads to the creation of complex hydrocarbons and nitriles that may eventually precipitate onto the moon's surface to form hydrocarbon-nitrile lakes or oceans. The investigation is also focused on the neutral and plasma environments of Saturn's ring system and icy moons and on the identification of positive ions and neutral species in Saturn's inner magnetosphere. Measurement of material sputtered from the satellites and the rings by magnetospheric charged particle and micrometeorite bombardment is expected to provide information about the formation of the giant neutral cloud of water molecules and water products that surrounds Saturn out to a distance of ∼12 planetary radii and about the genesis and evolution of the rings. The INMS instrument consists of a closed ion source and an open ion source, various focusing lenses, an electrostatic quadrupole switching lens, a radio frequency quadrupole mass analyzer, two secondary electron multiplier detectors, and the associated supporting electronics and power supply systems. The INMS will be operated in three different modes: a closed source neutral mode, for the measurement of non-reactive neutrals such as N2 and CH4; an open source neutral mode, for reactive neutrals such as atomic nitrogen; and an open source ion mode, for positive ions with energies less than 100 eV. Instrument sensitivity is greatest in the first mode, because the ram pressure of the inflowing gas can be used to enhance the density of the sampled non-reactive neutrals in the closed source antechamber. In this mode, neutral species with concentrations on the order of ≥104 cm -3 will be detected (compared with ≥105 cm-3 in the open source neutral mode). For ions the detection threshold is on the order of 10-2 cm-3 at Titan relative velocity (6 km sec-1). The INMS instrument has a mass range of 1-99 Daltons and a mass resolutionM/ΔM of 100 at 10% of the mass peak height, which will allow detection of heavier hydrocarbon species and of possible cyclic hydrocarbons such as C6H6. The INMS instrument was built by a team of engineers and scientists working at NASA's Goddard Space Flight Center (Planetary Atmospheres Laboratory) and the University of Michigan (Space Physics Research Laboratory). INMS development and fabrication were directed by Dr. Hasso B. Niemann (Goddard Space Flight Center). The instrument is operated by a Science Team, which is also responsible for data analysis and distribution. The INMS Science Team is led by Dr. J. Hunter Waite, Jr. (University of Michigan). © 2004 Kluwer Academic Publishers.
• Yelle, R. V. (2004). Aeronomy of extra-solar giant planets at small orbital distances. Icarus, 170(1), 167-179.
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  Abstract: One-dimensional aeronomical calculations of the atmospheric structure of extra-solar giant planets in orbits with semi-major axes from 0.01 to 0.1 AU show that the thermospheres are heated to over 10,000 K by the EUV flux from the central star. The high temperatures cause the atmosphere to escape rapidly, implying that the upper thermosphere is cooled primarily by adiabatic expansion. The lower thermosphere is cooled primarily by radiative emissions from H+3, created by photoionization of H2 and subsequent ion chemistry. Thermal decomposition of H2 causes an abrupt change in the composition, from molecular to atomic, near the base of the thermosphere. The composition of the upper thermosphere is determined by the balance between photoionization, advection, and H+ recombination. Molecular diffusion and thermal conduction are of minor importance, in part because of large atmospheric scale heights. The energy-limited atmospheric escape rate is approximately proportional to the stellar EUV flux. Although escape rates are large, the atmospheres are stable over time scales of billions of years. © 2004 Elsevier Inc. All rights reserved.
• Yelle, R. V., Soderblom, L. A., & Jokipii, J. R. (2004). Formation of jets in Comet 19P/Borrelly by subsurface geysers. Icarus, 167(1), 30-36.
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  Abstract: Observations of the inner coma of Comet 19P/Borrelly with the camera on the Deep Space 1 spacecraft revealed several highly collimated dust jets emanating from the nucleus. The observed jets can be produced by acceleration of evolved gas from a subsurface cavity through a narrow orifice to the surface. As long as the cavity is larger than the orifice, the pressure in the cavity will be greater than the ambient pressure in the coma and the flow from the geyser will be supersonic. The gas flow becomes collimated as the sound speed is approached and dust entrainment in the gas flow creates the observed jets. Outside the cavity, the expanding gas loses its collimated character, but the density drops rapidly decoupling the dust and gas, allowing the dust to continue in a collimated beam. The hypothesis proposed here can explain the jets seen in the inner coma of Comet 1P/Halley as well, and may be a primary mechanism for cometary activity. © 2003 Published by Elsevier Inc.
• Bétrémieux, Y., Yelle, R. V., & Griffith, C. A. (2003). HST observation of the atmospheric composition of Jupiter's equatorial region: Evidence for tropospheric C₂H₂. Icarus, 163(2), 414-427.
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  Abstract: This paper presents the first detailed analysis of acetylene absorption features observed longward of 190.0 nm in a jovian spectrum by the Faint Object Spectrograph on board the Hubble Space Telescope. The presence of two features located near 207.0 nm can only be explained by a substantial abundance of acetylene in the upper troposphere. Using a Rayleigh-Raman radiative transfer model, it was determined that the acetylene vertical profile is characterized by a decrease in the mole fraction with increasing pressure in the upper stratosphere, a minimum around 14 to 29 mbar, followed by an increase to about 1.5 × 10-7 in the upper troposphere. Longward of 220 nm, the relatively high contrast of Raman features to the continuum precludes the existence of an optically significant amount of clouds from 150 to 500 mbar unless they are highly reflective. Instead, the reflectivity at these long wavelengths is determined by stratospheric, not tropospheric, scatterers and absorbers. Analysis of the data also suggests that ammonia is extremely undersaturated at pressures below 700 mbar. However, no firm conclusions can be reached because of the uncertainties surrounding its cross section longward of 217.0 nm, which are due to vibrationally excited states. © 2003 Elsevier Science (USA). All rights reserved.
• Müller-Wodarg, I., Yelle, R. V., Mendillo, M. J., & Aylward, A. D. (2003). On the global distribution of neutral gases in Titan's upper atmosphere and its effect on the thermal structure. Journal of Geophysical Research A: Space Physics, 108(A12).
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  Abstract: Using a time-dependent general circulation model of Titan's thermosphere, we calculate the global distribution of neutral gases by winds and diffusion. Our calculations suggest that solar driven dynamics effectively redistribute constituents, causing considerable diurnal and seasonal changes in gas abundances. Subsidence causes an accumulation of lighter gases on the nightside, with nighttime CH4 mole fractions at equinox near 1400 km reaching up to 50%. The reverse happens on the dayside, where lighter gases are depleted, giving minimum CH4 mole fractions near 1400 km of around 12%. The vertical transport time scales are around 5-10% of a Titan day, so these extrema in gas abundances are shifted with respect to local noon and midnight by up to 4 hours Local Solar Time (LST). The strong horizontal variations in gas abundances, combined with the local time shifts of their extrema, have an important impact on the thermal structure and lead to a shift of the nighttime minimum from local midnight towards early morning hours (0330 LST). This coupling between gas distribution and thermal structure on the nightside occurs via dynamical processes, primarily through changes in adiabatic heating. The redistribution of gases effectively controls, through changes in mean molecular weight, the pressure gradients, which in turn control the horizontal and vertical winds, and thereby adiabatic heating and cooling. On the dayside, changes in solar EUV absorption due to the redistributed gases occur but are comparatively small. Although it is possible with our calculations to identify important processes, Voyager and ground based observations of Titan are currently not sufficient to constrain the dynamics of Titan's upper atmosphere, but comparisons with forthcoming Cassini observations are highly anticipated. Copyright 2003 by the American Geophysical Union.
• Yelle, R. V., & Griffith, C. A. (2003). HCN fluorescence on Titan. Icarus, 166(1), 107-115.
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  Abstract: The HCN emission features near 3 μm recently detected by Geballe et al. (2003, Astrophys. J. 583, L39) are analyzed with a model for fluorescence of sunlight in the ν3 band of HCN. The emission spectrum is consistent with current knowledge of the atmospheric temperature profile and the HCN distribution inferred from millimeter-wave observations. The spectrum is insensitive to the abundance of HCN in the thermosphere and the thousand-fold enhancement relative to photochemical models suggested by Geballe et al. (2003, Astrophys. J. 583, L39) is not required to explain the observations. We find that the spectrum can be matched with temperatures from 130 to 200 K, with slightly better fits at high temperature, contrary to the temperature determination of 130 ± 10 K of Geballe et al. (2003, Astrophys. J. 583, L39). The HCN emission spectrum is sensitive to the collisional de-excitation probability, P10, for the ν3 state and we determine a value of 10-5 with an accuracy of about a factor of two. Analysis of absorption lines in the C2H2 ν3 band near 3 μm, detected in the same spectrum, indicate a C2H2 mole fraction near 0.01 μbar of 10-5 for P10 = 10-4. The derived mole fraction, however, is dependent upon the value adopted for P10 and lower values are required if P10 at Titan temperatures is less than its room temperature value. © 2003 Elsevier Inc. All rights reserved.
• Boice, D. C., Soderblom, L. A., Britt, D. T., Brown, R. H., Sandel, B. R., Yelle, R. V., Buratti, B. J., Hicks, M. D., Nelson, R. M., Rayman, M. D., Oberst, J., & Thomas, N. (2002). The deep space 1 encounter with comet 19P/Borrelly. Earth, Moon and Planets, 89(1-4), 301-324.
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  Abstract: NASA's Deep Space 1 (DS1) spacecraft successfully encountered comet 19P/Borrelly near perihelion and the Miniature Integrated Camera and Spectrometer (MICAS) imaging system onboard DS1 returned the first high-resolution images of a Jupiter-family comet nucleus and surrounding environment. The images span solar phase angles from 88° to 52°, providing stereoscopic coverage of the dust coma and nucleus. Numerous surface features are revealed on the 8-km long nucleus in the highest resolution images (47-58 m/pixel). A smooth, broad basin containing brighter regions and mesa-like structures is present in the central part of the nucleus that seems to be the source of jet-like dust features seen in the coma. High ridges seen along the jagged terminator lead to rugged terrain on both ends of the nucleus containing dark patches and smaller series of parallel grooves. No evidence of impact craters with diameters larger than about 200-m are present, indicating a young and active surface. The nucleus is very dark with albedo variations from 0.007 to 0.035. Short-wavelength, infrared spectra from 1.3 to 2.6 μm revealed a hot, dry surface consistent with less than about 10% actively sublimating. Two types of dust features are seen: Broad fans and highly collimated "jets" in the sunward hemisphere that can be traced to the surface. The source region of the main jet feature, which resolved into at least three smaller "jets" near the surface, is consistent with an area around the rotation pole that is constantly illuminated by the sun during the encounter. Within a few nuclear radii, entrained dust is rapidly accelerated and fragmented and geometrical effects caused from extended source regions are present, as evidenced in radial intensity profiles centered on the jet features that show an increase in source strength with increasing cometocentric distance. Asymmetries in the dust from dayside to nightside are pronounced and may show evidence of lateral flow transporting dust to structures observed in the nightside coma. A summary of the initial results of the Deep Space 1 Mission is provided, highlighting the new knowledge that has been gained thus far.
• Buratti, B., Hicks, M., Soderblom, L., Britt, D., Boice, D., Brown, R., Nelson, R., Oberst, J., Owen, T., Sandel, B., Stern, S. A., Thomas, N., & Yelle, R. (2002). The nucleus of 19/P Borrelly as revealed by deep space 1. European Space Agency, (Special Publication) ESA SP, 545-547.
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  Abstract: The Deep Space 1 encounter with comet 19/P Borrelly offered the first close-up view of a comet unobscured by dust. The geometric albedo of the comet is 0.029±0.006 (with a size of 8.0 × 3.15 km), comparable to the low-albedo hemisphere of Iapetus, the lowest albedo C-type asteroids, and the Uranian rings. Albedo variegations on the body are substantial, far greater than on the handful of asteroids so far scrutinized by spacecraft. The Bond albedo of Borrelly is 0.009 ± 0.002, the lowest of any object in the Solar System. The physical photometric parameters of the comet are similar to asteroids, but the optically active portion of its regolith may be fluffier. Differences in macroscopic roughness exist on its surface: the older regions appear to be slightly less rough, as if low-lying regions are infilled with native dust. Regional differences in the single particle phase function exist, with small regions exhibiting almost isotropic functions.
• Jian, G. e., Ren, D., Lunine, J. I., Brown, R. H., Yelle, R. V., & Soderblom, L. A. (2002). A compact high-resolution 3-D imaging spectrometer for discovering Oases on Mars. Proceedings of SPIE - The International Society for Optical Engineering, 4859, 45-56.
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  Abstract: A new design for a very lightweight, very high throughput reflectance sectrometer enabled by two new technologies being developed is presented. These new technologies include integral field unit optics to enable simultaneous imaging and spectroscopy at high spatial resolution with an infrared (IR) array, and silicon grisms to enable compact and high-resolution spectroscopy.
• Müller-Wodarg, I., & Yelle, R. V. (2002). The effect of dynamics on the composition of Titan's upper atmosphere. Geophysical Research Letters, 29(23), 54-1.
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  Abstract: Using a global time-dependent general circulation model, we calculate the distribution of constituents in Titan's thermosphere resulting from transport caused by winds and molecular and eddy diffusion. Our simulations reveal that thermospheric winds effectively mix constituents in Titan's upper atmosphere. Consequently, the large eddy coefficients inferred from Voyager UVS observations may be a result of vigorous thermospheric circulation on Titan. Thermospheric winds also cause large diurnal variations in composition, with equatorial CH4 mole fractions near 1400 km ranging from ∼ 15% in the late afternoon to ∼ 58% in the early morning at equinox.
• Soderblom, L. A., Becker, T. L., Bennett, G., Boice, D. C., Britt, D. T., Brown, R. H., Buratti, B. J., Isbell, C., Giese, B., Hare, T., Hicks, M. D., Howington-Kraus, E., Kirk, R. L., Lee, M., Nelson, R. M., Oberst, J., Owen, T. C., Rayman, M. D., Sandel, B. R., , Stern, S. A., et al. (2002). Observations of comet 19P/Borrelly by the miniature integrated camera and spectrometer aboard deep space 1. Science, 296(5570), 1087-1091.
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  PMID: 11934989;Abstract: The nucleus of the Jupiter-family comet 19P/Borrelly was closely observed by the Miniature Integrated Camera and Spectrometer aboard the Deep Space 1 spacecraft on 22 September 2001. The 8-kilometer-long body is highly variegated on a scale of 200 meters, exhibiting large albedo variations (0.01 to 0.03) and complex geologic relationships. Short-wavelength infrared spectra (1.3 to 2.6 micrometers) show a slope toward the red and a hot, dry surface (≤345 kelvin, with no trace of water ice or hydrated minerals), consistent with ∼10% or less of the surface actively sublimating. Borrelly's coma exhibits two types of dust features: fans and highly collimated jets. At encounter, the near-nucleus coma was dominated by a prominent dust jet that resolved into at least three smaller jets emanating from a broad basin in the middle of the nucleus. Because the major dust jet remained fixed in orientation, it is evidently aligned near the rotation axis of the nucleus.
• Yelle, R. V., Griffith, C. A., & Young, L. A. (2001). Structure of the Jovian Stratosphere at the Galileo Probe Entry Site. Icarus, 152(2), 331-346.
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  Abstract: The structure of the Jovian stratosphere at the Galileo probe entry site is investigated through calculations of radiative heating rates based on measurements of the temperature profile, composition, and aerosol distribution. From analysis of mid-IR observations of Jupiter, we determine a C2H2 mole fraction of 1.1-4.3 × 10-6 at 0.01 mbar, and a C2H6 mole fraction of 2.8-6.5 × 10-6 at 0.4-10 mbar. The derived distributions imply that C2H6 and C2H2 are the most important coolants in the Jovian stratosphere from 0.004 to 10 mbar, and that the stratosphere is close to radiative equilibrium. In Jupiter's stratosphere, as in the stratosphere of the Earth, photochemical species play an essential role in the energy balance. © 2001 Academic Press.
• Young, L. A., Cook, J. C., Yelle, R. V., & Young, E. F. (2001). Upper limits on gaseous CO at Pluto and Triton from high-resolution near-IR spectroscopy. Icarus, 153(1), 148-156.
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  Abstract: We observed Pluto and Triton with the CSHELL echelle spectrograph on the Infrared Telescope Facility in April and July 1996, in an effort to detect the R(2), R(3), and R(4) rotational lines of the 2-0 vibrational transition of gaseous CO. As no lines were detected, we derived 3-σ upper limits on the average widths of these three lines of 0.040 cm-1 for Pluto and 0.028 cm-1 for Triton. The corresponding upper limits on the gaseous CO mole fractions depend on the assumed profiles of temperature and pressure in the atmospheres of these bodies. If Triton's atmosphere in 1996 resembles that measured by stellar occultation in 1997, we find a 3-σ upper limit to the CO mole fraction of 59%. If Pluto's atmosphere resembles the tropospheric model of J. A. Stansberry, J. I. Lunine, W. B. Hubbard, R. V. Yelle, and D. M. Hunten (1994), Icarus 11, 503-513, we find a 3-σ upper limit to the CO mole fraction of 6%. For Pluto, this limit to the gaseous mole fraction argues against intimate mixtures (e.g., "salt-and-pepper" mixtures, as opposed to solid solutions) of surface CO and N2 frost. © 2001 Academic Press.
• Griffith, C. A., & Yelle, R. V. (2000). Equilibrium chemistry in a brown dwarf's atmosphere: Cesium in Gliese 229B. Astrophysical Journal Letters, 532(1 PART 2), L59-L62.
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  Abstract: The distribution of Cs in Gliese 229B's atmosphere reveals how equilibrium chemistry establishes the atmospheric composition. The rapid kinetics of cesium chemistry keeps the Cs abundance in thermochemical equilibrium and renders Cs a sensitive measure of chemical processes in brown dwarf atmospheres. Observations of Gliese 229B indicate a subsolar bulk abundance of Cs, the depletion of alkali metals in the upper atmosphere from condensation, and a partitioning of heavy elements different from that of the Sun.
• Müller-Wodarg, I., Yelle, R. V., Mendillo, M., Young, L. A., & Aylward, A. D. (2000). The thermosphere of Titan simulated by a global three-dimensional time-dependent model. Journal of Geophysical Research A: Space Physics, 105(A9), 20833-20856.
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  Abstract: We present three-dimensional numerical simulations for dynamics and energetics of Titan's thermosphere. In so doing, we distinguish between the dynamics driven by solar insolation and those driven by vertical coupling to winds in Titan's middle atmosphere. Our calculations reveal that the solar-driven thermospheric dynamics are characterized by the balance between pressure gradients and viscosity, while the super-rotating zonal winds detected in Titan's stratosphere set up a balance between the pressure gradients, curvature and Coriolis forces. The day to night temperature gradients in the upper thermosphere (around 1300 km) typically lie around 20 (10) K for solar maximum (minimum), with peak solar-driven winds of around 60 (30) m/s. This difference decreases with height and virtually disappears below 1000 km as a result of dayside adiabatic cooling and nightside adiabatic heating. The model highlights unique features about the thermosphere on Titan, such as the important nighttime heating from mid-latitudes to high-latitudes caused by the relatively small size of the planet's shadow, leading to features in the wind profiles which are not found on Earth. Although the lack of measurement constraints prevents us from making predictions of actual wind profiles on Titan, the model does illustrate the physical processes driving the dynamics and suggests that anticipated thermospheric measurements from the Cassini spacecraft may provide constraints also for the dynamics at lower altitudes. Copyright 2000 by the American Geophysical Union.
• Rishbeth, H., Yelle, R. V., & Mendillo, M. (2000). Dynamics of Titan's thermosphere. Planetary and Space Science, 48(1), 51-58.
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  Abstract: We estimate the wind speeds in Titan's thermosphere by considering the various terms of the wind equation, without actually solving it, with a view to anticipating what might be observed by the Cassini spacecraft in 2004. The winds, which are driven by horizontal pressure gradients produced by solar heating, are controlled in the Earth's thermosphere by ion-drag and coriolis force, but in Titan's thermosphere they are mainly controlled by the nonlinear advection and curvature forces. Assuming a day night temperature difference of 20 K, we find that Titan's thermospheric wind speed is typically 60 m s-1. In contrast, the Earth's thermospheric winds, of order 50 m s-1, do not equalize day and night temperatures. We speculate on other factors, such as the electrodynamics of Titan's thermosphere and the tides due to Saturn. © 1999 Elsevier Science Ltd. All rights reserved.
• Spencer, J. R., Jessup, K. L., McGrath, M. A., Ballester, G. E., & Yelle, R. (2000). Discovery of gaseous S₂ in Io's Pele plume. Science, 288(5469), 1208-1210.
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  PMID: 10817990;Abstract: Spectroscopy of Io's Pele plume against Jupiter by the Hubble Space Telescope in October 1999 revealed absorption due to S2 gas, with a column density of 1.0 ± 0.2 x 1016 per square centimeter, and probably also SO2 gas with a column density of 7 ± 3 x 1016 per square centimeter. This SO2/S2 ratio (3 to 12) is expected from equilibration with silicate magmas near the quartz-fayalite-magnetite or wustite-magnetite buffers. Condensed S3 and S4, probable coloring agents in Pele's red plume deposits, may form by polymerization of the S2, which is unstable to ultraviolet photolysis. Diffuse red deposits near other Io volcanoes suggest that venting and polymerization of S2 gas is a widespread feature of Io volcanism.
• Bétremieux, Y., & Yelle, R. V. (1999). HST Detection of h₂ Raman Scattering in the Jovian Atmosphere. Icarus, 142(2), 324-341.
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  Abstract: Raman scattered features by molecular hydrogen have been detected in Hubble Space Telescope (HST) Faint Object Spectrograph (FOS) observations of Jupiter. The measurements were obtained with the G190H grating and red detector combination spanning 158.0-232.0 nm at about 0.3 nm resolution. The data were corrected for scattered light, and careful modeling of the line spread function (LSF) of the instrument was performed to accurately degrade the solar spectrum obtained by SOLSTICE (solar-stellar irradiance comparison experiment) to the spectral resolution of the FOS. A cross-correlation method was used to align features in the planetary spectra to those in the SOLSTICE solar spectrum. At all latitudes longward of 210.0 nm, the resulting I/F displayed discrete features up to 20% of the continuum level that anticorrelate with the solar spectrum. A radiative transfer code was developed to include the effect of rotational and vibrational multiple Raman scattering for the first few lowest energy rotational states of molecular hydrogen under the approximation that the Raman component of the scattering phase function is isotropic. Simulations show not only that the detected features are indeed due to Raman scattering by H2, but are sensitive to its ortho-para ratio as well. An analysis of the equatorial spectrum reveals that the features are consistent with an equilibrium or normal population of H2 at 130 K. © 1999 Academic Press.
• Griffith, C. A., & Yelle, R. V. (1999). Disequilibrium chemistry in a brown dwarf's atmosphere: Carbon monoxide in Gliese 229B. Astrophysical Journal Letters, 519(1 PART 2), L85-L88.
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  Abstract: The distribution of CO in Gliese 229B's atmosphere reveals how disequilibrium processes establish the atmospheric composition. The CO abundance derived from its spectral signature exceeds the equilibrium value by several orders of magnitude. Our investigation of the source for CO considers disequilibrium mechanisms common to planetary atmospheres and concludes that the CO abundance is sensitive to atmospheric dynamics. Convection is not required to explain the observed abundance; instead, the vertical transport rate at ∼6 bars may be similar to that in planetary stratospheres.
• Stansberry, J. A., & Yelle, R. V. (1999). Emissivity and the fate of Pluto's atmosphere. Icarus, 141(2), 299-306.
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  Abstract: We present a simplified model for seasonal changes in Pluto's surface-atmosphere system. The model demonstrates the potential importance of the solid-state phase transition between α-N2 and β-N2, and the accompanying change in emissivity, for predicting the seasonal bulk of Pluto's (and Triton's) atmosphere. Specifically, the model shows that under simplified but not unreasonable assumptions Pluto may have nearly the same atmospheric pressure at aphelion as it does now, near perihelion. The emissivity change which accompanies the α-β phase change should be included in the next generation of Pluto and Triton seasonal models for the purposes of understanding the evolution of their atmospheres over seasonal and climatic timescales. © 1999 Academic Press.
• Griffith, C. A., Yelle, R. V., & Marley, M. S. (1998). The dusty atmosphere of the brown dwarf Gliese 229B. Science, 282(5396), 2063-2067.
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  PMID: 9851924;Abstract: The brown dwarf Gliese 229B has an observable atmosphere too warm to contain ice clouds like those on Jupiter and too cool to contain silicate clouds like those on low-mass stars. These unique conditions permit visibility to higher pressures than possible in cool stars or planets. Gliese 229B's 0.85- to 1.0-micrometer spectrum indicates particulates deep in the atmosphere (10 to 50 bars) having optical properties of neither ice nor silicates. Their reddish color suggests an organic composition characteristic of aerosols in planetary stratospheres. The particles' mass fraction (10-7) agrees with a photochemical origin caused by incident radiation from the primary star and suggests the occurrence of processes native to planetary stratospheres.
• Fox, J. L., & Yelle, R. V. (1997). Hydrocarbon ions in the ionosphere of Titan. Geophysical Research Letters, 24(17), 2179-2182.
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  Abstract: We have constructed a new model of the ionosphere of Titan that includes 67 species and 626 reactions. Although N 2+ is the major ion produced over most of the ionosphere, the ionization flows to ions whose parent neutrals have lower ionization potentials and to ions formed from species with large proton affinities. In contrast to other models, which have predicted that HCNH + should be the major ion, our calculations suggest that the major ions at and below the ion peak are hydrocarbon ions, and H, C, and N-containing ions. Our predicted peak electron density for a solar zenith angle of 60° is about 7.5 × 103 cm -3 at an altitude of 1040 km. Copyright 1997 by the American Geophysical Union.
• Young, L. A., Yelle, R. V., Young, R., Seiff, A., & Kirk, D. B. (1997). Gravity waves in Jupiter's thermosphere. Science, 276(5309), 108-111.
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  Abstract: The Atmosphere Structure Instrument on the Galileo probe detected wavelike temperature fluctuations superimposed on a 700-kelvin temperature increase in Jupiter's thermosphere. These fluctuations are consistent with gravity waves that are viscously damped in the thermosphere. Moreover, heating by these waves can explain the temperature increase measured by the probe. This heating mechanism should be applicable to the thermospheres of the other giant planets and may help solve the long-standing question of the source of their high thermospheric temperatures.
• Beauchamp, P. M., Alkalai, L., Brown, R. H., Capps, R. W., Chen, G., Crisp, M. P., Cutts, J. A., Davidson, J. M., Petrick, S. W., Rodgers, D. H., Vane, G., Soderblom, L. A., & Yelle, R. V. (1996). Sciencecraft process. Proceedings of SPIE - The International Society for Optical Engineering, 2810, 22-30.
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  Abstract: In this paper, the authors propose a new process for the development and operation of unmanned vehicles for the exploration of space. We call the vehicle (and the process used to create it) sciencecraft. A Sciencecraft is an integrated unit that combines into a single system those elements (but no more) which are necessary to achieve the science objectives of the mission, including science instruments, electronics, telecommunications, power, and propulsion. the design of a sciencecraft begins with the definition of the mission science objectives. This is followed by the establishment of measurement goals and the definition of a critical data set. Next an observational sequence is developed, which will provide the data set. This step is followed by the design of the integrated sensor system that will make the observations. The final step in the development of a sciencecraft is the design of the hardware subsystems needed to deliver the sensor to its target and return the science data to the earth. This approach assures that the sciencecraft hardware design and overall architecture will be driven by the science objectives and the sensor requirements rather than the reverse, as has historically been the case. Throughout the design process, there is an emphasis on shared functionality, shared redundancy, and reduced cost. We illustrate the power of the sciencecraft approach by describing the Planetary Integrated Camera Spectrometer (PICS), an integrated sensor system in which the 'sciencecraft' process has been applied to the development of a single subsystem, which integrates multiple functionalities. PICS is a case-in-point where the sciencecraft process has been successfully demonstrated. We then describe a sciencecraft mission for exploration of the outer Solar System, including flybys of Uranus, Neptune, and an object in the Kuiper Belt. This mission, called the Kuiper Express, will use solar electric propulsion to shape its trajectory in the inner solar system and will use no nuclear power. The Kuiper Express is an example of how the sciencecraft approach can return 'voyager class science at ten cents on the dollar'.
• Beauchamp, P. M., Benoit, R. T., Brown, R. H., Bruce Jr., C. F., Chen, G., Crisp, M. P., Davidson, J. M., Fraschetti, G. A., Petrick, S. W., Rodgers, D. H., Sandel, B. R., Sepulveda, C. A., Soderblom, L. A., Wang, D., Soll, S. L., & Yelle, R. V. (1996). Planetary Integrated Camera Spectrometer (PICS): a new approach to developing a self-sequencing, integrated, multiwavelength instrument. Proceedings of SPIE - The International Society for Optical Engineering, 2744, 698-711.
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  Abstract: The planetary integrated camera-spectrometer, PICS, is a highly integrated sensor system which performs the functions of three optical instruments: a near infrared (IR) spectrometer, a visible imaging camera, and an ultraviolet (UV) spectrometer. Integration serves to minimize the mass and power required to operate a complex suite of instruments, and automatically yields a comprehensive data set, optimized for correlative analysis. This approach is useful for deep space missions such as Pluto Express and will also enable Galileo/Cassini class remote observations of any object within the solar system. In our baseline concept, a single set of lightweight multiwavelength foreoptics is shared by a UV imaging spectrometer (80 spectral channels 70 - 150 nm), a two-CCD visible imaging system (shuttered in two colors 300 - 500 nm and 500 - 1000 nm), and a near-IR imaging spectrometer (256 spectral channels 1300-2600 nm). The entire structure, including its optics, is built from silicon carbide (SiC) for thermal and dimensional stability. In addition, there are no moving parts and each spectrometer covers a single octave in wavelength. A separate port is provided for measurement of a UV solar occultation and for spectral radiance calibration of the IR and visible subsystems. The integrated science that the PICS will yield meets or exceeds all of the Priority-1A science objectives, and many Priority 1-B science objectives as well, for the Pluto Express Mission. This paper provides details of the PICs instrument design, fabrication and testing, both at the sub-assembly and the instrument level. In all tests, including optical, thermal vacuum, and structural/dynamics, the PICS hardware prototype met or exceeded functional requirements.
• Rodgers, D. H., Alkalai, L., Beauchamp, P. M., Chen, G., Crisp, M. P., Brown, R. H., Davidson, J. M., Huxtable, D. D., Penzo, P. A., Petrick, S. W., Soderblom, L. A., Stewart, A., Vane, G., & Yelle, R. V. (1996). Kuiper Express: a sciencecraft. Proceedings of SPIE - The International Society for Optical Engineering, 2810, 11-21.
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  Abstract: The Kuiper Express is a mission to achieve the first reconnaissance of one of the primitive objects that reside in the Kuiper Belt. The objects in the Kuiper Belt are the remnants of the planetesimal swarm that formed the four giant planets of the outer Solar System. These objects, because they are far from the Sun, have not been processed by solar heating and are essentially in their primordial state. This makes them unique objects and their study will provide information on the composition of the solar nebula that cannot be extracted from a study of other objects in the Solar System. The Kuiper Express is a sciencecraft mission. A sciencecraft is an integrated unit that combines into a single system the essential elements (but no more) necessary to achieve the science objectives of the mission, including science instruments, electronics, telecommunications, power, and propulsion. The design of a sciencecraft begins with the definition of mission science objectives and cost constraint. An observational sequence and sensor subsystem are then designed. This sensor subsystem in turn becomes the design driver for the sciencecraft architecture and hardware subsystems needed to deliver the sensor to its target and return the science data to the earth. Throughout the design process, shared functionality, shared redundancy, and reduced cost are strongly emphasized. The Kuiper Express will be launched using a Delta vehicle and will use solar electric propulsion to add velocity and shape its trajectory in the inner Solar System, executing two earth gravity-assist flybys. It will also execute flybys of main belt asteroids, Mars, Uranus, and Neptune/Triton en route to its target in the Kuiper belt, where it will arrive about ten years after launch. It will use no nuclear power. The surface constituents and morphology of the objects visited will be measured and their atmospheres will be characterized. The cost of the detailed design, fabrication, and launch of the Kuiper Express is consistent with the $150M limit set by the NASA Discovery Program.
• Stansberry, J. A., Pisano, D. J., & Yelle, R. V. (1996). The emissivity of volatile ices on Triton and Pluto. Planetary and Space Science, 44(9), 945-955.
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  Abstract: The Hapke theory is used to calculate the emissivity of a semi-infinite layer of granular N2 ice with CH4 and CO as contaminants. It is assumed that the layer is composed of grains which can be characterized as having a single size, and that temperature gradients in the emitting layers of the surface are negligible. The emission spectrum for β-N2, stable above 35.6 K, results from a very broad peak in the absorption spectrum centered at 154 μm, while two absorption peaks, at 143 and 204 μm, produce the emission spectrum of the lower temperature α-N2 phase. For a grain size of 1 cm the Planck-mean bolometric emissivity calculated for the pure β-N2 ice is 0.85. If the effective N2 grain size is 1 mm the emissivity is 0.40. Both are low enough to significantly affect surface energy balance calculations. The very narrow absorption features of α-N2 result in even smaller bolometric emissivities of only 0.11 and 0.30 for 1 mm and 1 cm grain sizes at 34 K. The effect of CH4 and CO, in solid solution with N2 or as separate, intimately mixed grains, on the emissivity is also estimated. It is found that the presence of either or both of these two molecules in solid solution with the N2 ice on Triton and Pluto only slightly increases the β-N2 emissivity. The emissivity of intimate mixtures of grains of CH4 and CO with N2 is much less certain, and probably much less applicable to Triton and Pluto. CH4 and CO in solid solution with α-N2 increase the emissivity by about 50%. For an α-N2 grain size of 1 cm, the addition of 2% each CH4 and CO in solid solution with the N2 increases the emissivity from 0.30 to 0.48 at 34 K. For a 1 mm grain size the emissivity of such a solid solution changes to 0.16 from 0.11. However, the emissivity of α-N2 even with CH4 and/or CO in solution is still considerably lower than for β-N2. Seasonal variations on Triton and Pluto could be strongly influenced by this emissivity contrast between the α and β phases. In the extreme case of pure N2 ice, Pluto's atmosphere could be prevented from freezing out, even at aphelion. Copyright © 1996 Elsevier Science Ltd.
• Stansberry, J. A., Spencer, J. R., Schmitt, B., Benchkoura, A., Yelle, R. V., & Lunine, J. I. (1996). A model for the overabundance of methane in the atmospheres of Pluto and Triton. Planetary and Space Science, 44(9), 1051-1063.
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  Abstract: A model for producing atmospheric CH4 mixing ratios larger than would be expected from simple vapor pressure equilibrium over a solid solution of N2 and CH4 is described. Laboratory experiments show that rapid sublimation of a dilute (0.2% mole fraction) solid solution of CH4 in α-N2 produces a residue of nearly pure CH4 grains. The CH4 grains begin to form very quickly, and most of the CH4 originally in solid solution with the N2 is taken up by the grainy residue rather than sublimating. If the same is true for the much slower sublimation rates on Pluto, patches of nearly pure CH4 ice grains will be built up on sub-seasonal timescales. Such CH4 patches will be in contact with Pluto's predomionatly N2 atmosphere. Further sublimation of these patches wil be controlled by molecular and turbulent diffusion, as will be the condensation of CH4 from the atmosphere in other areas. It is shown that the balance between diffusive sublimation and condensation can easily produce 1% mixing ratios of CH4 in the atmosphere, generally consistent with requirements for explaining Pluto's 100K upper-atmospheric temperature and producing a steep positive temperature gradient in the 2-3 μb region. The same mechanism can explain Triton's less elevated atmospheric CH4 mixing ratio. Copyright © 1996 Elsevier Science Ltd.
• Yelle, R. V. (1996). Structure of Jupiter's upper atmosphere: Predictions for Galileo. Journal of Geophysical Research E: Planets, 101(E1), 2149-2161.
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  Abstract: The Voyager mission to the outer solar system discovered that the thermospheres of all the giant planets are remarkably hot. To date, no convincing explanation for this phenomenon has been offered; however, there are a number of recent observational results which provide new information on the thermal structure of Jupiter's upper atmosphere that bear on this outstanding problem. We present an analysis of Jupiter's thermal structure using constraints from H3+ emissions, Voyager UVS occultation data, ground-based stellar occultation data, and the properties of the Jovian UV dayglow. Although the initial, separate analysis of these data sets produced contradictory results, our reanalysis shows that the observations are consistent and that the temperature profile in Jupiter's upper atmosphere is well constrained. We find that the data demand the presence of a large temperature gradient, of order 3-10 K/km, near a pressure of 0.3 μbar. Analysis of the temperature profile implies that an energy source of roughly 1 erg cm-2 s-1 is required to produce the high thermospheric temperature and that this energy must be deposited in the 0.1-1.0 μbar region. It is also necessary that this energy be deposited above the region where diffusive separation of CH4 occurs, so that the energy is not radiated away by CH4. We show that dissipation of gravity waves can supply the energy required and that this energy will be deposited in the proper region. Moreover, because the turbulent mixing caused by gravity waves determines the level at which diffusive separation of CH4 occurs, the location of the energy source (dissipation of waves) and the energy sink (radiation by CH4) are coupled. We show that the gravity waves will deposit their energy several scale heights above the CH4 layer; energy is carried downward by thermal conduction in the intervening region, causing the large temperature gradient. Thus dissipation of gravity waves appears to be a likely explanation for the high thermospheric temperature. Our arguments are general and should apply to Saturn, Uranus, and Neptune, as well as Jupiter. The model temperature profiles presented here and the relationship between the gravity wave flux and thermospheric temperature are directly testable by the Atmospheric Structure Instrument carried by the Galileo probe. Copyright 1996 by the American Geophysical Union.
• Yelle, R. V., & Mcgrath, M. A. (1996). Ultraviolet spectroscopy of the SL9 impact sites: I. The 175-230 nm region. Icarus, 119(1), 90-111.
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  Abstract: We present a comprehensive analysis of spectra in the 175-230 nm wavelength region obtained by the Faint Object Spectrograph of the Hubble Space Telescope (HST) to determine the abundance of molecular species in the vicinity of the G and L impact sites. Data were obtained on July 18, roughly 3 hr after the G impact, on August 9, and on August 23. All spectra clearly show signatures of aerosols and gaseous CS2 and NH3. The spectra obtained on July 18 also show the spectral signature of H2S. To determine the abundance of gases and aerosols we compare the observations with calculations based on the scattering properties of three-layer models for the atmosphere. We are able to fit the aerosol-dominated portions of the spectra with aerosol distributions similar to those derived from HST imaging observations by West et al. (Science, 267, 1296-1301, 1995). On all three dates we find that CS2 resides at lower pressures than H2S, NH3, and the bulk of the aerosols. The CS2 column abundance is approximately 10-7 g-cm-2 on July 18, a factor of 2-3 less on August 9, and another factor of 2 less on August 23. NH3 is confined to pressures greater than 5 mbar with a mole fraction of 1 × 10-7 on July 18 and August 9, decreasing to 3 × 10-8 on August 23. H2S is also confined to pressures greater than 5 mbar with a mole fraction of 5 × 10-8 on July 18. These mole fractions depend upon assumptions about the aerosol distribution and are derived from models with an aerosol column density of 2 × 109 cm-2 Using different aerosol models, it is possible to obtain adequate fits to the spectra with mole fractions of H2S and NH3 that are 2.5 and 7.5 times smaller. The spectra show no evidence for SO2 absorption and we derive an upper limit of 10-7 g-cm-2 for the July 18 spectrum, assuming that SO2 has the same altitude distribution as CS2. Using the same assumptions we derive upper limits of 10-6 and 3 × 10-8 for OCS and SO. There is no compelling evidence for either H2O or C2H2 but both can be tolerated with mole fractions of 1 × 10-7 and 3 × 10-7, respectively. The altitude distributions of CS2, H2S, and NH3 suggest that CS2 was created by chemistry in the plume but that H2S and NH3 were injected into the stratosphere from below by upwelling over spatial scales of thousands of kilometers associated with the impact. The presence of H2S on July 18 suggests that the G fragment penetrated at least as deep as the NH4-SH clouds. © 1996 Academic Press, Inc.
• Jaffel, L., Prangé, R., Sandel, B. R., Yelle, R. V., Emerich, C., Feng, D., & Hall, D. T. (1995). New Analysis of the Voyager UVS H Lyman-α Emission of Saturn. Icarus, 113(1), 91-102.
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  Abstract: The limb to limb Lyman-α reflectivities observed with the Voyager UVS instruments during the fly-by of Saturn are reanalyzed using a revised H Lyman-α sensitivity for the Voyager 1 instrument. The new sensitivity reconciles the measured intensities to those of Voyager 2 and gives a coherent set of data. To fit the UV airglow observations, four sources are considered: (i) H resonance and H2 Rayleigh scattering of solar Lyman-α radiation, (ii) the interplanetary Lyman-α radiation, (iii) a possible internal source of unknown origin, (iv) the possibility of atmospheric turbulence recently proposed to explain the Lyman-α bulge of Jupiter. The analysis supports neither a dominant collisional excitation source for the UV emissions nor the presence of strong atmospheric turbulence. The best fit, in terms of brightness but also in terms of shape of the limb to limb profile (that is to say independent on the absolute calibrations), is obtained for pure resonance and Rayleigh scattering of solar and interstellar wind line in an atmosphere enriched in atomic hydrogen up to three times the standard model. Influx of water from the rings of Saturn may provide a means for producing such enhanced H densities in the upper atmosphere. © 1995 Academic Press. All rights reserved.
• Noll, K. S., McGrath, M. A., Trafton, L. M., Atreya, S. K., Caldwell, J. J., Weaver, H. A., Yelle, R. V., Barnet, C., & Edgington, S. (1995). HST spectroscopic observations of Jupiter after the collision of comet Shoemaker-Levy 9. Science, 267(5202), 1307-1313.
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  PMID: 7871428;Abstract: Ultraviolet spectra obtained with the Hubble Space Telescope identified at least 10 molecules and atoms in the perturbed stratosphere near the G impact site, most never before observed in Jupiter. The large mass of sulfur-containing material, more than 1014 grams in S2 alone, indicates that many of the sulfur-containing molecules S2, CS2, CS, H2S, and S+ may be derived from a sulfur-bearing parent molecule native to Jupiter. If so, the fragment must have penetrated at least as deep as the predicted NH4SH cloud at a pressure of approximately 1 to 2 bars. Stratospheric NH3 was also observed, which is consistent with fragment penetration below the cloud tops. Approximately 107 grams of neutral and ionized metals were observed in emission, including Mg II, Mg I, Si I, Fe I, and Fe II. Oxygen-containing molecules were conspicuous by their absence; upper limits for SO2, SO, CO, SiO, and H2O are derived.
• Wang, Y., & Yelle, R. V. (1995). Methane heating rates in outer planet atmospheres. Journal of Quantitative Spectroscopy and Radiative Transfer, 54(5), 819-826.
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  Abstract: We have reviewed and expanded the techniques for calculating the heating rates of CH4 near-i.r. bands (3.3, 2.3 and 1.7 μm) for conditions relevant to the atmospheres of the outer planets. We consider all pressures below 100 mbar and temperatures from 77 to 300 K. We find that using the correlated-k method, the heating rates of the stronger band, 3.3 μm band, at all pressures and the heating rates for the other two bands at pressures below 0.1 mbar can be accurately calculated. We also find that the application of the general random band models which depend on the line listings is not appropriate to these three bands. © 1995.
• Yelle, R. V., Herbert, F., Sandel, B. R., Vervack Jr., R. J., & Wentzel, T. M. (1993). The Distribution Hydrocarbons in Neptune's Upper Atmosphere. Icarus, 104(1), 38-59.
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  Abstract: Data from the Voyager UVS solar occultation experiment are analyzed to determine the distribution of hydrocarbons in the upper atmosphere of Neptune. Objective inversion techniques are used to infer densities from the transmission properties of the atmosphere measured by the UVS. Densities of H2, CH4, and C2H6 are determined over limited altitude ranges and constraints are placed on the abundances of C2H2 and C2H4. The inferred densities when used in conjunction with models for the transport of CH4 and C2H6 imply that the CH4 mole fraction in the stratosphere is between 6 × 10-4 and 5 × 10-3, i.e., more than one order of magnitude larger than expected if the tropopause cold trap is operating. This conclusion is independent of assumptions about the temperature profile in the stratosphere. C2H6 is produced from CH4 photolysis with an efficiency of 30-50%. The eddy diffusion coefficient has a value of 1.0 ± 0.3 × 105 cm2 sec-1 at and below 300 km, rising exponentially to a value of 2.4+0.7-0.5 × 106 cm2sec-1 at 550 km. The C2H6 densities determined from this study are consistent with results from groundbased and Voyager infrared observations. The H2 densities are in excellent agreement with results from the Voyager radio occultation. © 1993 Academic Press. All rights reserved.
• Stansberry, J. A., Yelle, R. V., Lunine, J. I., & McEwen, A. S. (1992). Triton's surface-atmosphere energy balance. Icarus, 99(2), 242-260.
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  Abstract: We explore the energetics of Triton's surface-atmosphere system using a model that includes the turbulent transfer of sensible heat as well as insolation, reradiation, and latent heat transport. The model relies on a 1° by 1° resolution hemispheric bolometric albedo map of Triton for determining the atmospheric temperature, the N2 frost emissivity, and the temperatures of unfrosted portions of the surface consistent with a frost temperature of ≅38 K. For a physically plausible range of heat transfer coefficients, we find that the atmospheric temperature roughly 1 km above the surface is approximately 1 to 3 K hotter than the surface. Atmospheric temperatures of 48 K suggested by early analysis of radio occultation data cannot be obtained for plausible values of the heat transfer coefficients. Our calculations indicate that Triton's N2 frosts must have an emissivity well below unity in order to have a temperature of ≅38 K, consistent with previous results. We also find that convection over small hot spots does not significantly cool them off, so they may be able to act as continous sources of buoyancy for convective plumes, but have not explored whether the convection is vigorous enough to entrain particulate matter thereby forming a dust devil. Our elevated atmospheric temperatures make geyser driven plumes with initial upward velocities ≤10 m s-1 stagnate in the lower atmosphere. These "wimpy" plumes provide a possible explanation for Triton's "wind streaks.". © 1992.
• Yelle, R. V. (1992). The effect of surface roughness on Triton's volatile distribution. Science, 255(5051), 1553-1555.
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  PMID: 17820167;Abstract: Calculations of radiative equilibrium temperatures on Triton's rough surface suggest that significant condensation of N2 may be occurring in the northern equatorial regions, despite their relatively dark appearance. The bright frost is not apparent in the Voyager images because it tends to be concentrated in relatively unilluminated facets of the surface. This patchwork of bright frost-covered regions and darker bare ground may be distributed on scales smaller than that of the Voyager resolution; as a result the northern equatorial regions may appear relatively dark. This hypothesis also accounts for the observed wind direction in the southern hemisphere because it implies that the equatorial regions are warmer than the south polar regions.
• Majeed, T., McConnell, J. C., & Yelle, R. V. (1991). Vibrationally excited H₂ in the outer planets thermosphere: Fluorescence in the Lyman and Werner bands. Planetary and Space Science, 39(11), 1591-1606.
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  Abstract: We have considered the impact of fluorescence of ground state H2 on the distribution of the vibrational levels of Ha in the upper atmospheres of Jupiter and Saturn for non-auroral latitudes. For v ≥ 3, for the conditions studied, this is the most important source of vibrationally excited H2 compared with other sources, such as photoelectron induced fluorescence, dissociative recombination of H+3 ions, and direct vibrational excitation of H2 by photoelectron impact. Combining the Voyager limb observations of H2 band emissions on Saturn, theoretical calculations of the H2 fluoresence distribution, and column constraints of Jovian H2 airglow, we estimate that some of the higher vibrational levels may have effective temperatures > 3000 K on both Jupiter and Saturn. In turn, the vibrational population of v ≥ 4 levels are sufficiently increased by the fluorescence source that the chemical sink for the ionization is enhanced. As a result, ionospheric densities may be greatly affected. We also show that the vertical ion flows induced by horizontal neutral winds or dynamo electric fields must play some role in maintaining the plasma peaks at higher altitudes. © 1991.
• Yelle, R. V. (1991). Non-LTE models of Titan's upper atmosphere. Astrophysical Journal Letters, 383(1), 380-400.
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  Abstract: Models for the thermal structure of Titan's upper atmosphere, between 0.1 mbar and 10-2 nbar are presented. The calculations include non-LTE heating/cooling in the rotation-vibration bands of CH4, C2H2, and C2H6, absorption of solar IR radiation in the near-IR bands of CH4 and subsequent cascading to the ν4 band of CH4, absorption of solar EUV and UV radiation, thermal conduction and cooling by HCN rotational lines. Unlike earlier models, the calculated exospheric temperature agrees well with observations, because of the importance of HCN cooling. The calculations predict a well-developed mesopause with a temperature of 135-140 K at an altitude of approximately 600 km and pressure of ∼0.1 μbar. The mesopause is at a higher pressure than predicted by earlier calculations because non-LTE radiative transfer in the rotation-vibration bands of CH4, C2H2, and C2H6 is treated in an accurate manner. The accuracy of the LTE approximation for source functions and heating rates is discussed. It found that C2H6 acts as a heat source near the mesopause by absorbing radiation from the warm stratosphere; consequently, the temperature at the mesopause depends sensitively on the C2H6 abundance. Because of the strong coupling between photochemistry and thermal structure the agreement between calculated and observed temperatures lend support to the photochemical model of Yung et al.
• Yelle, R. V., Lunine, J. I., & Hunten, D. M. (1991). Energy balance and plume dynamics in Triton's lower atmosphere. Icarus, 89(2), 347-358.
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  Abstract: Consideration of the roles of thermal conduction, eddy mixing, condensation, and radiative heating in the thermal balance of Triton's lower atmosphere results in the conclusion that the temperature gradient is negative in the lower atmosphere but becomes positive at higher altitudes. The negative temperature gradient is caused by eddy mixing, which drives the atmosphere toward the dry adiabat. The positive gradient at higher altitudes is a result of the downward conduction of heat produced in the ionosphere. The low concentrations of thermally active molecules and the small aerosol optical depths imply that radiative processes have a negligible effect on the thermal structure. We show that this temperature profile is reasonably consistent with the data from the radio-occultation experiment. Based on the height of the geyser-like plumes seen by Voyager we suggest that the convective and conductive regions of the atmosphere join at a tropopause near 10 km. We suggest that the eddy diffusion and heat-transport coefficients are about 106 cm2/s below 8 km, dropping to about 300 cm2/sec just above, for a profile that resembles the Earth's. Rather modest geyser action in the subliming nitrogen ice cap triggers moist convective plumes which must have diameters of at least 1 km and may have velocities up to 100 m/sec; they stop within about 1 km of the tropopause. © 1991.
• Majeed, T., McConnell, J. C., & Yelle, R. V. (1990). Vibrationally excited H₂ in the upper atmosphere of Saturn. Advances in Space Research, 10(1), 131-134.
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  Abstract: We have considered the impact of resonance fluorescense of solar EUV radiation by H2 on the distribution of the vibrational levels of H2 in the upper atmosphere of Saturn. This source has not been considered to date. It appears that, for v ≥ 3, this is the most important source, more important than those due to photoelectron induced fluorescence, recombination of molecular ions such as H3+, and vibrational excitation of H2 by photoelectron impact. Based on the Voyager limb observations of H2 band emission we estimate that some of the higher vibrational levels may have effective temperatures ∼ 3500 K. Such high vibrational densities may have an impact on ionospheric densities. © 1989.
• Broadfoot, A. L., Atreya, S. K., Bertaux, J. L., Blamont, J. E., Dessler, A. J., Donahue, T. M., Forrester, W. T., Hall, D. T., Herbert, F., Holberg, J. B., Hunten, D. M., Krasnopolsky, V. A., Linick, S., Lunine, J. I., McConnell, J. C., Moos, H. W., Sandel, B. R., Schneider, N. M., Shemansky, D. E., , Smith, G. R., et al. (1989). Ultraviolet spectrometer observations of Neptune and Triton. Science, 246(4936), 1459-1466.
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  Abstract: Results from the occultation of the sun by Neptune imply a temperature of 750±150 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane, acetylene, and ethane at lower levels. The ultraviolet spectrum of the sunlit atmosphere of Neptune resembles the spectra of the Jupiter, Saturn, and Uranus atmospheres in that it is dominated by the emissions of H Lyman α (340±20 rayleighs) and molecular hydrogen. The extreme ultraviolet emissions in the range from 800 to 1100 angstroms at the four planets visited by Voyager scale approximately as the inverse square of their heliocentric distances. Weak auroral emissions have been tentatively identified on the night side of Neptune. Airglow and occultation observations of Triton's atmosphere show that it is composed mainly of molecular nitrogen, with a trace of methane near the surface. The temperature of Triton's upper atmosphere is 95±5 kelvins, and the surface pressure is roughly 14 microbars.
• Yelle, R. V., & Lunine, J. I. (1989). Evidence for a molecule heavier than methane in the atmosphere of Pluto. Nature, 339(6222), 288-290.
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  Abstract: THE recent occultation of a 12th magnitude star by Pluto provided a unique opportunity for studying its atmosphere. Analyses of measurements made at the Hobart observatory in Australia and the Kuiper Airborne Observatory (KAO) have been published1,2. It is generally agreed that Pluto possesses a substantial atmosphere, with a surface pressure of ∼l0bar. Both occultation measurements are sensitive primarily to the atmospheric conditions at the 1-μbar level. Analysis of the Hobart data reveals an atmospheric scale height of 46-57 km at a radial distance of 1,240-1,290 km, whereas the scale height derived from the KAO data is 59.7 ± 1.5 km at 1,214 ± 20 km. The existence of an optically thick dust layer along the line of sight at the limb has been inferred from the KAO measurements, raising doubts about the true surface radius of Pluto2. The measured scale heights are consistent with a purely methane atmosphere at a temperature of 50-61 K for the Hobart data1 and 67±6K for the KAO data2. These values are close to the surface temperature3. Here we examine the energy balance in the atmosphere and conclude that the temperature near 1 μ bar is ∼100K rather than the surface temperature; consequently, the mean molecular weight of the atmosphere is close to 25a.m.u., and a molecule heavier than (and in addition to) methane must be present in the atmosphere. © 1989 Nature Publishing Group.
• Yelle, R. V., McConnell, J. C., Strobel, D. F., & Doose, L. R. (1989). The far ultraviolet reflection spectrum of Uranus: Results from the Voyager encounter. Icarus, 77(2), 439-456.
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  Abstract: The spectrum of Uranus in the 1250- to 1700-Å region, as measured by the Voyager ultraviolet spectrometer (UVS), is analyzed as primarily solar reflected light from an H2 Rayleigh and Raman scattering atmosphere with small but measurable absorption by hydrocarbons. CH4 and C2H2 are expected to be the primary absorbers in the 1250- to 1700-Å region. The UVS spectra definitely show a C2H2 absorption signature and the effects of CH4 are evident as well. By comparison of the observed subsolar spectra to synthetic spectra based on Rayleigh-Raman scattering in some simple three-layer model atmospheres we infer a CH4 mixing ratio of ∼1-3 × 10-7 and C2H2 mixing ratio of ∼0.6-1.2 × 10-8 above 3 mbar. Between 3 and 5 mbar these respective mixing ratios are ∼3 × 10-7and 0.6-2 × 10-8. Previously, the UVS spectra in the 1250- to 1700-Å region were interpreted as evidence for excitation of H2 by very low energy electrons (A. L. Broadfoot et al. 1986, Science 233, 74), but it is possible to explain the observations without any severe anomalies in the H2 emission spectrum. The hydrocarbon abundances determined in our analysis are far below the abundances at comparable levels in the atmospheres of Jupiter or Saturn. We suggest that, in a 1-D view, this is due to a combination of diffusive separation and photochemical depletion caused by a very low eddy diffusion coefficient, on the order of 100 cm2 sec-1 or less. For these low values of the eddy coefficient the hydrocarbon mixing ratios should decrease rapidly with height; however, analysis of the UVS solar occultation experiment by F. Herbert, B. R. Sandel, R. V. Yelle, J. B. Holberg, A. L. Broadfoot, D. E. Shemansky, S. K. Atreya, and P. N. Romani (1987, J. Geophys. Res. 92, 15093) which occurred near the terminator, suggest a C2H2 mixing ratio of ∼10-8 near 100 μbar. In addition, the UV albedo in the 1338- to 1523-Å range is enhanced over the pole relative to low-latitude regions. Therefore, all available evidence suggests strong latitudinal variations in the hydrocarbon abundances, with substantial depletions in the subsolar, polar stratosphere. We discuss the possibility that the meridional circulation of the Uranian stratosphere inferred by F. M. Flasar, B. J. Conrath, P. J. Gierash, and J. A. Pirraglia (1987, J. Geophys. Res. 92, 15011) could cause the inferred latitudinal variations. © 1989.
• Broadfoot, A. L., Holberg, J. B., Sandel, B. R., Shemansky, D. E., & Yelle, R. V. (1987). Ultraviolet spectrometer observations of Uranus. Advances in Space Research, 7(12), 259-263.
• Yelle, R. V., Doose, L. R., & Tomasko, M. G. (1987). ANALYSIS OF RAMAN SCATTERED LY- alpha EMISSIONS FROM THE ATMOSPHERE OF URANUS.. Geophysical Research Letters, 14(5), 483-486.
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  Abstract: A line at 1280 Angstrom, due to Raman scattering of solar Lyman alpha (Ly- alpha ) in the atmosphere of Uranus, has been detected by the Voyager Ultraviolet Spectrometer. The measured intensity of 40 plus or minus 20 R implies that 200 R to 500 R of the measured 1500 R Ly- alpha intensity at the sub-solar point is due to Rayleigh scattering of the solar line. The presence of Rayleigh and Raman scattering at 1216 Angstrom suggests that the Uranian atmosphere is largely devoid of absorbing hydrocarbons above the 0. 5 mbar level. The most natural explanation of this depletion is very weak vertical mixing equivalent to an eddy coefficient on the order of 200 cm**2 s** minus **1 between 0. 5 mbar and 100 mbar.
• Broadfoot, A. L., Herbert, F., Holberg, J. B., Hunten, D. M., Kumar, S., Sandel, B. R., Shemansky, D. E., Smith, G. R., Yelle, R. V., Strobel, D. F., Moos, H. W., Donahue, T. M., Atreya, S. K., Bertaux, J. L., & Blamont, J. E. (1986). ULTRAVIOLET SPECTROMETER OBSERVATIONS OF URANUS.. Science, 233(4759), 74-79.
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  Abstract: Data from solar and stellar occulations of Uranus indicate a temperature of about 750 kelvins in the upper levels of the atmosphere (composed mostly of atomic and molecular hydrogen) and define the distributions of methane and acetylene in the lower levels. The ultraviolet spectrum of the sunlit hemisphere is dominated by emissions from atomic and molecular hydrogen, which are known as electroglow emissions. The high temperature of the atmosphere, the small size of Uranus, and the number density of hydrogen atoms in the thermosphere imply an extensive thermal hydrogen corona that reduces the orbital lifetime of ring particles and biases the size distribution toward larger particles. An initial estimate of the acetylene volume mixing ratios, as judged from measurements of the far ultraviolet albedo, is about 2 multiplied by 10** minus **7 at a vertical column abundance of molecular hydrogen of 10**2**3 per sq. cm.
• Yelle, R. V., & Sandel, B. R. (1986). URANIAN H LY- alpha EMISSION: THE INTERSTELLAR WIND SOURCE.. Geophysical Research Letters, 13(2), 89-92.
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  Abstract: IUE observation of Uranian emissions in hydrogen Lyman alpha (H Ly- alpha ) is considered. The purpose of this paper is to call attention to an important source not yet considered in these analyses, namely, the reflection of H Ly- alpha emissions from the interstellar wind (ISW). The calculations presented below demonstrate that the ISW source exceeds the solar source on the sunlit atmosphere of Uranus and in fact may comprise a substantial fraction of the observed intensity.
• Roesler, F. L., Scherb, F., Magee, K., Harlander, J., Reynolds, R. J., Yelle, R. V., Broadfoot, A. L., & Oliversen, R. J. (1985). High spectral resolution line profiles and images of comet Halley. Advances in Space Research, 5(12), 279-282.
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  Abstract: High-spectral-resolution line profiles and images of comet Halley were obtained in 1986 at the National Solar Observatory McMath telescope, using a dual-etalon Fabry-Perot spectrometer. The spectrometer was designed to obtain data in four distinct modes: (1) high-resolution (R = 200,000) scanning, (2) high-resolution imaging, (3) moderate resolution (R = 30,000) scanning, and (4) moderate resolution imaging. This paper describes the instrument and some examples of data obtained in the high-resolution scanning mode. © 1985.
• Budny, R., Cavallo, A., Cohen, S., Daughney, C., Efthimion, P., Fonck, R., Hulse, R., Hwang, D., Manos, D., Pecquet, A. L., Ruzic, D., Schivell, J., Smith, B., & Yelle, R. (1982). PERTURBATION OF TOKAMAK EDGE PLASMA BY LASER BLOW-OFF IMPURITY INJECTION.. Journal of Vacuum Science and Technology A: Vacuum, Surfaces and Films, 1(2 pt 2), 837-840.
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  Abstract: Measurements are described of tokamak edge plasma perturbations caused by rapid injection of impurities such as Sc, Fe, and Mo into Ohmically heated PLT discharges. The temporal evolution of the radiated power, convected power, charge-exchange neutral flux, electron temperature, density, and floating potentials were monitored using recently developed fast bolometers, directional calorimeters, and other diagnostics. The central radiated power can be doubled without plasma disruptions. Radiation from the edge then increases tenfold for approximately 2 ms. Neutral efflux at the limiter decreases up to two orders of magnitude for approximately 20 ms after injection. The electrical potential of the limiters increases, approaching the potential of the vacuum vessel. MHD activity in the m equals 1 (sawtooth) mode tends to increase while that in the m equals 2 and m equals 3 modes decreases. The power flowing along field lines in the limiter shadow region sometimes increases or sometimes decreases by more than a factor of 4. The density and electron temperature in the limiter shadow region, generally, does not change more than 50%.
• Fonck, R. J., Bell, M., Bol, K., Brau, K., Budny, R., Cecchi, J. L., Cohen, S., Davis, S., Dylla, H. F., Goldston, R., Grek, B., Hawryluk, R. J., Hirschberg, J., Johnson, D., Hülse, R., Kaita, R., Kaye, S., Knize, R. J., Kugel, H., , Manos, D., et al. (1982). Impurity levels and power loading in the pdx tokamak with high power neutral beam injection. Journal of Nuclear Materials, 111-112(C), 343-354.
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  Abstract: The PDX tokamak provides an experimental facility for the direct comparison of various impurity control techniques under reactor-like conditions. Four neutral beam lines inject > 6 MW for 300 ms. Carbon rail limiter discharges have been used to test the effectiveness of perpendicular injection, but non-disruptive full power operation for > 100 ms is difficult without extensive conditioning. Initial tests of a toroidal bumper limiter indicate reduced power loading and roughly similar impurity levels compared to the carbon rail limiter discharges. Poloidal divertor discharges with up to 5 MW of injected power are cleaner than similar circular discharges, and the power is deposited in a remote divertor chamber. High density divertor operation indicates a reduction of impurity flow velocity in the divertor and enhanced recycling in the divertor region during neutral injection. © 1982.
• Fonck, R. J., Ramsey, A. T., & Yelle, R. V. (1982). MULTICHANNEL GRAZING-INCIDENCE SPECTROMETER FOR PLASMA IMPURITY DIAGNOSIS - SPRED.. Applied Optics, 21(12), 2115-2123.
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  PMID: 20395992;Abstract: A compact vacuum ultraviolet spectrometer system was developed to provide time-resolved impurity spectra from tokmak plasmas. Two interchangeable aberration-corrected toroidal diffraction gratings with flat focal fields provide simultaneous coverage over the ranges 100-1100 A or 160-1700 A. The detector is an intensified self-scanning photodiode array. Spectral resolution is 2 A with the higher dispersion grating. Minimum readout time for a full spectrum is 20 msec, but up to seven individual spectral lines can be measured with a 1-msec time resolution.

Proceedings Publications

• Cangi, E., Chaffin, M., Yelle, R., Gregory, B., Mayyasi, M., Clarke, J., & Deighan, J. (2023, oct). Seasonal variation in atomic D/H of the upper Mars atmosphere is a tracer for lower atmospheric dynamics. In AAS/Division for Planetary Sciences Meeting Abstracts, 55.
• Gupta, S., Yelle, R., Schneider, N., Jain, S., Verdier, L., Braude, A., Montmessin, F., Mayyasi, M., Deighan, J., & Curry, S. (2023, oct). Day-Night Differences in the Martian Upper Atmospheric Molecular Oxygen. In AAS/Division for Planetary Sciences Meeting Abstracts, 55.
• Hanley, K., McFadden, J., Mitchell, D., Fowler, C., Stone, S., Mayyasi, M., Yelle, R., Benna, M., Elrod, M., Ergun, R., Andersson, L., Espley, J., & Curry, S. (2023, oct). In situ observations of ion temperatures and suprathermal ions in the Martian ionosphere. In AAS/Division for Planetary Sciences Meeting Abstracts, 55.
• Jain, S., Deighan, J., Chaffin, M., Holsclaw, G., Lillis, R., Fillingim, M., England, S., Al, M. H., Lootah, F., Yelle, R., Gupta, S., Schneider, N., & Al, M. H. (2023, may). The first EMM/EMUS stellar occultation measurements of the Martian atmosphere in both extreme and far ultraviolet wavelengths. In EGU General Assembly Conference Abstracts.
• Stephenson, P., Koskinen, T., Brown, Z., Quemerais, E., Lavvas, P., Moses, J., Sandel, B., & Yelle, R. (2023, oct). Seasonal variation of Saturn's Lyman alpha airglow and upper atmospheric hydrogen. In AAS/Division for Planetary Sciences Meeting Abstracts, 55.
• Stone, S., Benna, M., Tolson, R., Yelle, R., Lugo, R., Bougher, S., Elrod, M., & Zurek, R. (2023, oct). Synergistic Science from the MAVEN Accelerometer and Mass Spectrometer. In AAS/Division for Planetary Sciences Meeting Abstracts, 55.
• Braude, A., Montmessin, F., Schneider, N., Gupta, S., Jain, S., Lef{\`evre}, F., Verdier, L., Flimon, Z., Jiang, F., Yelle, R., & Deighan, J. (2022, dec). Seasonal and longitudinal variations in ozone and aerosol vertical distribution on Mars from MAVEN/IUVS. In AAS/Division for Planetary Sciences Meeting Abstracts, 54.
• Braude, A., Montmessin, F., Verdier, L., Flimon, Z., Lefevre, F., Gupta, S., Jain, S., Schneider, N., Deighan, J., Jiang, F., & Yelle, R. (2022, jun). Monitoring Ozone and Aerosol in the Martian Mesosphere from MAVEN/IUVS Stellar Occultation Observations between MY 32 and 36. In Seventh International Workshop on the Mars Atmosphere: Modelling and Observations.
• Cangi, E., Chaffin, M., Deighan, J., Gregory, B., & Yelle, R. (2022, jun). Fully-Coupled Photochemical Modeling of the Deuterated Ionosphere and Non-Thermal Escape of D. In Seventh International Workshop on the Mars Atmosphere: Modelling and Observations.
• Gupta, S., Schneider, N., Jain, S., Deighan, J., Yelle, R., Jiang, F., Verdier, L., Braude, A., & Montmessin, F. (2022, jun). Diurnal Variations in the Martian Atmosphere from Enhanced MAVEN/IUVS Stellar Occultation Dataset. In Seventh International Workshop on the Mars Atmosphere: Modelling and Observations.
• Stone, S., Villanueva, G., Liuzzi, G., Benna, M., Mahaffy, P., Elrod, M., & Yelle, R. (2022, jun). Transport and Escape of Water: Toward a More Complete Interpretation with TGO NOMAD and MAVEN NGIMS. In Seventh International Workshop on the Mars Atmosphere: Modelling and Observations.
• Stone, S., Villanueva, G., Yelle, R., Benna, M., Liuzzi, G., Elrod, M., & Mahaffy, P. (2022, dec). Isotope Ratios in the Martian Upper Atmosphere Measured by MAVEN NGIMS. In AAS/Division for Planetary Sciences Meeting Abstracts, 54.
• Stone, S., Villanueva, G., Yelle, R., Benna, M., Liuzzi, G., Elrod, M., & Mahaffy, P. (2022, sep). Isotope Ratios in the Martian Upper Atmosphere Measured by MAVEN NGIMS. In European Planetary Science Congress.
• Deighan, J., Crismani, M., Matta, M., Stevens, M., Mcclintock, W., Clarke, J., Yelle, R., Montmessin, F., Lefevre, F., Jakosky, B., Holsclaw, G., Schneider, N., Chaffin, M., Evans, J. S., Jain, S., Lo, D., Milby, Z., Stewart, A., & Hughes, A. (2021, jan). Mars in the Ultraviolet: Highlights from (nearly) Six Years of Observations by the MAVEN Imaging UltraViolet Spectrograph. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• Hanley, K., McFadden, J., Mitchell, D., Fowler, C., Stone, S., Yelle, R., Mayyasi, M., Ergun, R., Andersson, L., Benna, M., Elrod, M., & Curry, S. (2021, oct). Unexpected Enhancement of Ion Temperatures in Mars' Lower Ionosphere. In AAS/Division for Planetary Sciences Meeting Abstracts, 53.
• Lillis, R., Bougher, S., Combi, M., Cravens, T., Fox, J. L., Yelle, R., Deighan, J., Rahmati, A., Lee, Y., Gacesa, M., & Lo, D. (2021, jan). Photochemical escape from Mars as constrained by MAVEN in situ measurements and recent modeling. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• Mahieux, A., Piccialli, A., Robert, S., Vandaele, A. C., Yelle, R., Chamberlain, S., Trompet, L., & Aoki, S. (2021, jan). Water and its isotopologues mesospheric distributions at the Venus terminator as measured by SOIR/VEx. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• Brain, D., Chaffin, M., Curry, S., Egan, H., Ramstad, R., Jakosky, B., Luhmann, J., Dong, C., & Yelle, R. (2020, feb). Atmospheric Escape from Mars: Lessons for Studies of Exoplanets. In Exoplanets in Our Backyard: Solar System and Exoplanet Synergies on Planetary Formation, Evolution, and Habitability, 2195.
• Serigano, J., Horst, S., He, C., Gautier, T., Yelle, R., & Koskinen, T. (2020, feb). Investigating the Interactions Between Saturn's Upper Atmosphere and Rings from Cassini INMS Measurements. In Exoplanets in Our Backyard: Solar System and Exoplanet Synergies on Planetary Formation, Evolution, and Habitability, 2195.
• Serigano, J., Horst, S., He, C., Gautier, T., Yelle, R., Koskinen, T., & Trainer, M. (2020, oct). Compositional Measurements of Saturn's Upper Atmosphere and Rings from Cassini INMS. In AAS/Division for Planetary Sciences Meeting Abstracts, 52.
• Chaufray, J., Gonzalez-Galindo, F., Lopez-Valverde, M., Forget, F., Qu{\'emerais}, E., Bertaux, J., Montmessin, F., Chaffin, M., Schneider, N., Clarke, J., Leblanc, F., Modolo, R., & Yelle, R. (2019, Sep). Study of the hydrogen escape rate at Mars during Martian years 28 and 29 from comparisons between SPICAM/Mars Express observations and GCM-LMD simulations. In EPSC-DPS Joint Meeting 2019, 2019.
• Clarke, J., Mayyasi, M., Bhattacharyya, D., Schneider, N., Chaufray, J., Bertaux, J., Chaffin, M., Jakosky, B., Deighan, J., Jain, S., McClintock, B., & Yelle, R. (2019, Sep). The D/H Ratio in the Martian Upper Atmosphere. In EPSC-DPS Joint Meeting 2019, 2019.
• Lillis, R., Lo, D., Deighan, J., Fox, J., Yelle, R., Elrod, M., Lee, Y., Benna, M., & Jakosky, B. (2019, Mar). Photochemical Escape of Carbon from Mars: Greater Than Previously Thought?. In Lunar and Planetary Science Conference.
• Palmer, M., Yelle, R., & Koskinen, T. (2019, Sep). Latitudinal variations in Titan's atmosphere: UVIS observations of three simultaneous stellar occultations. In EPSC-DPS Joint Meeting 2019, 2019.
• Serigano, J., Horst, S., Yelle, R., Koskinen, T., & He, C. (2019, Sep). Investigating the Interactions between Saturn's Upper Atmosphere and Rings from Cassini INMS Measurements. In EPSC-DPS Joint Meeting 2019, 2019.
• Siddle, A., M{\"uller-Wodarg}, I., Bruinsma, S., Marty, J., Svedhem, H., Yelle, R., & Stone, S. (2019, Sep). Thermosphere structure and variability as inferred from the ExoMars Trace Gas Orbiter aerobraking campaign and in-situ MAVEN NGIMS observations. In EPSC-DPS Joint Meeting 2019, 2019.
• Stone, S., Yelle, R., Benna, M., Elrod, M., & Mahaffy, P. (2019, Sep). Transport of Water to the Martian Upper Atmosphere amid Regional and Global Dust Storms. In EPSC-DPS Joint Meeting 2019, 2019.
• Vriesema, J., Koskinen, T., Yelle, R., & M{\"uller-Wodarg}, I. (2019, Sep). Results from an Improved Model of Electrodynamics in Saturn's Upper Atmosphere. In EPSC-DPS Joint Meeting 2019, 2019.
• Yoshida, N., Nakagawa, H., Terada, N., Schneider, N., Evans, S., Jain, S., Fujiwara, H., Imamura, T., Deighan, J., Stewart, I., Stevens, M., & Yelle, R. (2019, Apr). Seasonal variations of N2/CO2 at 140 km altitude derived from MAVEN/IUVS. In EGU General Assembly Conference Abstracts.
• Chaffin, M., Crismani, M., McClintock, W., Stevens, M., Stewart, I., Clarke, J., Schneider, N., Yelle, R., Montmessin, F., Lefevre, F., Jakosky, B., Gr{\"oller}, H., Holsclaw, G., Mayyasi, M., Chaufray, J., Deighan, J., Stiepen, A., Evans, J., Lo, D., & Jain, S. (2018, jul). Highlights from Imaging Ultraviolet Spectroscopy of the Mars Atmosphere with MAVEN/IUVS. In 42nd COSPAR Scientific Assembly, 42.
• Chaffin, M., Schneider, N., Deighan, J., Jain, S., Stiepen, A., Lefevre, F., Groller, H., Crismani, M., Stewart, A., Clarke, J., Mayyasi, M., Evans, J., Stevens, M., Chaufray, J., McClintock, W., Holsclaw, G., Montmessin, F., Yelle, R., Lo, D., & Jakosky, B. (2018, feb). MAVEN IUVS Remote Sensing Highlights Relevant to Upcoming TGO Observations. In From Mars Express to ExoMars.
• Chaufray, J., Gonzalez-Galindo, F. .., Lopez-Valverde, M., Forget, F., Chaffin, M., Bertaux, J., Qu{\'emerais}, E., Clarke, J., Leblanc, F., Montmessin, F., Yelle, R., & Schneider, N. (2018, feb). Hydrogen escape at Mars : Comparisons from MGCM-LMD simulations and observations from Mars-Express/SPICAM. In From Mars Express to ExoMars.
• Crismani, M., Schneider, N., Plane, J., Evans, S., Jain, S., Deighan, J., & Yelle, R. (2018, apr). Martian Metallic Ions Deposited by Comet Siding Spring Defy Expectations. In EGU General Assembly Conference Abstracts, 20.
• Lo, D., Lillis, R., Yelle, R., & Team, M. (2018, mar). Modeling Carbon Production and Densities in the Martian Atmosphere Under MAVEN Deep Dip 2 Conditions. In Lunar and Planetary Science Conference, 49.
• Perry, M., Waite, H., Perryman, R., Mitchell, D., Cravens, T., Moore, L., Miller, K., Yelle, R., Teolis, B., & McNutt, R. (2018, apr). The flow of material inward from Saturn's rings. In EGU General Assembly Conference Abstracts, 20.
• Perry, M., Waite, J., Perryman, R., Mitchell, D., Cravens, T., Moore, L., Miller, K., Yelle, R., Teolis, B., & McNutt, R. (2018, mar). A New Understanding of the Interaction Between Saturn and Its Rings. In Lunar and Planetary Science Conference, 49.
• Serigano}, J., Horst, S., Yelle, R., Koskinen, T., He, C., Perry, M., Cravens, T., Perryman, R., Waite, J., & Team, {. I. (2018, oct). The Composition and Thermal Structure of Saturn's Upper Atmosphere from Cassini/INMS Measurements. In AAS/Division for Planetary Sciences Meeting Abstracts \#50, 50.
• Vuitton, V., Yelle, R., Klippenstein, S., Horst, S., & Lavvas, P. (2018, jun). Modeling the Chemical Complexity in Titan's Atmosphere. In American Astronomical Society Meeting Abstracts \#232, 232.
• Chaffin, M., Schneider, N., Deighan, J., Jain, S., McClintock, B., Stewart, I., Clarke, J., Holsclaw, G., Montmessin, F., Lefevre, F., Chaufray, J., Stiepen, A., Crismani, M., Mayyasi, M., Evans, S., Stevens, M., Yelle, R., Groller, H., Lo, D., & Jakosky, B. (2017, apr). Highlights from two years of remote sensing at Mars with MAVEN's Imaging Ultraviolet Spectrograph. In EGU General Assembly Conference Abstracts, 19.
• Crismani, M., Schneider, N., Jain, S., Plane, J., Deighan, J., Evans, S., Yelle, R., & Carrillo-Sanchez, J. (2017, apr). The Metals Delivered by Comet Siding Spring to Mars. In EGU General Assembly Conference Abstracts, 19.
• Crismani, M., Schneider, N., Plane, J., Evans, J., Jain, S., Carrillo-Sanchez, J., Stevens, M., Deighan, J., Chaffin, M., Yelle, R., Stewart, A., McClintock, W., Clarke, J., Stiepen, A., Holsclaw, G., Montmessin, F., & Jakosky, B. (2017, jan). A Persistent Meteoric Metal Layer in Mars' Atmosphere. In The Mars Atmosphere: Modelling and observation.
• Crismani, M., Schneider, N., Plane, J., Jain, S., Deighan, J., Yelle, R., & Evans, J. (2017, oct). Martian Metallic Ions Deposited by Comet Siding Spring Defy Expectations. In AAS/Division for Planetary Sciences Meeting Abstracts \#49, 49.
• Deighan, J., Stevens, M., Schneider, N., Jain, S., Lef{\`evre}, F., Wolff, M., Montmessin, F., Stiepen, A., Evans, J., Chaffin, M., Crismani, M., Yelle, R., Lo, D., Stewart, A., McClintock, W., Clarke, J., Holsclaw, G., & Jakosky, B. (2017, jan). Characterization of High Altitude Clouds at the Martian Limb and Terminator Using MAVEN IUVS Observations. In The Mars Atmosphere: Modelling and observation.
• Erwin, J., Yelle, R., & Koskinen, T. (2017, apr). Escape of Hydrogen from HD209458b. In EGU General Assembly Conference Abstracts, 19.
• Gr{\"oller}, H., Yelle, R., Koskinen, T., Montmessin, F., Lacombe, G., Jain, S., Deighan, J., Schneider, N., Nakagawa, H., & Medvedev, A. (2017, jan). Temperature Profiles and Wave Structures Observed with IUVS/MAVEN Stellar Occultations. In The Mars Atmosphere: Modelling and observation.
• Lef{\`evre}, F., Montmessin, F., Schneider, N., Deighan, J., Jain, S., Stewart, A., Chaffin, M., Crismani, M., McClintock, W., Holsclaw, G., Jakosky, B., Stiepen, A., Lo, D., Yelle, R., & Clarke, J. (2017, jan). Mars Ozone mapping with MAVEN IUVS. In The Mars Atmosphere: Modelling and observation.
• Nixon, C., Achterberg, R., Buch, A., Clark, R., Coll, P., Flasar, F., Hayes, A., Iess, L., Lorenz, R., Lopes, R., Mastroguiseppe, M., Raulin, F., Smith, T., Solomidou, A., Sotin, C., Strobel, D., Turtle, E., Vuitton, V., West, R., & Yelle, R. (2017, feb). Riddles of the Sphinx: Titan Science Questions at the End of Cassini-Huygens. In Planetary Science Vision 2050 Workshop, 1989.
• Schneider, N., Connour, K., Forget, F., Deighan, J., Jain, S., Vals, M., Wolff, M., Chaffin, M., Crismani, M., Stewart, A., McClintock, W., Holsclaw, G., Lefevre, F., Montmessin, F., Stiepen, A., Stevens, M., Evans, J., Yelle, R., Lo, D., , Clarke, J., et al. (2017, oct). Mars topographic clouds: MAVEN/IUVS observations and LMD MGCM predictions. In AAS/Division for Planetary Sciences Meeting Abstracts \#49, 49.
• Schneider, N., Deighan, J., Jain, S., Derks, A., Chaffin, M., Crismani, M., Stewart, A., McClintock, W., Holsclaw, G., Jakosky, B., Lef{\`evre}, F., Montmessin, F., Wolff, M., Stiepen, A., Stevens, M., Evans, J., Yelle, R., Lo, D., & Clarke, J. (2017, jan). Global UV Imaging by MAVEN/IUVS: Diurnal Cloud Formation, Dust Storms and Atmospheric Scattering. In The Mars Atmosphere: Modelling and observation.
• Slipski, M., Jakosky, B., Benna, M., Mahaffy, P., Elrod, M., Yelle, R., Stone, S., & Alsaeed, N. (2017, jan). Total Atmospheric Loss from Upper-Atmospheric Structure of \^}$\{$36$\}$Ar/\^{$\{$38$\}$Ar Observed by MAVEN. In The Mars Atmosphere: Modelling and observation.
• Sotin, C., Hayes, A., Malaska, M., Nimmo, F., Trainer, M., Mastrogiuseppe, M., Soderblom, J., Tortora, P., Hofgartner, J., Aharonson, O., Barnes, J., Hodyss, R., Iess, L., Kirk, R., Lavvas, P., Lorenz, R., Lunine, J., Mazarico, E., McEwen, A., , Neish, C., et al. (2017, mar). Oceanus: A New Frontiers Orbiter to Study Titan's Potential Habitability. In Lunar and Planetary Science Conference, 48.
• Vriesema, J., Koskinen, T., & Yelle, R. (2017, oct). Resistive Heating and Ion Drag in Saturn's Thermosphere. In AAS/Division for Planetary Sciences Meeting Abstracts \#49, 49.
• Yelle, R., Gr{\"oller}, H., Koskinen, T., Montmessin, F., Lacombe, G., Lef{\`evre}, F., Jain, S., Deighan, J., & Schneider, N. (2017, jan). Atmospheric Abundances Retrieved from IUVS/MAVEN Stellar Occultations. In The Mars Atmosphere: Modelling and observation.
• Yelle, R., Koskinen, T., & Lavvas, P. (2017, oct). Non-LTE Models for the Thermal Structure of Hot Jupiters. In AAS/Division for Planetary Sciences Meeting Abstracts \#49, 49.
• Esman}, T., {Yelle}, R., {Stone}, S., {Andersson}, L., {Fowler}, C., {Benna}, M., {Eparvier}, F., {Mahaffy}, P., {Ergun}, B., {Elrod}, M.,
• Stone}, S., {Yelle}, R., {Mahaffy}, P., {Benna}, M., {Elrod}, M., {Bougher}, S.,
• Yelle, R. V. (2016, jun). Titan Aeromony and Climate Workshop. In Titan Aeronomy and Climate.
• {Benna}, M., {Yelle}, R., {Grebowsky}, J., {Fox}, J., , P. (2016, jul). Structure of the martian ionosphere as revealed by the Neutral Gas and Ion Mass Spectrometer during the first two years of the MAVEN mission. In 41st COSPAR Scientific Assembly, 41.
• {Chaffin}, M., {Schneider}, N., {McClintock}, B., {Stewart}, I., {Deighan}, J., {Jain}, S., {Clarke}, J., {Holsclaw}, G., {Montmessin}, F., {Lefevre}, F., {Chaufray}, J., {Stiepen}, A., {Crismani}, M., {Mayyasi}, M., {Evans}, S., {Stevens}, M., {Yelle}, R., , B. (2016, apr). MAVEN Imaging UV Spectrograph Results on the Mars Atmosphere and Atmospheric Escape. In EGU General Assembly Conference Abstracts, 18.
• {Chaufray}, J., {Forget}, F., {Modolo}, M., {Leblanc}, F., {Lopez-Valverde}, M., {Clarke}, J., {Chaffin}, M., {Schneider}, N., {Yelle}, R., {Gonzalez-Galindo}, F., {Jakosky}, B., {Deighan}, J., , R. (2016, jul). Modeling of the Martian exosphere and first comparisons to IUVS/MAVEN observations. In 41st COSPAR Scientific Assembly, 41.
• {Elrod}, M., {Bougher}, S., {Benna}, M., {Yelle}, R., {Jakosky}, B., {Bell}, J., {Mahaffy}, P., , S. (2016, jul). He Bulge Detection by MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS) in the Upper Atmosphere of Mars. In 41st COSPAR Scientific Assembly, 41.
• {Gr{\"o}ller}, H., {Yelle}, R., {Koskinen}, T., {Montmessin}, F., {Lacombe}, G., {Kass}, D., {Kleinb{\"o}hl}, A., {Schofield}, T., {McCleese}, D., {Schneider}, N., {Deighan}, J., {Jain}, S., , B. (2016, mar). Martian Temperature Profiles Measured by MAVEN and MRO from 20 to 160 km. In Lunar and Planetary Science Conference, 47.
• {Gr{\"o}ller}, H., {Yelle}, R., {Montmessin}, F., {Lacombe}, G., {Schneider}, N., {Deighan}, J., {Jain}, S., {Nakagawa}, H., , B. (2016, oct). IUVS/MAVEN Stellar Occultations. In AAS/Division for Planetary Sciences Meeting Abstracts, 48.
• {Jakosky}, B., {Slipski}, M., {Benna}, M., {Mahaffy}, P., {Elrod}, M., {Yelle}, R., {Stone}, S., , N. (2016, oct). Mars Atmospheric History Derived from Upper-Atmospheric Structure of $^{38}$Ar/$^{36}$Ar Measured From MAVEN. In AAS/Division for Planetary Sciences Meeting Abstracts, 48.
• {Lo}, D., {Yelle}, R., {Schneider}, N., {Jain}, S., {Stewart}, A., {Deighan}, J., {Stiepen}, A., {Evans}, J., {Stevens}, M., {Chaffin}, M., {Crismani}, M., {McClintock}, W., {Clarke}, J., {Holsclaw}, G., {Lefevre}, F., , B. (2016, mar). Twilight Limb Observations of Aerosols in the Martian Atmosphere by MAVEN IUVS. In Lunar and Planetary Science Conference, 47.
• {Lo}, D., {Yelle}, R., {Schneider}, N., {Jain}, S., {Stewart}, I., {Deighan}, J., {Stiepen}, A., {Evans}, S., {Stevens}, M., {Chaffin}, M., {Crismani}, M., {McClintock}, W., {Clarke}, J., {Holsclaw}, G., {Lefevre}, F., , B. (2016, oct). Twilight Limb Observations of the Martian North Polar Hood by MAVEN IUVS. In AAS/Division for Planetary Sciences Meeting Abstracts, 48.
• {Vriesema}, J., {Koskinen}, T., , R. (2016, oct). Resistive Heating in Saturn's Thermosphere. In AAS/Division for Planetary Sciences Meeting Abstracts, 48.
• {Vuitton}, V., {Carrasco}, N., {Flandinet}, L., {Horst}, S., {Klippenstein}, S., {Lavvas}, P., {Orthous-Daunay}, F., {Quirico}, E., {Thissen}, R., , R. (2016, oct). Titan's Oxygen Chemistry and its Impact on Haze Formation. In AAS/Division for Planetary Sciences Meeting Abstracts, 48.
• {Vuitton}, V., {Carrasco}, N., {Flandinet}, L., {Horst}, S., {Klippenstein}, S., {Lavvas}, P., {Orthous-Daunay}, F., {Thissen}, R., , R. (2016, jun). Titan's Oxygen Chemistry and its Impact on Haze Formation. In Titan Aeronomy and Climate.
• {Yelle}, R., {Koskinen}, T., {Withers}, P., {Schinder}, P., {Moses}, J., , I. (2016, oct). The Effect of Diurnal Variations on Ionospheric Radio Occultations. In AAS/Division for Planetary Sciences Meeting Abstracts, 48.
• {Yelle}, R., {Vuitton}, V., {Lavvas}, P., {Klippenstein}, S., , S. (2016, jun). Coupled, Nitrogen, Oxygen, Carbon and Ion Chemistry on Titan. In Titan Aeronomy and Climate.
• {Cui}, J., {Galand}, M., {Yelle}, R., , Y. (2015, apr). Day-to-night transport in the Martian ionosphere. In EGU General Assembly Conference Abstracts, 17.
• {Deighan}, J., {Jain}, S., {Lo}, D., {Stewart}, A., {Schneider}, N., {Stiepen}, A., {Evans}, J., {Stevens}, M., {Yelle}, R., {England}, S., {Chaffin}, M., {Crismani}, M., {McClintock}, W., {Clarke}, J., {Holsclaw}, G., {Lefevre}, F., {Montmessin}, F., {Thiemann}, E., {Eparvier}, F., , B. (2015, nov). Structure and variability of the Martian upper atmosphere: Ultraviolet dayglow observations by MAVEN/IUVS. In AAS/Division for Planetary Sciences Meeting Abstracts, 47.
• {Elrod}, M., {Mahaffy}, P., {Yelle}, R., {Stone}, S., {Benna}, M., , B. (2015, nov). He Bulge Detection by MAVEN Neutral Gas and Ion Mass Spectrometer (NGIMS) in the Upper Atmosphere of Mars. In AAS/Division for Planetary Sciences Meeting Abstracts, 47.
• {Erwin}, J., {Koskinen}, T., , R. (2015, nov). Radiative equilibrium and escape of Pluto's atmosphere. In AAS/Division for Planetary Sciences Meeting Abstracts, 47.
• {Koskinen}, T., {Erwin}, J., , R. (2015, nov). Predictions for the escape of CH$_{4}$ from Pluto. In AAS/Division for Planetary Sciences Meeting Abstracts, 47.
• {Mahaffy}, P., {Benna}, M., {Elrod}, M., {Bougher}, S., {Yelle}, R., , B. (2015, mar). Early Composition, Structure, and Isotope Measurements in the Upper Atmosphere of Mars from MAVEN's Neutral Gas and Ion Mass Spectrometer. In Lunar and Planetary Science Conference, 46.
• {Mahieux}, A., {Erwin}, J., {Chamberlain}, S., {Robert}, S., {Carine Vandaele}, A., {Wilquet}, V., {Thomas}, I., {Yelle}, R., , J. (2015, apr). A 1-D radiative conductive model to study the SOIR/VEx thermal profiles. In EGU General Assembly Conference Abstracts, 17.
• {Montmessin}, F., {Groeller}, H., {Lacombe}, G., {Schneider}, N., {Yelle}, R., {Stewart}, I., {Deighan}, J., {Clarke}, J., {Lefevre}, F., {Baggio}, L., {McClintock}, W., {Holsclaw}, G., , B. (2015, nov). Combined analysis of Far UV and Mid UV spectra obtained by the MAVEN IUVS instrument in a Stellar Occultation Mode. In AAS/Division for Planetary Sciences Meeting Abstracts, 47.
• {Montmessin}, F., {Schneider}, N., {Stewart}, A., {Deighan}, J., {Yelle}, R., {Lefevre}, F., {McClintock}, W., {Clarke}, J., {Holsclaw}, G., , B. (2015, mar). MAVEN IUVS in Stellar Occultation Mode: A First Look at Martian Atmospheric Density and Temperature Profiles. In Lunar and Planetary Science Conference, 46.
• {Schneider}, N., {Stewart}, A., {McClintock}, W., {Mahaffy}, P., {Benna}, M., {Deighan}, J., {Jain}, S., {Stiepen}, A., {Elrod}, M., {Chaffin}, M., {Crismani}, M., {Plane}, J., {Sanchez}, J., {Yelle}, R., {Lo}, D., {Evans}, J., {Stevens}, M., {Combi}, M., {Clarke}, J., , {Holsclaw}, G., et al. (2015, mar). MAVEN IUVS Observations of the Aftermath of Comet Siding Spring's Meteor Shower. In Lunar and Planetary Science Conference, 46.
• {Yelle}, R., {Benna}, M., {Mahaffy}, P., {Elrod}, M., {Epsley}, J., {Rahmati}, A., {Cravens}, T., {Larson}, D., , B. (2015, mar). MAVEN Observations of the Effect of Comet Siding Spring on the Mars Atmosphere. In Lunar and Planetary Science Conference, 46.
• {Clarke}, J., {Bhattacharyya}, D., {Bertaux}, J., {Chaufray}, J., {Matta}, M., {Deighan}, J., {Yelle}, R., , D. (2014, nov). HST Observations of the Martian Hydrogen and Oxygen Exosphere. In AAS/Division for Planetary Sciences Meeting Abstracts, 46.
• {Cui}, J., {Yelle}, R., {Li}, T., {Lian}, Y., {Mueller-Wodarg}, I., , D. (2014, may). Density waves in Titan's upper atmosphere. In EGU General Assembly Conference Abstracts, 16.
• {Erwin}, J., {Yelle}, R., , T. (2014, nov). Escaping hydrogen from HD209458b. In AAS/Division for Planetary Sciences Meeting Abstracts, 46.
• {Gr{\"o}ller}, H., {Sandel}, B., {Yelle}, R., {Koskinen}, T., {Lewis}, N., {Bertaux}, J., {Montmessin}, F., merais}, E. (2014, nov). O$_{2}$ abundance on Mars observed by Mars Express / SPICAM. In AAS/Division for Planetary Sciences Meeting Abstracts, 46.
• {Koskinen}, T., {Sandel}, B., , R. (2014, nov). The variability of Saturn's thermosphere from Cassini/UVIS occultations. In AAS/Division for Planetary Sciences Meeting Abstracts, 46.
• {Lavvas}, P., {Koskinen}, T., , R. (2014, nov). Electron densities and alkali atoms in exoplanet atmospheres. In AAS/Division for Planetary Sciences Meeting Abstracts, 46.
• {Mahieux}, A., {Erwin}, J., {Chamberlain}, S., {Funke}, B., {L{\'o}pez Puertas}, M., {L{\'o}pez Valverde}, M., {Robert}, S., {Vandaele}, A., {Wilquet}, V., {Yelle}, R., , J. (2014, nov). Thermal structure at the Venus terminator: Comparison of SOIR/Vex profiles with a radiative transfer model. In AAS/Division for Planetary Sciences Meeting Abstracts, 46.
• {Vuitton}, V., {Yelle}, R., {Klippenstein}, S., {H{\"o}rst}, S., , P. (2014, nov). A Coupled Ion-Neutral Photochemical Model for the Titan Atmosphere. In AAS/Division for Planetary Sciences Meeting Abstracts, 46.
• {Yelle}, R., {Gonzalez-Galindo}, F., {Chaufray}, J., {Forget}, F., , A. (2014, nov). Perturbation of the Mars atmosphere by Siding Spring. In AAS/Division for Planetary Sciences Meeting Abstracts, 46.
• {Yelle}, R., {Mahieux}, A., {Morrison}, S., {Vuitton}, V., rst}, S. (2014, may). Model simulation of the perturbation of the Mars atmosphere by the near-collision Comet C/2013 A1 (Siding Spring). In EGU General Assembly Conference Abstracts, 16.
• {Yelle}, R., {Mahieux}, A., {Morrisson}, S., {Vuitton}, V., , S. (2014, jul). Perturbation of the Mars Atmosphere by Comet C/2013 A1. In Eighth International Conference on Mars, 1791.

Poster Presentations

• Cui, J., Yelle, R., Li, T., Snowden, D., & Mueller-Wodarg, I. (2013, September). Density waves in Titan's upper atmosphere. European Planetary Science Congress. London, UK.
• Hörst, S., Klippenstein, S., Lavvas, P., Vuitton, V., & Yelle, R. (2013, September). Titan's Oxygen Chemistry: An UpDates. European Planetary Science Congress. London, UK.
• Koskinen, T., Sandel, B., Yelle, R., Capalbo, F., Holsclaw, G., McClintock, B., & Edgington, S. (2013, Fall). The thermosphere of Saturn from Cassini UVIS occultations. American Astronomical Society, DPS meeting #45.
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  #512.03
• Lavvas, P., Campbell, L., Yelle, R., Galand, M., & Brunger, M. (2013, September). N2 states population and airglow in Titan's atmosphere. European Planetary Science Congress. London, UK.
• Lavvas, P., Koskine, T., & Yelle, R. (2013, September). Chemical evolution of hot-Jupiters at different orbital distances. European Planetary Science Congress. London, UK.
• Lavvas, P., Koskinen, T., & Yelle, R. (2013, September). Aerosol properties in exoplanet atmospheres. European Planetary Science Congress. London, UK.
• Morrison, S., & Yelle, R. (2013, Fall). Thermal Effects from Comet 2013 A1 (Siding Spring) on Mars’ Upper Atmosphere. American Astronomical Society, DPS meeting #45.
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  #313.18
• Müller-Wodarg, I., Newman, C., Yelle, R., & Cui, J. (2013, September). Vertical coupling in Titan's upper atmosphere. European Planetary Science Congress. London, UK.
• Snowden, D., & Yelle, R. (2013, Fall). Estimates of energy sources and sinks in Titan's upper atmosphere. American Geophysical Union.
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  Abstract #: P53C-1875
• Vigren, E., Galand, M., Mueller-Wodarg, I., Wellbrock, A., Coates, A., Yelle, R., Snowden, D., Cui, J., Lavvas, P., Ĺgren, K., Wahlund, J., & Vuitton, V. (2013, September). Thermal electron balance in Titan's nightside ionosphere. European Planetary Science Congress. London, UK.
• Vuitton, V., Yelle, R., Klippenstein, S., Horst, S., & Lavvas, P. (2013, Fall). A coupled ion-neutral photochemical model for the Titan atmosphere. American Geophysical Union.
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  Abstract #: P53C-1876
• Vuitton, V., Yelle, R., Klippenstein, S., Lavvas, P., Hörst, S., & Bazin, A. (2013, September). Hydrogen isocyanide, HNC, in Titan's ionosphere. European Planetary Science Congress. London, UK.
• Capalbo, F., Benilan, Y., Koskinen, T., & Yelle, R. (2012, Fall). Titan’s Higher Atmosphere Composition from EUV Solar Occultation Measurements. American Astronomical Society, DPS meeting #44.
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  #300.08
• Cui, J., Lian, Y., Mueller-Wodarg, I., & Yelle, R. (2012, Fall). Compositional Effects In Titan's Thermospheric Gravity Waves. American Astronomical Society, DPS meeting #44.
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  #312.09
• Koskinen, T., Sandel, B., Yelle, R., & Capalbo, F. (2012, Fall). Temperature And Density Structure In Saturn's Thermosphere From Cassini/uvis Solar Occultations. American Astronomical Society, DPS meeting #44.
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  #412.09
• Koskinen, T., Yelle, R., Harris, M., & Lavvas, P. (2012, Fall). The escape of exoplanetary atmospheres under strong irradiation. American Geophysical Union.
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  Abstract #: P24A-02
• Lavvas, P., Koskinen, T., & Yelle, R. (2012, September). Si chemistry in the atmospheres of extrasolar giant planets. European Planetary Science Congress. Madrid, Spain.
• Lavvas, P., Yelle, R., Koskinen, T., Bazin, A., Vuitton, V., Vigren, E., Galand, M., Wellbrock, A., Coates, A., Wahlund, J., Crary, F., & Snowden, D. (2012, Fall). Aerosol growth in Titan's ionosphere through particle charging. American Geophysical Union.
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  Abstract #: P24C-04
• Snowden, D., Yelle, R., Galand, M., Coates, A., Jones, G., Wellbrock, A., & Lavvas, P. (2012, Fall). A Global Model of the Precipitation of Magnetically-mirroring Magnetospheric Electrons in Titan’s Ionosphere. DPS meeting #44American Astronomical Society.
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  #303.03
• Vigren, E., Galand, M., Yelle, R., Cui, J., Wahlund, J., Ĺgren, K., Lavvas, P., Mueller-Wodarg, I., & Strobel, D. (2012, September). The effective recombination coefficient in Titan's sunlit upper atmosphere. European Planetary Science Congress. Madrid, Spain.
• Yelle, R. (2012, July). Titan's Upper Atmosphere and Ionosphere. 39th COSPAR Scientific Assembly. Mysore, India.
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  Abstract: A1.1-15-12; Page: 2210

Others

• {Kelley}, M., {Bauer}, J., {Bodewits}, D., {Farnham}, T., {Stevenson}, R., , R. (2014). CO2 Impacts on the Martian Atmosphere.