Kristopher G Klein

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Journals/Publications

• Broeren, T., & Klein, K. (2024). Constrained Wave-telescope Technique. Research Notes of the American Astronomical Society, 8(5), 130.
• Broeren, T., & Klein, K. (2024). Multi-Point Gradient Estimation in Turbulence. arXiv e-prints, arXiv:2405.14889.
• Broeren, T., Klein, K., & TenBarge, J. (2024). Multi-Spacecraft Magnetic Field Reconstructions: A Cross-Scale Comparison of Methods. Earth and Space Science, 11(3), e2023EA003369.
• Chen, X., Giacalone, J., Guo, F., & Klein, K. G. (2024). Parallel Diffusion Coefficient of Energetic Charged Particles in the Inner Heliosphere from the Turbulent Magnetic Fields Measured by Parker Solar Probe. \apj, 965(1), 61.
• Holmes, J., Kasper, J., Klein, K. G., Lepri, S. T., & Raines, J. M. (2024). Zone of Preferential Heating for Minor Ions in the Solar Wind. \apj, 964(1), 19.
• Huang, J., Kasper, J., Larson, D. E., McManus, M. D., Whittlesey, P., Livi, R., Rahmati, A., Romeo, O., Klein, K., Sun, W., Holst, B., Huang, Z., Jian, L. K., Szabo, A., Verniero, J., Chen, C., Lavraud, B., Liu, M., Badman, S. T., , Niembro, T., et al. (2024). Erratum: ``Parker Solar Probe Observations of High Plasma \ensuremath{\beta} Solar Wind from the Streamer Belt'' (2023, ApJS, 265, 47). \apjs, 271(2), 66.
• Johnson, E., Maruca, B., McManus, M., Stevens, M., Klein, K., & Mostafavi, P. (2024). Application of collisional analysis to the differential velocity of solar wind ions. Frontiers in Astronomy and Space Sciences, 10, 1284913.
• Klein, K. G. (2024). Application of collisional analysis to the differential velocity of solar wind ions. Frontiers in Astronomy and Space Sciences.
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  Collisional analysis combines the effects of collisional relaxation and large-scale expansion to quantify how solar wind parameters evolve as the plasma expands through the heliosphere. Though previous studies have applied collisional analysis to the temperature ratio between protons (ionized hydrogen) and α-particles (fully ionized helium), this is the first study to explore α-proton differential flow with collisional analysis. First, the mathematical model for the collisional analysis of differential flow was derived. Then, this model was applied to individual in-situ observations from Parker Solar Probe (PSP; r = 0.1–0.27 au) to generate predictions of the α-proton differential flow in the near-Earth solar wind. A comparison of these predicted values with contemporaneous measurements from the Wind spacecraft (r = 1.0 au) shows strong agreement, which may imply that the effects of expansion and Coulomb collisions have a large role in governing the evolution of differential flow through the inner heliosphere....
• Klein, K. G. (2024). Diagnosing collisionless energy transfer using field-particle correlations: gyrokinetic turbulence.
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  Determining the physical mechanisms that extract energy from turbulent fluctuations in weakly collisional magnetized plasmas is necessary for a more complete characterization of the behavior of a variety of space and astrophysical plasmas. Such a determination is complicated by the complex nature of the turbulence as well as observational constraints, chiefly that in situ measurements of such plasmas are typically only available at a single point in space. Recent work has shown that correlations between electric fields and particle velocity distributions constructed from single-point measurements produce a velocity-dependent signature of the collisionless damping mechanism. We extend this work by constructing field-particle correlations using data sets drawn from single points in strongly driven, turbulent, electromagnetic gyrokinetic simulations to demonstrate that this technique can identify the collisionless mechanisms operating in such systems. The correlation's velocity-space structure agrees with expectations of resonant mechanisms transferring energy collisionlessly in turbulent systems. This work motivates the eventual application of field-particle correlations to spacecraft measurements in the solar wind, with the ultimate goal to determine the physical mechanisms that dissipate magnetized plasma turbulence.
• Klein, K. G. (2024). Mind the Gap: Nonlocal Cascades and Preferential Heating in High-$\beta$ Alfvénic Turbulence. arXiv e-prints.
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  Characterizing the thermodynamics of turbulent plasmas is key to decoding observable signatures from astrophysical systems. In magnetohydrodynamic (MHD) turbulence, nonlinear interactions between counter-propagating Alfvén waves cascade energy to smaller spatial scales where dissipation heats the protons and electrons. When the thermal pressure far exceeds the magnetic pressure, linear theory predicts a spectral gap at perpendicular scales near the proton gyroradius where Alfvén waves become non-propagating. For simple models of an MHD turbulent cascade that assume only local nonlinear interactions, the cascade halts at this gap, preventing energy from reaching smaller scales where electron dissipation dominates, leading to an overestimate of the proton heating rate. In this work, we demonstrate that nonlocal contributions to the cascade, specifically large scale shearing and small scale diffusion, can bridge the non-propagating gap, allowing the cascade to continue to smaller scales. We provide an updated functional form for the proton-to-electron heating ratio accounting for this nonlocal energy transfer by evaluating a nonlocal weakened cascade model over a range of temperature and pressure ratios. In plasmas where the thermal pressure dominates the magnetic pressure, we observe that the proton heating is moderated compared to the significant enhancement predicted by local models....
• Klein, K. G. (2024). Mind the gap: Nonlocal cascades and preferential heating in high-β Alfvénic turbulence. Monthly Notices of the Royal Astronomical Society: Letters.
• Klein, K. G. (2024). Multi‐Spacecraft Magnetic Field Reconstructions: A Cross‐Scale Comparison of Methods. Earth and Space Science.
• Klein, K. G. (2024). Proton- and Alpha-driven Instabilities in an Ion Cyclotron Wave Event. The Astrophysical Journal.
• Klein, K. G. (2024). Velocity-space Signatures of Resonant Energy Transfer between Whistler Waves and Electrons in the Earth’s Magnetosheath. The Astrophysical Journal.
• Klein, K. G. (2024). Zone of Preferential Heating for Minor Ions in the Solar Wind. The Astrophysical Journal.
• Klein, K., Broeren, T., Roberts, O., & Schulz, L. (2024). Wave-Telescope Analysis for Multipoint Observatories: Impact of Timing and Spatial Uncertainties. Journal of Geophysical Research (Space Physics), 129(12), 2024JA033428.
• Lewis, H. C., Stawarz, J. E., Matteini, L., Franci, L., Klein, K. G., Wicks, R. T., Salem, C. S., Horbury, T. S., & Wang, J. H. (2024). Turbulent Energy Conversion Associated With Kinetic Microinstabilities in Earth's Magnetosheath. \grl, 51(24), 2024GL112038.
• Martinovi{\'c}, M. M., Klein, K. G., De, M. R., Verscharen, D., D'Amicis, R., & Bruno, R. (2024). Impact of Two-Population $\alpha$-particle Distributions on Plasma Stability. arXiv e-prints, arXiv:2412.04885.
• McManus, M. D., Klein, K. G., Bale, S. D., Bowen, T. A., Huang, J., Larson, D., Livi, R., Rahmati, A., Romeo, O., Verniero, J., & Whittlesey, P. (2024). Proton- and Alpha-driven Instabilities in an Ion Cyclotron Wave Event. \apj, 961(1), 142.
• Pecora, F., Pucci, F., Malara, F., Klein, K. G., Marcucci, M. F., Retin{\`o}, A., & Matthaeus, W. (2024). Evaluation of Scale-dependent Kurtosis with HelioSwarm. \apjl, 970(2), L36.
• Roberts, O., Klein, K., V{\"or\"os}, Z., Nakamura, R., Li, X., Narita, Y., Schmid, D., Bandyopadhyay, R., & Matthaeus, W. (2024). Measurement of the Taylor Microscale and the Effective Magnetic Reynolds Number in the Solar Wind With Cluster. Journal of Geophysical Research (Space Physics), 129(11), 2024JA032968.
• Shankarappa, N., Klein, K. G., Martinovi{\'c}, M. M., & Bowen, T. A. (2024). Estimated Heating Rates Due to Cyclotron Damping of Ion-scale Waves Observed by the Parker Solar Probe. \apj, 973(1), 20.
• Walters, J., Klein, K. G., Lichko, E., Juno, J., & TenBarge, J. M. (2024). Electron Influence on the Parallel Proton Firehose Instability in 10-moment, Multifluid Simulations. \apj, 975(2), 290.
• Yoon, P. H., Salem, C. S., Klein, K. G., Martinovi{\'c}, M. M., L{\'opez}, R. A., Seough, J., Sarfraz, M., Lazar, M., & Shaaban, S. M. (2024). Regulation of Solar Wind Electron Temperature Anisotropy by Collisions and Instabilities. \apj, 975(1), 105.
• Yoon, P., Lazar, M., Salem, C., Seough, J., Martinovi{\'c}, M., Klein, K., & L{\'opez}, R. (2024). Boundary of the Distribution of Solar Wind Proton Beta versus Temperature Anisotropy. \apj, 969(2), 77.
• Zhang, M. F., Kunz, M. W., Squire, J., & Klein, K. G. (2024). Extreme heating of minor ions in imbalanced solar-wind turbulence. arXiv e-prints, arXiv:2408.04703.
• Zheng, X., Martinovi{\'c}, M. M., Pierrard, V., Klein, K. G., Liu, M., Abraham, J. B., Liu, Y., Luo, J., Lin, X., Liu, G., & Li, J. (2024). Radial Evolution of Non-Maxwellian Electron Populations Derived from Quasi-thermal Noise Spectroscopy: Parker Solar Probe Observations. \apj, 977(1), 39.
• Klein, K. G. (2023). Analyses of ∼0.05–2 MeV Ions Associated with the 2022 February 16 Energetic Storm Particle Event Observed by Parker Solar Probe. The Astrophysical Journal.
• Klein, K. G. (2023). Anterograde Collisional Analysis of Solar Wind Ions. The Astrophysical Journal.
• Klein, K. G. (2023). Data-driven Uncertainty Quantification of the Wave Telescope Technique: General Equations and Demonstration Using HelioSwarm. The Astrophysical Journal Supplement Series.
• Klein, K. G. (2023). Erratum: “The Statistical Properties of Solar Wind Temperature Parameters Near 1 au” (2018, ApJS, 236, 41). The Astrophysical Journal Supplement Series.
• Klein, K. G. (2023). Estimation of Turbulent Proton and Electron Heating Rates via Landau Damping Constrained by Parker Solar Probe Observations. The Astrophysical Journal.
• Klein, K. G. (2023). Estimation of the Error in the Calculation of the Pressure‐Strain Term: Application in the Terrestrial Magnetosphere. Journal of Geophysical Research: Space Physics.
• Klein, K. G. (2023). HelioSwarm: A Multipoint, Multiscale Mission to Characterize Turbulence. Space Science Reviews.
• Klein, K. G. (2023). HelioSwarm: A Multipoint, Multiscale Mission to Characterize Turbulence. arXiv e-prints.
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  HelioSwarm (HS) is a NASA Medium-Class Explorer mission of the Heliophysics Division designed to explore the dynamic three-dimensional mechanisms controlling the physics of plasma turbulence, a ubiquitous process occurring in the heliosphere and in plasmas throughout the universe. This will be accomplished by making simultaneous measurements at nine spacecraft with separations spanning magnetohydrodynamic and sub-ion spatial scales in a variety of near-Earth plasmas. In this paper, we describe the scientific background for the HS investigation, the mission goals and objectives, the observatory reference trajectory and instrumentation implementation before the start of Phase B. Through multipoint, multiscale measurements, HS promises to reveal how energy is transferred across scales and boundaries in plasmas throughout the universe....
• Klein, K. G. (2023). Magnetospheric Multiscale measurements of turbulent electric fields in earth's magnetosheath: How do plasma conditions influence the balance of terms in generalized Ohm's law?. Physics of Plasmas.
• Klein, K. G. (2023). Near-Sun In Situ and Remote-sensing Observations of a Coronal Mass Ejection and its Effect on the Heliospheric Current Sheet. The Astrophysical Journal.
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  During the thirteenth encounter of the Parker Solar Probe (PSP) mission, the spacecraft traveled through a topologically complex interplanetary coronal mass ejection (ICME) beginning on 2022 September 5. PSP traversed through the flank and wake of the ICME while observing the event for nearly two days. The Solar Probe ANalyzer and FIELDS instruments collected in situ measurements of the plasma particles and magnetic field at ~13.3 R S from the Sun. We observe classical ICME signatures, such as a fast-forward shock, bidirectional electrons, low proton temperatures, low plasma β, and high alpha particle to proton number density ratios. In addition, PSP traveled through two magnetic inversion lines, a magnetic reconnection exhaust, and multiple sub-Alfvénic regions. We compare these in situ measurements to remote-sensing observations from the Wide-field Imager for Solar PRobe Plus instrument on board PSP and the Sun Earth Connection Coronal and Heliospheric Investigation on the Solar Terrestrial Relations Observatory. Based on white-light coronagraphs, two CMEs are forward modeled to best fit the extent of the event. Furthermore, Air Force Data Assimilative Flux Transport magnetograms modeled from Global Oscillation Network Group magnetograms and Potential Field Source Surface modeling portray a global reconfiguration of the heliospheric current sheet (HCS) after the CME event, suggesting that these eruptions play a significant role in the evolution of the HCS....
• Klein, K. G. (2023). Parker Solar Probe Observations of High Plasma β Solar Wind from the Streamer Belt. The Astrophysical Journal Supplement Series.
• Klein, K. G. (2023). Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum. Space Science Reviews.
• Klein, K. G. (2023). Phase-space Energization of Ions in Oblique Shocks. The Astrophysical Journal.
• Klein, K. G. (2023). Proton and Alpha Driven Instabilities in an Ion Cyclotron Wave Event. arXiv e-prints.
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  Ion scale wave events or "wave storms" in the solar wind are characterised by enhancements in magnetic field fluctuations as well as coherent magnetic field polarisation signatures at or around the local ion cyclotron frequencies. In this paper we study in detail one such wave event from Parker Solar Probe's (PSP) fourth encounter, consisting of an initial period of left-handed (LH) polarisation abruptly transitioning to a strong period of right-handed (RH) polarisation, accompanied by clear core-beam structure in both the alpha and proton velocity distribution functions. A linear stability analysis shows that the LH polarised waves are anti-Sunward propagating Alfvén/ion cyclotron (A/IC) waves primarily driven by a proton cyclotron instability in the proton core population, and the RH polarised waves are anti-Sunward propagating fast magnetosonic/whistler (FM/W) waves driven by a firehose-like instability in the secondary alpha beam population. The abrupt transition from LH to RH is caused by a drop in the proton core temperature anisotropy. We find very good agreement between the frequencies and polarisations of the unstable wave modes as predicted by linear theory and those observed in the magnetic field spectra. Given the ubiquity of ion scale wave signatures observed by PSP, this work gives insight into which exact instabilities may be active and mediating energy transfer in wave-particle interactions in the inner heliosphere, as well as highlighting the role a secondary alpha population may play as a rarely considered source of free energy available for producing wave activity....
• Klein, K. G. (2023). Quantifying the Energy Budget in the Solar Wind from 13.3 to 100 Solar Radii. The Astrophysical Journal.
• Klein, K. G. (2023). The Effects of Nonequilibrium Velocity Distributions on Alfvén Ion-cyclotron Waves in the Solar Wind. The Astrophysical Journal.
• Klein, K. G. (2023). Three-Dimensional Energy Transfer in Space Plasma Turbulence from Multipoint Measurement. Physical Review Letters.
• Klein, K. G. (2023). Vol. 55, Issue 3 (Heliophysics 2024 Decadal Whitepapers). Bulletin of the AAS.
• Bowen, T. A., Chandran, B. D., Squire, J., Bale, S. D., Duan, D., Klein, K. G., Larson, D., Mallet, A., McManus, M. D., Meyrand, R., Verniero, J. L., & Woodham, L. D. (2022). In Situ Signature of Cyclotron Resonant Heating in the Solar Wind. \prl, 129(16), 165101.
• Howes, G. G., Verniero, J. L., Larson, D. E., Bale, S. D., Kasper, J. C., Goetz, K., Klein, K. G., Whittlesey, P. L., Livi, R., Rahmati, A., Chen, C. H., Wilson, L. B., Alterman, B. L., & Wicks, R. T. (2022). Revolutionizing Our Understanding of Particle Energization in Space Plasmas Using On-Board Wave-Particle Correlator Instrumentation. Frontiers in Astronomy and Space Sciences, 9, 912868.
• Jiang, W., Verscharen, D., Li, H., Wang, C., & Klein, K. G. (2022). Whistler Waves as a Signature of Converging Magnetic Holes in Space Plasmas. \apj, 935(2), 169.
• Klein, K. G. (2022). A Case for Electron-Astrophysics. Experimental Astronomy.
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  The smallest characteristic scales, at which electron dynamics determines the plasma behaviour, are the next frontier in space and astrophysical plasma research. The analysis of astrophysical processes at these scales lies at the heart of the research theme of electron-astrophysics. Electron scales are the ultimate bottleneck for dissipation of plasma turbulence, which is a fundamental process not understood in the electron-kinetic regime. In addition, plasma electrons often play an important role for the spatial transfer of thermal energy due to the high heat flux associated with their velocity distribution. The regulation of this electron heat flux is likewise not understood. By focussing on these and other fundamental electron processes, the research theme of electron-astrophysics links outstanding science questions of great importance to the fields of space physics, astrophysics, and laboratory plasma physics. In this White Paper, submitted to ESA in response to the Voyage 2050 call, we review a selection of these outstanding questions, discuss their importance, and present a roadmap for answering them through novel space-mission concepts....
• Klein, K. G. (2022). In Situ Signature of Cyclotron Resonant Heating in the Solar Wind. Physical Review Letters.
• Klein, K. G. (2022). Patches of Magnetic Switchbacks and Their Origins. The Astrophysical Journal.
• Klein, K. G. (2022). Plasma Parameters From Quasi‐Thermal Noise Observed by Parker Solar Probe: A New Model for the Antenna Response. Journal of Geophysical Research: Space Physics.
• Klein, K. G. (2022). Revolutionizing Our Understanding of Particle Energization in Space Plasmas Using On-Board Wave-Particle Correlator Instrumentation. Frontiers in Astronomy and Space Sciences.
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  A leap forward in our understanding of particle energization in plasmas throughout the heliosphere is essential to answer longstanding questions in heliophysics, including the heating of the solar corona, acceleration of the solar wind, and energization of particles that lead to observable phenomena, such as the Earth's aurora. The low densities and high temperatures of typical heliospheric environments lead to weakly collisional plasma conditions. Under these conditions, the energization of particles occurs primarily through collisionless interactions between the electromagnetic fields and the individual plasma particles with energies characteristic of a particular interaction. To understand how the plasma heating and particle acceleration impacts the macroscopic evolution of the heliosphere, impacting phenomena such as extreme space weather, it is critical to understand these collisionless wave-particle interactions on the characteristic ion and electron kinetic timescales. Such understanding requires high-cadence measurements of both the electromagnetic fields and the three-dimensional particle velocity distributions. Although existing instrument technology enables these measurements, a major challenge to maximize the scientific return from these measurements is the limited amount of data that can be transmitted to the ground due to telemetry constraints. A valuable, but underutilized, approach to overcome this limitation is to compute on-board correlations of the maximum-cadence field and particle measurements to improve the sampling time by several orders of magnitude. Here we review the fundamentals of the innovative field-particle correlation technique, present a formulation of the technique that can be implemented as an on-board wave-particle correlator, and estimate results that can be achieved with existing instrumental capabilities for particle velocity distribution measurements....
• Klein, K. G. (2022). Strong Perpendicular Velocity-space Diffusion in Proton Beams Observed by Parker Solar Probe. The Astrophysical Journal.
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  The SWEAP instrument suite on Parker Solar Probe (PSP) has detected numerous proton beams associated with coherent, circularly polarized, ion-scale waves observed by PSP's FIELDS instrument suite. Measurements during PSP Encounters 4-8 revealed pronounced complex shapes in the proton velocity distribution functions (VDFs), in which the tip of the beam undergoes strong perpendicular diffusion, resulting in VDF level contours that resemble a "hammerhead." We refer to these proton beams, with their attendant "hammerhead" features, as the ion strahl. We present an example of these observations occurring simultaneously with a 7 hr ion-scale wave storm and show results from a preliminary attempt at quantifying the occurrence of ion-strahl broadening through three-component ion VDF fitting. We also provide a possible explanation of the ion perpendicular scattering based on quasilinear theory and the resonant scattering of beam ions by parallel-propagating, right circularly polarized, fast magnetosonic/whistler waves....
• Klein, K. G. (2022). The Solar Probe ANalyzer—Ions on the Parker Solar Probe. The Astrophysical Journal.
• Klein, K. G. (2022). Wind/Waves Antenna Length Determined Using Quasi-Thermal Noise Spectroscopy. Research Notes of the AAS.
• Livi, R., Larson, D. E., Kasper, J. C., Abiad, R., Case, A., Klein, K. G., Curtis, D. W., Dalton, G., Stevens, M., Korreck, K. E., Ho, G., Robinson, M., Tiu, C., Whittlesey, P. L., Verniero, J. L., Halekas, J., McFadden, J., Marckwordt, M., Slagle, A., , Abatcha, M., et al. (2022). The Solar Probe ANalyzer-Ions on the Parker Solar Probe. \apj, 938(2), 138.
• Martinovi{\'c}, M. M., Dordevi{\'c}, A. R., Klein, K. G., Maksimovi{\'c}, M., Issautier, K., Liu, M., Pulupa, M., Bale, S. D., Halekas, J. S., & McManus, M. D. (2022). Plasma Parameters From Quasi-Thermal Noise Observed by Parker Solar Probe: A New Model for the Antenna Response. Journal of Geophysical Research (Space Physics), 127(4), e30182.
• Martinovi{\'c}, M. M., Klein, K. G., & Krishnan, H. G. (2022). Wind/Waves Antenna Length Determined Using Quasi-Thermal Noise Spectroscopy. Research Notes of the American Astronomical Society, 6(8), 166.
• Matthaeus, W., Adhikari, S., Bandyopadhyay, R., Brown, M., Bruno, R., Borovsky, J., Carbone, V., Caprioli, D., Chasapis, A., Chhiber, R., Dasso, S., Dmitruk, P., Del Zanna, L., Dmitruk, P., Franci, L., Gary, S., Goldstein, M., Gomez, D., Greco, A., , Horbury, T., et al. (2022). The essential role of multi-point measurements in investigations of turbulence, three-dimensional structure, and dynamics: the solar wind beyond single scale and the Taylor Hypothesis. arXiv e-prints, arXiv:2211.12676.
• Shi, C., Panasenco, O., Velli, M., Tenerani, A., Verniero, J. L., Sioulas, N., Huang, Z., Brosius, A., Bale, S. D., Klein, K., Kasper, J., Wit, T. D., Goetz, K., Harvey, P. R., MacDowall, R. J., Malaspina, D. M., Pulupa, M., Larson, D., Livi, R., , Case, A., et al. (2022). Patches of Magnetic Switchbacks and Their Origins. \apj, 934(2), 152.
• Verniero, J., Chandran, B., Larson, D., Paulson, K., Alterman, B., Badman, S., Bale, S., Bonnell, J., Bowen, T., Wit, T. D., Kasper, J., Klein, K., Lichko, E., Livi, R., McManus, M., Rahmati, A., Verscharen, D., Walters, J., & Whittlesey, P. (2022). Strong Perpendicular Velocity-space Diffusion in Proton Beams Observed by Parker Solar Probe. \apj, 924(2), 112.
• Verscharen, D., Wicks, R. T., Alexandrova, O., Bruno, R., Burgess, D., Chen, C. H., D'Amicis, R., De, K. J., Wit, T. D., Franci, L., He, J., Henri, P., Kasahara, S., Khotyaintsev, Y., Klein, K. G., Lavraud, B., Maruca, B. A., Maksimovic, M., Plaschke, F., , Poedts, S., et al. (2022). A Case for Electron-Astrophysics. Experimental Astronomy, 54(2-3), 473-519.
• Bowen, T. A., Badman, S. T., Bale, S. D., Wit, T., Horbury, T. S., Klein, K. G., Larson, D., Mallet, A., Matteini, L., McManus, M. D., & Squire, J. (2021). Nonlinear Interactions in Spherically Polarized Alfv\'enic Turbulence. arXiv e-prints, arXiv:2110.11454.
• Bowen, T. A., Squire, J., Bale, S. D., Chandran, B., Duan, D., Klein, K. G., Larson, D., Mallet, A., McManus, M. D., Meyrand, R., Verniero, J., & Woodham, L. D. (2021). The In Situ Signature of Cyclotron Resonant Heating. arXiv e-prints, arXiv:2111.05400.
• Broeren, T., Klein, K., TenBarge, J., Dors, I., Roberts, O., & Verscharen, D. (2021). Magnetic Field Reconstruction for a Realistic Multi-Point, Multi-Scale Spacecraft Observatory. Frontiers in Astronomy and Space Sciences, 8, 144.
• Chen, C., Chandran, B., Woodham, L., Jones, S., Perez, J., Bourouaine, S., Bowen, T., Klein, K., Moncuquet, M., Kasper, J., & Bale, S. (2021). The near-Sun streamer belt solar wind: turbulence and solar wind acceleration. \aap, 650, L3.
• Esman, T., Espley, J., Gruesbeck, J., Klein, K., & Giacalone, J. (2021). Plasma Waves in the Distant Martian Environment: Implications for Mars' Sphere of Influence. Journal of Geophysical Research (Space Physics), 126(11), e29686.
• Halekas, J., Whittlesey, P., Larson, D., McGinnis, D., Bale, S., Berthomier, M., Case, A., Chandran, B., Kasper, J., Klein, K., Korreck, K., Livi, R., MacDowall, R., Maksimovic, M., Malaspina, D., Matteini, L., Pulupa, M., & Stevens, M. (2021). Electron heat flux in the near-Sun environment. \aap, 650, A15.
• Juno, J., Howes, G. G., TenBarge, J. M., Wilson, L. B., Spitkovsky, A., Caprioli, D., Klein, K. G., & Hakim, A. (2021). A field-particle correlation analysis of a perpendicular magnetized collisionless shock. Journal of Plasma Physics, 87(3), 905870316.
• Klein, K. G. (2021). A field–particle correlation analysis of a perpendicular magnetized collisionless shock. Journal of Plasma Physics.
• Klein, K. G. (2021). Detection of small magnetic flux ropes from the third and fourth Parker Solar Probe encounters. Astronomy and Astrophysics.
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  Context. Aims: We systematically search for magnetic flux rope structures in the solar wind to within the closest distance to the Sun of ~0.13 AU, using data from the third and fourth orbits of the Parker Solar Probe. Methods: We extended our previous magnetic helicity-based technique of identifying magnetic flux rope structures. The method was improved upon to incorporate the azimuthal flow, which becomes larger as the spacecraft approaches the Sun. Results: A total of 21 and 34 magnetic flux ropes are identified during the third (21-day period) and fourth (17-day period) orbits of the Parker Solar Probe, respectively. We provide a statistical analysis of the identified structures, including their relation to the streamer belt and heliospheric current sheet crossing....
• Klein, K. G. (2021). Experimental Determination of Ion Acoustic Wave Dispersion Relation With Interferometric Analysis. Journal of Geophysical Research: Space Physics.
• Klein, K. G. (2021). How Alfvén waves energize the solar wind: heat versus work. Journal of Plasma Physics.
• Klein, K. G. (2021). Inferred Linear Stability of Parker Solar Probe Observations Using One- and Two-component Proton Distributions. The Astrophysical Journal.
• Klein, K. G. (2021). Ion-driven Instabilities in the Inner Heliosphere. I. Statistical Trends. The Astrophysical Journal.
• Klein, K. G. (2021). Magnetic Field Reconstruction for a Realistic Multi-Point, Multi-Scale Spacecraft Observatory. Frontiers in Astronomy and Space Sciences.
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  Future in situ space plasma investigations will likely involve spatially distributed observatories comprised of multiple spacecraft, beyond the four and five spacecraft configurations currently in operation. Inferring the magnetic field structure across the observatory, and not simply at the observation points, is a necessary step towards characterizing fundamental plasma processes using these unique multi-point, multi-scale data sets. We propose improvements upon the classic first-order reconstruction method, as well as a second-order method, utilizing magnetometer measurements from a realistic nine-spacecraft observatory. The improved first-order method, which averages over select ensembles of four spacecraft, reconstructs the magnetic field associated with simple current sheets and numerical simulations of turbulence accurately over larger volumes compared to second-order methods or first-order methods using a single regular tetrahedron. Using this averaging method on data sets with fewer than nine measurement points, the volume of accurate reconstruction compared to a known magnetic vector field improves approximately linearly with the number of measurement points....
• Klein, K. G. (2021). Multiscale Solar Wind Turbulence Properties inside and near Switchbacks Measured by the Parker Solar Probe. The Astrophysical Journal.
• Klein, K. G. (2021). PATCH: Particle Arrival Time Correlation for Heliophysics. Journal of Geophysical Research: Space Physics.
• Klein, K. G. (2021). Parker Solar Probe Enters the Magnetically Dominated Solar Corona. Physical Review Letters.
• Klein, K. G. (2021). The Near-Sun Streamer Belt Solar Wind: Turbulence and Solar Wind Acceleration. arXiv e-prints.
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  The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 Rs, allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP's fourth solar encounter, likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with spectral index close to -5/3 rather than -3/2), a lower Alfvénicity, and a "1/f" break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ~4° from the HCS, suggesting ~8° as the full-width of the streamer belt wind at these distances. While the majority of the Alfvénic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind....
• Klein, K. G. (2021). The near-Sun streamer belt solar wind: turbulence and solar wind acceleration. Astronomy & Astrophysics.
• Klein, K. G. (2021). Wave-particle energy transfer directly observed in an ion cyclotron wave. Astronomy and Astrophysics.
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  Context. The first studies with Parker Solar Probe (PSP) data have made significant progress toward understanding of the fundamental properties of ion cyclotron waves in the inner heliosphere. The survey mode particle measurements of PSP, however, did not make it possible to measure the coupling between electromagnetic fields and particles on the time scale of the wave periods. Aims: We present a novel approach to study wave-particle energy exchange with PSP. Methods: We used the Flux Angle operation mode of the Solar Probe Cup in conjunction with the electric field measurements and present a case study when the Flux Angle mode measured the direct interaction of the proton velocity distribution with an ion cyclotron wave. Results: Our results suggest that the energy transfer from fields to particles on the timescale of a cyclotron period is equal to approximately 3-6% of the electromagnetic energy flux. This rate is consistent with the hypothesis that the ion cyclotron wave was locally generated in the solar wind....
• Klein, K., Verniero, J., Alterman, B., Bale, S., Case, A., Kasper, J., Korreck, K., Larson, D., Lichko, E., Livi, R., McManus, M., Martinovi{\'c}, M., Rahmati, A., Stevens, M., & Whittlesey, P. (2021). Inferred Linear Stability of Parker Solar Probe Observations Using One- and Two-component Proton Distributions. \apj, 909(1), 7.
• Martinovi{\'c}, M. M., Klein, K. G., Huang, J., Chandran, B. D., Kasper, J. C., Lichko, E., Bowen, T., Chen, C. H., Matteini, L., Stevens, M., Case, A. W., & Bale, S. D. (2021). Multiscale Solar Wind Turbulence Properties inside and near Switchbacks Measured by the Parker Solar Probe. \apj, 912(1), 28.
• Martinovi{\'c}, M. M., Klein, K. G., {\v{D}urovcov\'a}, T., & Alterman, B. L. (2021). Ion-driven Instabilities in the Inner Heliosphere. I. Statistical Trends. \apj, 923(1), 116.
• Perez, J. C., Chandran, B. D., Klein, K. G., & Martinovi{\'c}, M. M. (2021). How Alfv\'en waves energize the solar wind: heat versus work. Journal of Plasma Physics, 87(2), 905870218.
• Vech, D., Malaspina, D. M., Cattell, C., Schwartz, S. J., Ergun, R. E., Klein, K. G., Kromyda, L., & Chasapis, A. (2021). Experimental Determination of Ion Acoustic Wave Dispersion Relation With Interferometric Analysis. Journal of Geophysical Research (Space Physics), 126(11), e29221.
• Vech, D., Martinovi{\'c}, M., Klein, K., Malaspina, D., Bowen, T., Verniero, J., Paulson, K., Wit, T., Kasper, J., Huang, J., Stevens, M., Case, A., Korreck, K., Mozer, F., Goodrich, K., Bale, S., Whittlesey, P., Livi, R., Larson, D., , Pulupa, M., et al. (2021). Wave-particle energy transfer directly observed in an ion cyclotron wave. \aap, 650, A10.
• Vech, D., Stevens, M., Paulson, K., Malaspina, D., Case, A., Klein, K., & Kasper, J. (2021). A powerful machine learning technique to extract proton core, beam, and \ensuremath{\alpha}-particle parameters from velocity distribution functions in space plasmas. \aap, 650, A198.
• Verniero, J., Howes, G., Stewart, D., & Klein, K. (2021). Determining Threshold Instrumental Resolutions for Resolving the Velocity Space Signature of Ion Landau Damping. Journal of Geophysical Research (Space Physics), 126(5), e28361.
• Verniero, J., Howes, G., Stewart, D., & Klein, K. (2021). PATCH: Particle Arrival Time Correlation for Heliophysics. Journal of Geophysical Research (Space Physics), 126(5), e28940.
• Verscharen, D., Wicks, R. T., Alexandrova, O., Bruno, R., Burgess, D., Chen, C. H., D'Amicis, R., De, K. J., Wit, T. D., Franci, L., He, J., Henri, P., Kasahara, S., Khotyaintsev, Y., Klein, K. G., Lavraud, B., Maruca, B. A., Maksimovic, M., Plaschke, F., , Poedts, S., et al. (2021). A Case for Electron-Astrophysics. Experimental Astronomy.
• Zhao, L. -., Zank, G., Hu, Q., Telloni, D., Chen, Y., Adhikari, L., Nakanotani, M., Kasper, J., Huang, J., Bale, S., Korreck, K., Case, A., Stevens, M., Bonnell, J., Wit, T., Goetz, K., Harvey, P., MacDowall, R., Malaspina, D., , Pulupa, M., et al. (2021). Detection of small magnetic flux ropes from the third and fourth Parker Solar Probe encounters. \aap, 650, A12.
• Adhikari, L., Zank, G., Zhao, L. -., Kasper, J., Korreck, K., Stevens, M., Case, A., Whittlesey, P., Larson, D., Livi, R., & Klein, K. (2020). Turbulence Transport Modeling and First Orbit Parker Solar Probe (PSP) Observations. \apjs, 246(2), 38.
• Bandyopadhyay, R., Goldstein, M., Maruca, B., Matthaeus, W., Parashar, T., Ruffolo, D., Chhiber, R., Usmanov, A., Chasapis, A., Qudsi, R., Bale, S. D., Bonnell, J., Wit, T., Goetz, K., Harvey, P. R., MacDowall, R. J., Malaspina, D. M., Pulupa, M., Kasper, J., , Korreck, K., et al. (2020). Enhanced Energy Transfer Rate in Solar Wind Turbulence Observed near the Sun from Parker Solar Probe. \apjs, 246(2), 48.
• Bowen, T. A., Mallet, A., Huang, J., Klein, K. G., Malaspina, D. M., Stevens, M., Bale, S. D., Bonnell, J., Case, A. W., Chandran, B. D., Chaston, C., Chen, C. H., Wit, T., Goetz, K., Harvey, P. R., Howes, G. G., Kasper, J., Korreck, K. E., Larson, D., , Livi, R., et al. (2020). Ion-scale Electromagnetic Waves in the Inner Heliosphere. \apjs, 246(2), 66.
• Case, A., Kasper, J. C., Stevens, M. L., Korreck, K. E., Paulson, K., Daigneau, P., Caldwell, D., Freeman, M., Henry, T., Klingensmith, B., Bookbinder, J., Robinson, M., Berg, P., Tiu, C., Wright Jr., ., Reinhart, M. J., Curtis, D., Ludlam, M., Larson, D., , Whittlesey, P., et al. (2020). The Solar Probe Cup on the Parker Solar Probe. \apjs, 246(2), 43.
• Chen, C., Bale, S., Bonnell, J., Borovikov, D., Bowen, T., Burgess, D., Case, A., Chandran, B., Wit, T. D., Goetz, K., Harvey, P., Kasper, J., Klein, K., Korreck, K., Larson, D., Livi, R., MacDowall, R., Malaspina, D., Mallet, A., , McManus, M., et al. (2020). The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere. \apjs, 246(2), 53.
• Giacalone, J., Mitchell, D., Allen, R., Hill, M., McNutt Jr., ., Szalay, J., Desai, M., Rouillard, A., Kouloumvakos, A., McComas, D., Christian, E., Schwadron, N., Wiedenbeck, M., Bale, S., Brown, L., Case, A., Chen, X., Cohen, C., Joyce, C., , Kasper, J., et al. (2020). Solar Energetic Particles Produced by a Slow Coronal Mass Ejection at ̃0.25 au. \apjs, 246(2), 29.
• Halekas, J., Whittlesey, P., Larson, D., McGinnis, D., Maksimovic, M., Berthomier, M., Kasper, J., Case, A., Korreck, K., Stevens, M., Klein, K., Bale, S., MacDowall, R., Pulupa, M., Malaspina, D., Goetz, K., & Harvey, P. (2020). Electrons in the Young Solar Wind: First Results from the Parker Solar Probe. \apjs, 246(2), 22.
• Horbury, T. S., Woolley, T., Laker, R., Matteini, L., Eastwood, J., Bale, S. D., Velli, M., Chandran, B. D., Phan, T., Raouafi, N. E., Goetz, K., Harvey, P. R., Pulupa, M., Klein, K., Wit, T., Kasper, J. C., Korreck, K. E., Case, A., Stevens, M. L., , Whittlesey, P., et al. (2020). Sharp Alfv\'enic Impulses in the Near-Sun Solar Wind. \apjs, 246(2), 45.
• Kim, T., Pogorelov, N., Arge, C., Henney, C., Jones-Mecholsky, S., Smith, W., Bale, S., Bonnell, J., Wit, T., Goetz, K., Harvey, P., MacDowall, R., Malaspina, D., Pulupa, M., Kasper, J., Korreck, K., Stevens, M., Case, A., Whittlesey, P., , Livi, R., et al. (2020). Predicting the Solar Wind at the Parker Solar Probe Using an Empirically Driven MHD Model. \apjs, 246(2), 40.
• Klein, K. G. (2020). A Field-Particle Correlation Analysis of a Perpendicular Magnetized Collisionless Shock: II. Vlasov Simulations. arXiv e-prints.
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  Using the field-particle correlation technique, we examine the particle energization in a 1D-2V continuum Vlasov-Maxwell simulation of a perpendicular magnetized collisionless shock. The combination of the field-particle correlation technique, with the high fidelity representation of the particle distribution function provided by a direct discretization of the Vlasov equation, allows us to ascertain the details of the exchange of energy between the electromagnetic fields and the particles in phase space. We are able to identify the velocity-space signatures of shock-drift acceleration of the ions and adiabatic heating of the electrons due to the perpendicular collisionless shock. These energization signatures are compared to the analytical predictions of a companion study Howes et al., and the deviations of our self-consistent simulation from the idealized model considered in Howes et al. are explored. We determine the impact the finite shock-width and the cross-shock component of the electric field have on both the energization of the ions by shock-drift acceleration and the energization of the electrons by adiabatic heating. In doing so, we are able to completely characterize the energy transfer in the perpendicular collisionless shock considered here and provide predictions for the application of the field-particle correlation technique to spacecraft measurements of collisionless shocks....
• Klein, K. G. (2020). Alfvénic Slow Solar Wind Observed in the Inner Heliosphere by Parker Solar Probe. arXiv e-prints.
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  The slow solar wind is typically characterized as having low Alfvénicity. However, Parker Solar Probe (PSP) observed predominately Alfvénic slow solar wind during several of its initial encounters. From its first encounter observations, about 55.3\% of the slow solar wind inside 0.25 au is highly Alfvénic ($|\sigma_C| > 0.7$) at current solar minimum, which is much higher than the fraction of quiet-Sun-associated highly Alfvénic slow wind observed at solar maximum at 1 au. Intervals of slow solar wind with different Alfvénicities seem to show similar plasma characteristics and temperature anisotropy distributions. Some low Alfvénicity slow wind intervals even show high temperature anisotropies, because the slow wind may experience perpendicular heating as fast wind does when close to the Sun. This signature is confirmed by Wind spacecraft measurements as we track PSP observations to 1 au. Further, with nearly 15 years of Wind measurements, we find that the distributions of plasma characteristics, temperature anisotropy and helium abundance ratio ($N_\alpha/N_p$) are similar in slow winds with different Alfvénicities, but the distributions are different from those in the fast solar wind. Highly Alfvénic slow solar wind contains both helium-rich ($N_\alpha/N_p\sim0.045$) and helium-poor ($N_\alpha/N_p\sim0.015$) populations, implying it may originate from multiple source regions. These results suggest that highly Alfvénic slow solar wind shares similar temperature anisotropy and helium abundance properties with regular slow solar winds, and they thus should have multiple origins....
• Klein, K. G. (2020). Creation of large temperature anisotropies in a laboratory plasma. Physics of Plasmas.
• Klein, K. G. (2020). Dependence of kinetic plasma waves on ion-to-electron mass ratio and light-to-Alfvén speed ratio. Monthly Notices of the Royal Astronomical Society.
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  The magnetization |Ωe|/ωe is an important parameter in plasma astrophysics, where Ωe and ωe are the electron gyro-frequency and electron plasma frequency, respectively. It depends only on the mass ratio mI/me and the light-to-Alfvén speed ratio c/VAi, where mI (me) is the ion (electron) mass, c is the speed of light, and VAi is the ion Alfvén speed. Non-linear numerical plasma models such as particle-in-cell simulations must often assume unrealistic values for mI/me and for c/VAi. Because linear theory yields exact results for parametric scalings of wave properties at small amplitudes, we use linear theory to investigate the dispersion relations of Alfvén/ion-cyclotron and fast-magnetosonic/whistler waves as prime examples for collective plasma behaviour depending on mI/me and c/VAi. We analyse their dependence on mI/me and c/VAi in quasi-parallel and quasi-perpendicular directions of propagation with respect to the background magnetic field for a plasma with βj ∼ 1, where βj is the ratio of the thermal to magnetic pressure for species j. Although their dispersion relations are largely independent of c/VAi for c/VAi ≳ 10, the mass ratio mI/me has a strong effect at scales smaller than the ion inertial length. Moreover, we study the impact of relativistic electron effects on the dispersion relations. Based on our results, we recommend aiming for a more realistic value of mI/me than for a more realistic value of c/VAi in non-relativistic plasma simulations if such a choice is necessary, although relativistic and sub-Debye-length effects may require an additional adjustment of c/VAi....
• Klein, K. G. (2020). Dependence of kinetic plasma waves on ion-to-electron mass ratio and light-to-Alfvén speed ratio. arXiv e-prints.
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  The magnetization $|\Omega_{\mathrm e}|/\omega_{\mathrm{e}}$ is an important parameter in plasma astrophysics, where $\Omega_{\mathrm e}$ and $\omega_{\mathrm{e}}$ are the electron gyro-frequency and electron plasma frequency, respectively. It only depends on the mass ratio $m_{\mathrm i}/m_{\mathrm e}$ and the light-to-Alfvén speed ratio $c/v_{\mathrm{Ai}}$, where $m_{\mathrm i}$ ($m_{\mathrm e}$) is the ion (electron) mass, $c$ is the speed of light, and $v_{\mathrm{Ai}}$ is the ion Alfvén speed. Nonlinear numerical plasma models such as particle-in-cell simulations must often assume unrealistic values for $m_{\mathrm i}/m_{\mathrm e}$ and for $c/v_{\mathrm{Ai}}$. Because linear theory yields exact results for parametric scalings of wave properties at small amplitudes, we use linear theory to investigate the dispersion relations of Alfvén/ion-cyclotron and fast-magnetosonic/whistler waves as prime examples for collective plasma behaviour depending on $m_{\mathrm i}/m_{\mathrm e}$ and $c/v_{\mathrm{Ai}}$. We analyse their dependence on $m_{\mathrm i}/m_{\mathrm e}$ and $c/v_{\mathrm{Ai}}$ in quasi-parallel and quasi-perpendicular directions of propagation with respect to the background magnetic field for a plasma with $\beta_j\sim1$, where $\beta_j$ is the ratio of the thermal to magnetic pressure for species $j$. Although their dispersion relations are largely independent of $c/v_{\mathrm{Ai}}$ for $c/v_{\mathrm{Ai}}\gtrsim 10$, the mass ratio $m_{\mathrm i}/m_{\mathrm e}$ has a strong effect at scales smaller than the ion inertial length. Moreover, we study the impact of relativistic electron effects on the dispersion relations. Based on our results, we recommend aiming for a more realistic value of $m_{\mathrm i}/m_{\mathrm e}$ than for a more realistic value of $c/v_{\mathrm{Ai}}$ in non-relativistic plasma simulations if such a choice is necessary, although $\dots$...
• Klein, K. G. (2020). Diagnosing collisionless energy transfer using field-particle correlations: Alfven-Ion Cyclotron Turbulence. arXiv e-prints.
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  We apply field-particle correlations -- a technique that tracks the time-averaged velocity-space structure of the energy density transfer rate between electromagnetic fields and plasma particles -- to data drawn from a hybrid Vlasov-Maxwell simulation of Alfvén Ion-Cyclotron turbulence. Energy transfer in this system is expected to include both Landau and cyclotron wave-particle resonances, unlike previous systems to which the field-particle correlation technique has been applied. In this simulation, the energy transfer rate mediated by the parallel electric field $E_\parallel$ comprises approximately $60\%$ of the total rate, with the remainder mediated by the perpendicular electric field $E_\perp$. The parallel electric field resonantly couples to protons, with the canonical bipolar velocity-space signature of Landau damping identified at many points throughout the simulation. The energy transfer mediated by $E_\perp$ preferentially couples to particles with $v_{tp} \lesssim v_\perp \lesssim 3 v_{tp}$ in agreement with the expected formation of a cyclotron diffusion plateau. Our results demonstrate clearly that the field-particle correlation technique can distinguish distinct channels of energy transfer using single-point measurements, even at points in which multiple channels act simultaneously, and can be used to determine quantitatively the rates of particle energization in each channel....
• Klein, K. G. (2020). Electron heat flux in the near-Sun environment. arXiv e-prints.
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  We survey the electron heat flux observed by the Parker Solar Probe (PSP) in the near-Sun environment at heliocentric distances of 0.125-0.25 AU. We utilized measurements from the Solar Wind Electrons Alphas and Protons and FIELDS experiments to compute the solar wind electron heat flux and its components and to place these in context. The PSP observations reveal a number of trends in the electron heat flux signatures near the Sun. The magnitude of the heat flux is anticorrelated with solar wind speed, likely as a result of the lower saturation heat flux in the higher-speed wind. When divided by the saturation heat flux, the resulting normalized net heat flux is anticorrelated with plasma beta on all PSP orbits, which is consistent with the operation of collisionless heat flux regulation mechanisms. The net heat flux also decreases in very high beta regions in the vicinity of the heliospheric current sheet, but in most cases of this type the omnidirectional suprathermal electron flux remains at a comparable level or even increases, seemingly inconsistent with disconnection from the Sun. The measured heat flux values appear inconsistent with regulation primarily by collisional mechanisms near the Sun. Instead, the observed heat flux dependence on plasma beta and the distribution of suprathermal electron parameters are both consistent with theoretical instability thresholds associated with oblique whistler and magnetosonic modes....
• Klein, K. G. (2020). Enhanced Energy Transfer Rate in Solar Wind Turbulence Observed near the Sun from Parker Solar Probe. The Astrophysical Journal Supplement Series.
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  Direct evidence of an inertial-range turbulent energy cascade has been provided by spacecraft observations in heliospheric plasmas. In the solar wind, the average value of the derived heating rate near 1 au is $\sim {10}^{3}\,{\rm{J}}\,{\mathrm{kg}}^{-1}\,{{\rm{s}}}^{-1}$ , an amount sufficient to account for observed departures from adiabatic expansion. Parker Solar Probe, even during its first solar encounter, offers the first opportunity to compute, in a similar fashion, a fluid-scale energy decay rate, much closer to the solar corona than any prior in situ observations. Using the Politano─Pouquet third-order law and the von Kármán decay law, we estimate the fluid-range energy transfer rate in the inner heliosphere, at heliocentric distance R ranging from 54 R☉ (0.25 au) to 36 R☉ (0.17 au). The energy transfer rate obtained near the first perihelion is about 100 times higher than the average value at 1 au, which is in agreement with estimates based on a heliospheric turbulence transport model. This dramatic increase in the heating rate is unprecedented in previous solar wind observations, including those from Helios, and the values are close to those obtained in the shocked plasma inside the terrestrial magnetosheath....
• Klein, K. G. (2020). Ion versus Electron Heating in Compressively Driven Astrophysical Gyrokinetic Turbulence. Physical Review X.
• Klein, K. G. (2020). Ion versus electron heating in compressively driven astrophysical gyrokinetic turbulence. arXiv e-prints.
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  The partition of irreversible heating between ions and electrons in compressively driven (but subsonic) collisionless turbulence is investigated by means of nonlinear hybrid gyrokinetic simulations. We derive a prescription for the ion-to-electron heating ratio $Q_\rmi/Q_\rme$ as a function of the compressive-to-Alfvénic driving power ratio $P_\compr/P_\AW$, of the ratio of ion thermal pressure to magnetic pressure $\beta_\rmi$, and of the ratio of ion-to-electron background temperatures $T_\rmi/T_\rme$. It is shown that $Q_\rmi/Q_\rme$ is an increasing function of $P_\compr/P_\AW$. When the compressive driving is sufficiently large, $Q_\rmi/Q_\rme$ approaches $\simeq P_\compr/P_\AW$. This indicates that, in turbulence with large compressive fluctuations, the partition of heating is decided at the injection scales, rather than at kinetic scales. Analysis of phase-space spectra shows that the energy transfer from inertial-range compressive fluctuations to sub-Larmor-scale kinetic Alfvén waves is absent for both low and high $\beta_\rmi$, meaning that the compressive driving is directly connected to the ion entropy fluctuations, which are converted into ion thermal energy. This result suggests that preferential electron heating is a very special case requiring low $\beta_\rmi$ and no, or weak, compressive driving. Our heating prescription has wide-ranging applications, including to the solar wind and to hot accretion disks such as M87 and Sgr A*....
• Klein, K. G. (2020). Kinetic-scale Spectral Features of Cross Helicity and Residual Energy in the Inner Heliosphere. The Astrophysical Journal Supplement Series.
• Klein, K. G. (2020). Parker Solar Probe Observations of Proton Beams Simultaneous with Ion-scale Waves. The Astrophysical Journal Supplement Series.
• Klein, K. G. (2020). Proton core behaviour inside magnetic field switchbacks. Monthly Notices of the Royal Astronomical Society.
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  During Parker Solar Probe's first two orbits, there are widespread observations of rapid magnetic field reversals known as switchbacks. These switchbacks are extensively found in the near-Sun solar wind, appear to occur in patches, and have possible links to various phenomena such as magnetic reconnection near the solar surface. As switchbacks are associated with faster plasma flows, we questioned whether they are hotter than the background plasma and whether the microphysics inside a switchback is different to its surroundings. We have studied the reduced distribution functions from the Solar Probe Cup instrument and considered time periods with markedly large angular deflections to compare parallel temperatures inside and outside switchbacks. We have shown that the reduced distribution functions inside switchbacks are consistent with a rigid velocity space rotation of the background plasma. As such, we conclude that the proton core parallel temperature is very similar inside and outside of switchbacks, implying that a temperature-velocity (T-V) relationship does not hold for the proton core parallel temperature inside magnetic field switchbacks. We further conclude that switchbacks are consistent with Alfvénic pulses travelling along open magnetic field lines. The origin of these pulses, however, remains unknown. We also found that there is no obvious link between radial Poynting flux and kinetic energy enhancements suggesting that the radial Poynting flux is not important for the dynamics of switchbacks....
• Klein, K. G. (2020). Small-scale Magnetic Flux Ropes in the First Two Parker Solar Probe Encounters. The Astrophysical Journal.
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  Small-scale magnetic flux ropes (SFRs) are a type of structure in the solar wind that possess helical magnetic field lines. In a recent report we presented the radial variations of the properties of SFRs from 0.29 to 8 au using in situ measurements from the Helios, Advanced Composition Explorer/WIND (ACE/Wind), Ulysses, and Voyager spacecrafts. With the launch of the Parker Solar Probe (PSP), we extend our previous investigation further into the inner heliosphere. We apply a Grad-Shafranov-based algorithm to identify SFRs during the first two PSP encounters. We find that the number of SFRs detected near the Sun is much less than at larger radial distances, where magnetohydrodynamic (MHD) turbulence may act as the local source to produce these structures. The prevalence of Alfvénic structures significantly suppresses the detection of SFRs at closer distances. We compare the SFR event list with other event identification methods, yielding a dozen well-matched events. The cross-section maps of two selected events confirm the cylindrical magnetic flux-rope configuration. The power-law relation between the SFR magnetic field and heliocentric distances seems to hold down to 0.16 au....
• Klein, K. G. (2020). Solar Energetic Particles Produced by a Slow Coronal Mass Ejection at ̃0.25 au. The Astrophysical Journal Supplement Series.
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  We present an analysis of Parker Solar Probe (PSP) IS☉IS observations of ̃30─300 keV n−1 ions on 2018 November 11 when PSP was about 0.25 au from the Sun. Five hours before the onset of a solar energetic particle (SEP) event, a coronal mass ejection (CME) was observed by STEREO-A/COR2, which crossed PSP about a day later. No shock was observed locally at PSP, but the CME may have driven a weak shock earlier. The SEP event was dispersive, with higher energy ions arriving before the lower energy ones. Timing suggests the particles originated at the CME when it was at ̃7.4R☉. SEP intensities increased gradually from their onset over a few hours, reaching a peak, and then decreased gradually before the CME arrived at PSP. The event was weak, having a very soft energy spectrum (−4 to −5 spectral index). The earliest arriving particles were anisotropic, moving outward from the Sun, but later, the distribution was observed to be more isotropic. We present numerical solutions of the Parker transport equation for the transport of 30─300 keV n−1 ions assuming a source comoving with the CME. Our model agrees well with the observations. The SEP event is consistent with ion acceleration at a weak shock driven briefly by the CME close to the Sun, which later dissipated before arriving at PSP, followed by the transport of ions in the interplanetary magnetic field....
• Klein, K. G. (2020). Solar Wind Electron Parameters Determination on Wind Spacecraft Using Quasi‐Thermal Noise Spectroscopy. Journal of Geophysical Research: Space Physics.
• Klein, K. G. (2020). The Heliospheric Current Sheet and Plasma Sheet during Parker Solar Probe's First Orbit. The Astrophysical Journal.
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  We present heliospheric current sheet (HCS) and plasma sheet (HPS) observations during Parker Solar Probe's (PSP) first orbit around the Sun. We focus on the eight intervals that display a true sector boundary (TSB; based on suprathermal electron pitch angle distributions) with one or several associated current sheets. The analysis shows that (1) the main density enhancements in the vicinity of the TSB and HCS are typically associated with electron strahl dropouts, implying magnetic disconnection from the Sun, (2) the density enhancements are just about twice that in the surrounding regions, suggesting mixing of plasmas from each side of the HCS, (3) the velocity changes at the main boundaries are either correlated or anticorrelated with magnetic field changes, consistent with magnetic reconnection, (4) there often exists a layer of disconnected magnetic field just outside the high-density regions, in agreement with a reconnected topology, (5) while a few cases consist of short-lived density and velocity changes, compatible with short-duration reconnection exhausts, most events are much longer and show the presence of flux ropes interleaved with higher-β regions. These findings are consistent with the transient release of density blobs and flux ropes through sequential magnetic reconnection at the tip of the helmet streamer. The data also demonstrate that, at least during PSP's first orbit, the only structure that may be defined as the HPS is the density structure that results from magnetic reconnection, and its byproducts, likely released near the tip of the helmet streamer....
• Klein, K. G. (2020). Turbulence Characteristics of Switchback and Nonswitchback Intervals Observed by Parker Solar Probe. The Astrophysical Journal Letters.
• Klein, K. G. (2020). Turbulence characteristics of switchbacks and non-switchbacks intervals observed by \emph{Parker Solar Probe}. arXiv e-prints.
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  We use \emph{Parker Solar Probe} (\emph{PSP}) in-situ measurements to analyze the characteristics of solar wind turbulence during the first solar encounter covering radial distances between $35.7R_\odot$ and $41.7R_\odot$. In our analysis we isolate so-called switchback (SB) intervals (folded magnetic field lines) from non-switchback (NSB) intervals, which mainly follow the Parker spiral field. Using a technique based on conditioned correlation functions, we estimate the power spectra of Elsasser, magnetic and bulk velocity fields separately in the SB and NSB intervals. In comparing the turbulent energy spectra of the two types of intervals, we find the following characteristics: 1) The decorrelation length of the backward-propagating Elsasser field $z^-$ is larger in the NSB intervals than the one in the SB intervals; 2) the magnetic power spectrum in SB intervals is steeper, with spectral index close to -5/3, than in NSB intervals, which have a spectral index close to -3/2; 3) both SB and NSB turbulence are imbalanced with NSB having the largest cross-helicity, 4) the residual energy is larger in the SB intervals than in NSB, and 5) the analyzed fluctuations are dominated by Alfvénic fluctuations that are propagating in the \emph{sunward} (\emph{anti-sunward}) direction for the SB (NSB) turbulence. These observed features provide further evidence that the switchbacks observed by \emph{PSP} are associated with folded magnetic field lines giving insight into their turbulence nature....
• Klein, K. G., Howes, G. G., TenBarge, J. M., & Valentini, F. (2020). Diagnosing collisionless energy transfer using field-particle correlations: Alfv\'en-ion cyclotron turbulence. Journal of Plasma Physics, 86(4), 905860402.
• Martinovi{\'c}, M. M., Klein, K. G., Kasper, J. C., Case, A. W., Korreck, K. E., Larson, D., Livi, R., Stevens, M., Whittlesey, P., Chandran, B. D., Alterman, B. L., Huang, J., Chen, C. H., Bale, S. D., Pulupa, M., Malaspina, D. M., Bonnell, J. W., Harvey, P. R., Goetz, K., , Wit, T., et al. (2020). The Enhancement of Proton Stochastic Heating in the Near-Sun Solar Wind. \apjs, 246(2), 30.
• Tenerani, A., Velli, M., Matteini, L., R{\'eville}, V., Shi, C., Bale, S. D., Kasper, J. C., Bonnell, J. W., Case, A. W., Wit, T. D., Goetz, K., Harvey, P. R., Klein, K. G., Korreck, K., Larson, D., Livi, R., MacDowall, R. J., Malaspina, D. M., Pulupa, M., , Stevens, M., et al. (2020). Magnetic Field Kinks and Folds in the Solar Wind. \apjs, 246(2), 32.
• Vech, D., Kasper, J. C., Klein, K. G., Huang, J., Stevens, M. L., Chen, C. H., Case, A. W., Korreck, K., Bale, S. D., Bowen, T. A., Whittlesey, P. L., Livi, R., Larson, D. E., Malaspina, D., Pulupa, M., Bonnell, J., Harvey, P., Goetz, K., Wit, T., & MacDowall, R. (2020). Kinetic-scale Spectral Features of Cross Helicity and Residual Energy in the Inner Heliosphere. \apjs, 246(2), 52.
• Verscharen, D., Parashar, T. N., Gary, S. P., & Klein, K. G. (2020). Dependence of kinetic plasma waves on ion-to-electron mass ratio and light-to-Alfv\'en speed ratio. arXiv e-prints, arXiv:2004.01676.
• Whittlesey, P. L., Larson, D. E., Kasper, J. C., Halekas, J., Abatcha, M., Abiad, R., Berthomier, M., Case, A., Chen, J., Curtis, D. W., Dalton, G., Klein, K. G., Korreck, K. E., Livi, R., Ludlam, M., Marckwordt, M., Rahmati, A., Robinson, M., Slagle, A., , Stevens, M., et al. (2020). The Solar Probe ANalyzers\textemdashElectrons on the Parker Solar Probe. \apjs, 246(2), 74.
• Zhao, L. -., Zank, G., Adhikari, L., Hu, Q., Kasper, J., Bale, S., Korreck, K., Case, A., Stevens, M., Bonnell, J., Wit, T., Goetz, K., Harvey, P., MacDowall, R., Malaspina, D., Pulupa, M., Larson, D., Livi, R., Whittlesey, P., & Klein, K. (2020). Identification of Magnetic Flux Ropes from Parker Solar Probe Observations during the First Encounter. \apjs, 246(2), 26.
• , J. M., , O. A., , S. B., , F. C., , S. S., , C. H., , G. G., , T. H., , P. A., , H. J., , K. G., , C. K., , M. K., , N. F., , A. M., , B. A., , W. H., , R. M., , E. Q., , , J. C., et al. (2019). [Plasma 2020 Decadal] Disentangling the Spatiotemporal Structure of Turbulence Using Multi-Spacecraft Data.
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  This white paper submitted for 2020 Decadal Assessment of Plasma Scienceconcerns the importance of multi-spacecraft missions to address fundamentalquestions concerning plasma turbulence. Plasma turbulence is ubiquitous in theuniverse, and it is responsible for the transport of mass, momentum, and energyin such diverse systems as the solar corona and wind, accretion discs, planetformation, and laboratory fusion devices. Turbulence is an inherentlymulti-scale and multi-process phenomenon, coupling the largest scales of asystem to sub-electron scales via a cascade of energy, while simultaneouslygenerating reconnecting current layers, shocks, and a myriad of instabilitiesand waves. The solar wind is humankind's best resource for studying thenaturally occurring turbulent plasmas that permeate the universe. Sincelaunching our first major scientific spacecraft mission, Explorer 1, in 1958,we have made significant progress characterizing solar wind turbulence. Yet,due to the severe limitations imposed by single point measurements, we areunable to characterize sufficiently the spatial and temporal properties of thesolar wind, leaving many fundamental questions about plasma turbulenceunanswered. Therefore, the time has now come wherein making significantadditional progress to determine the dynamical nature of solar wind turbulencerequires multi-spacecraft missions spanning a wide range of scalessimultaneously. A dedicated multi-spacecraft mission concurrently covering awide range of scales in the solar wind would not only allow us to directlydetermine the spatial and temporal structure of plasma turbulence, but it wouldalso mitigate the limitations that current multi-spacecraft missions face, suchas non-ideal orbits for observing solar wind turbulence. Some of thefundamentally important questions that can only be addressed by in situmultipoint measurements are discussed.[Journal_ref: ]
• , K. G., , O. A., , J. B., , D. C., , A. W., , B. D., , L. J., , T. H., , L. J., , J. C., , O. L., , B. A., , W. M., , A. R., , O. R., , A. S., , R. S., , C. S., , J. S., , , H. S., et al. (2019). [Plasma 2020 Decadal] Multipoint Measurements of the Solar Wind: A Proposed Advance for Studying Magnetized Turbulence.
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  A multi-institutional, multi-national science team will soon submit a NASAproposal to build a constellation of spacecraft to fly into the near-Earthsolar wind in a swarm spanning a multitude of scales in order to obtaincritically needed measurements that will reveal the underlying dynamics ofmagnetized turbulence. This white paper, submitted to the Plasma 2020 DecadalSurvey Committee, provides a brief overview of turbulent systems thatconstitute an area of compelling plasma physics research, including why thismission is needed, and how this mission will achieve the goal of revealing howenergy is transferred across scales and boundaries in plasmas throughout theuniverse.[Journal_ref: ]
• , W. H., , R. B., , M. R., , J. B., , V. C., , D. C., , A. C., , R. C., , S. D., , P. D., , L. D., , P. A., , L. F., , S. P., , M. L., , D. G., , A. G., , T. S., , H. J., , , J. C., et al. (2019). [Plasma 2020 Decadal] The essential role of multi-point measurements in turbulence investigations: the solar wind beyond single scale and beyond the Taylor Hypothesis.
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  This paper briefly reviews a number of fundamental measurements that need tobe made in order to characterize turbulence in space plasmas such as the solarwind. It has long been known that many of these quantities require simultaneousmultipoint measurements to attain a proper characterization that would revealthe fundamental physics of plasma turbulence. The solar wind is an ideal plasmafor such an investigation, and it now appears to be technologically feasible tocarry out such an investigation, following the pioneering Cluster and MMSmissions. Quantities that need to be measured using multipoint measurementsinclude the two-point, two-time second correlation function of velocity,magnetic field and density, and higher order statistical objects such as thirdand fourth order structure functions. Some details of these requirements aregiven here, with a eye towards achieving closure on fundamental questionsregarding the cascade rate, spectral anisotropy, characteristic coherentstructures, intermittency, and dissipation mechanisms that describe plasmaturbuelence, as well as its variability with plasma parameters in the solarwind. The motivation for this discussion is the current planning for a proposedHelioswarm mission that would be designed to make these measurements,leading tobreakthrough understanding of the physics of space and astrophysicalturbulence.[Journal_ref: ]
• Chen, C., Klein, K. G., & Howes, G. G. (2019). Evidence for electron Landau damping in space plasma turbulence. NATURE COMMUNICATIONS, 10.
• Kasper, J. C., & Klein, K. G. (2019). Strong Preferential Ion Heating is Limited to within the Solar Alfv\'en Surface. The Astrophysical Journal Letters, 877(2), L35.
• Kasper, J. C., Bale, S. D., Belcher, J. W., Berthomier, M., Case, A. W., Chandran, B., Curtis, D. W., Gallagher, D., Gary, S. P., Golub, L., Halekas, J. S., Ho, G. C., Horbury, T. S., Hu, Q., Huang, J., Klein, K. G., Korreck, K. E., Larson, D. E., Livi, R., , Maruca, B., et al. (2019). Alfvenic velocity spikes and rotational flows in the near-Sun solar wind. NATURE, 576(7786), 228-+.
• Klein, K. G. (2019). A Case for Electron-Astrophysics. arXiv e-prints.
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  A grand-challenge problem at the forefront of physics is to understand how energy is transported and transformed in plasmas. This fundamental research priority encapsulates the conversion of plasma-flow and electromagnetic energies into particle energy, either as heat or some other form of energisation. The smallest characteristic scales, at which electron dynamics determines the plasma behaviour, are the next frontier in space and astrophysical plasma research. The analysis of astrophysical processes at these scales lies at the heart of the field of electron-astrophysics. Electron scales are the ultimate bottleneck for dissipation of plasma turbulence, which is a fundamental process not understood in the electron-kinetic regime. Since electrons are the most numerous and most mobile plasma species in fully ionised plasmas and are strongly guided by the magnetic field, their thermal properties couple very efficiently to global plasma dynamics and thermodynamics....
• Klein, K. G. (2019). Alfvénic velocity spikes and rotational flows in the near-Sun solar wind. Nature.
• Klein, K. G. (2019). Challenges and the next transformative steps in understanding plasma turbulence from the perspective of multi-spacecraft measurements. arXiv e-prints.
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  We have become heavily reliant on electrical technologies, from power grids to GPS to wireless communication. Any disruption of these systems will have severe global consequences. A major natural hazard for such electrical disruption is caused by solar wind disturbances that have dramatic geospace impact.Estimates are that a solar storm of the magnitude of the 1859 Carrington Solar Superstorm would cause over $2 trillion in damage today. In July 23, 2012, we had a near miss of a solar Superstorm that could have broken the record of largest such storms at Earth. To enable pre-emptive measures, developing accurate space weather forecasts is urgent. At the core of space weather forecasts is plasma physics, and kinetic turbulence, in particular. For example, the intense turbulence stirred up at the bow shock and foreshock have been shown to open up pathways for high velocity solar wind parcels to bypass the protective shield of the terrestrial magnetosphere and create disturbances in the ionosphere and lower atmosphere. A primary challenge in understanding kinetic turbulence and its global implications is its multi-scale nature, spanning from electron scales to macro scales of the magnetosphere. Current four-spacecraft missions with 3D formations, the Magnetospheric Multiscale (MMS) and Cluster, have made progress in our understanding of such turbulence. Yet the limitation of a fixed spacecraft formation size at a given time prohibits probing the multi-scale nature as well as the dynamical evolution of the phenomena. A transformative leap in our understanding of turbulence is expected with in-situ probes populating a 3D volume and forming multiple 'n-hedrons (n > 4)' in MHD to kinetic scales....
• Klein, K. G. (2019). Evidence for Electron Landau Damping in Space Plasma Turbulence. arXiv e-prints.
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  How turbulent energy is dissipated in weakly collisional space and astrophysical plasmas is a major open question. Here, we present the application of a field-particle correlation technique to directly measure the transfer of energy between the turbulent electromagnetic field and electrons in the Earth's magnetosheath, the region of solar wind downstream of the Earth's bow shock. The measurement of the secular energy transfer from the parallel electric field as a function of electron velocity shows a signature consistent with Landau damping. This signature is coherent over time, close to the predicted resonant velocity, similar to that seen in kinetic Alfv\'en turbulence simulations, and disappears under phase randomisation. This suggests that electron Landau damping could play a significant role in turbulent plasma heating, and that the technique is a valuable tool for determining the particle energisation processes operating in space and astrophysical plasmas....
• Klein, K. G. (2019). Evidence for electron Landau damping in space plasma turbulence. Nature Communications.
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  How turbulent energy is dissipated in weakly collisional space and astrophysical plasmas is a major open question. Here, we present the application of a field-particle correlation technique to directly measure the transfer of energy between the turbulent electromagnetic field and electrons in the Earth's magnetosheath, the region of solar wind downstream of the Earth's bow shock. The measurement of the secular energy transfer from the parallel electric field as a function of electron velocity shows a signature consistent with Landau damping. This signature is coherent over time, close to the predicted resonant velocity, similar to that seen in kinetic Alfven turbulence simulations, and disappears under phase randomisation. This suggests that electron Landau damping could play a significant role in turbulent plasma heating, and that the technique is a valuable tool for determining the particle energisation processes operating in space and astrophysical plasmas...
• Klein, K. G. (2019). Interplay between intermittency and dissipation in collisionless plasma turbulence. Journal of Plasma Physics.
• Klein, K. G. (2019). Predictions for the First Parker Solar Probe Encounter. The Astrophysical Journal Letters.
• Klein, K. G. (2019). Radial Evolution of Stochastic Heating in Low-β Solar Wind. The Astrophysical Journal.
• Klein, K. G. (2019). Solar Wind Plasma Parameter Distributions at 1 au. Research Notes of the AAS.
• Klein, K. G. (2019). Solar Wind Temperature Isotropy. Physical Review Letters.
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  Reliable models of the solar wind in the near-Earth space environment may constrain conditions close to the Sun. This is relevant to NASA's contemporary innerheliospheric mission Parker Solar Probe. Among the outstanding issues is how to explain the solar wind temperature isotropy. Perpendicular and parallel proton and electron temperatures near 1 AU are theoretically predicted to be unequal, but in situ observations show quasi-isotropy sufficiently below the instability threshold condition. This has not been satisfactorily explained. The present Letter shows that the dynamical coupling of electrons and protons via collisional processes and instabilities may contribute toward the resolution of this problem....
• Klein, K. G. (2019). The multi-scale nature of the solar wind. Living Reviews in Solar Physics.
• Klein, K. G. (2019). The multi-scale nature of the solar wind. arXiv e-prints.
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  The solar wind is a magnetized plasma and as such exhibits collective plasma behavior associated with its characteristic spatial and temporal scales. The characteristic length scales include the size of the heliosphere, the collisional mean free paths of all species, their inertial lengths, their gyration radii, and their Debye lengths. The characteristic timescales include the expansion time, the collision times, and the periods associated with gyration, waves, and oscillations. We review the past and present research into the multi-scale nature of the solar wind based on in-situ spacecraft measurements and plasma theory. We establish the notion that couplings of processes across scales are important for the global dynamics and thermodynamics of the solar wind. We describe methods to measure in-situ properties of particles and fields. We then discuss the role of expansion effects, non-equilibrium distribution functions, collisions, waves and turbulence, and kinetic microinstabilities for the multi-scale plasma evolution....
• Klein, K. G. (2019). Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence. Physics of Plasmas.
• Klein, K. G. (2019). Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence. arXiv e-prints.
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  Using kinetic particle-in-cell (PIC) simulations, we simulate reconnection conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta (ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic shear (strong guide field). Changing the simulation domain size, we find that the ion response varies greatly. For reconnecting regions with scales comparable to the ion Larmor radius, the ions do not respond to the reconnection dynamics leading to ''electron-only'' reconnection with very large quasi-steady reconnection rates. The transition to more traditional ''ion-coupled'' reconnection is gradual as the reconnection domain size increases, with the ions becoming frozen-in in the exhaust when the magnetic island width in the normal direction reaches many ion inertial lengths. During this transition, the quasi-steady reconnection rate decreases until the ions are fully coupled, ultimately reaching an asymptotic value. The scaling of the ion outflow velocity with exhaust width during this electron-only to ion-coupled transition is found to be consistent with a theoretical model of a newly reconnected field line. In order to have a fully frozen-in ion exhaust with ion flows comparable to the reconnection Alfvén speed, an exhaust width of at least several ion inertial lengths is needed. In turbulent systems with reconnection occurring between magnetic bubbles associated with fluctuations, using geometric arguments we estimate that fully ion-coupled reconnection requires magnetic bubble length scales of at least several tens of ion inertial lengths....
• Klein, K. G. (2019). [Plasma 2020 Decadal] The essential role of multi-point measurements in turbulence investigations: the solar wind beyond single scale and beyond the Taylor Hypothesis. arXiv e-prints.
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  This paper briefly reviews a number of fundamental measurements that need to be made in order to characterize turbulence in space plasmas such as the solar wind. It has long been known that many of these quantities require simultaneous multipoint measurements to attain a proper characterization that would reveal the fundamental physics of plasma turbulence. The solar wind is an ideal plasma for such an investigation, and it now appears to be technologically feasible to carry out such an investigation, following the pioneering Cluster and MMS missions. Quantities that need to be measured using multipoint measurements include the two-point, two-time second correlation function of velocity, magnetic field and density, and higher order statistical objects such as third and fourth order structure functions. Some details of these requirements are given here, with a eye towards achieving closure on fundamental questions regarding the cascade rate, spectral anisotropy, characteristic coherent structures, intermittency, and dissipation mechanisms that describe plasma turbuelence, as well as its variability with plasma parameters in the solar wind. The motivation for this discussion is the current planning for a proposed Helioswarm mission that would be designed to make these measurements,leading to breakthrough understanding of the physics of space and astrophysical turbulence....
• Klein, K. G., Martinovi{\'c}, M., Stansby, D., & Horbury, T. S. (2019). Linear Stability in the Inner Heliosphere: Helios Re-evaluated. The Astrophysical Journal, 887(2), 234.
• Klein, K., & Vech, D. (2019). Solar Wind Plasma Parameter Distributions at 1 au. Research Notes of the American Astronomical Society, 3(7), 107.
• Li, T. C., Howes, G. G., Klein, K. G., Liu, Y., & Tenbarge, J. M. (2019). Collisionless energy transfer in kinetic turbulence: field-particle correlations in Fourier space. Journal of Plasma Physics, 85(4), 905850406.
• Mallet, A., Klein, K. G., {Chand, r., Gro{\v{s}elj}, D., Hoppock, I. W., Bowen, T. A., Salem, C. S., & Bale, S. D. (2019). Interplay between intermittency and dissipation in collisionless plasma turbulence. Journal of Plasma Physics, 85(3), 175850302.
• Martinovi{\'c}, M. M., Klein, K. G., & Bourouaine, S. (2019). Radial Evolution of Stochastic Heating in Low-\ensuremath{\beta} Solar Wind. The Astrophysical Journal, 879(1), 43.
• Sharma Pyakurel, P., Shay, M., Phan, T., Matthaeus, W., Drake, J., TenBarge, J., Haggerty, C., Klein, K., Cassak, P., Parashar, T., Swisdak, M., & Chasapis, A. (2019). Transition from ion-coupled to electron-only reconnection: Basic physics and implications for plasma turbulence. Physics of Plasmas, 26(8), 082307.
• Verscharen, D., Klein, K. G., & Maruca, B. A. (2019). The multi-scale nature of the solar wind. Living Reviews in Solar Physics, 16(1), 5.
• Yoon, P., Seough, J., Salem, C., & Klein, K. (2019). Solar Wind Temperature Isotropy. Physical Review Letters, 123(14), 145101.
• van, d., Manchester, W. B., Klein, K. G., & Kasper, J. C. (2019). Predictions for the First Parker Solar Probe Encounter. ASTROPHYSICAL JOURNAL LETTERS, 872(2).
• Hoppock, I. W., Chandran, B., Klein, K. G., Mallet, A., & Verscharen, D. (2018). Stochastic proton heating by kinetic-Alfven-wave turbulence in moderately high-beta plasmas. JOURNAL OF PLASMA PHYSICS, 84(6).
• Howes, G. G., McCubbin, A. J., & Klein, K. G. (2018). Spatially localized particle energization by Landau damping in current sheets produced by strong Alfven wave collisions. JOURNAL OF PLASMA PHYSICS, 84(1).
• Klein, K. G. (2018). ALPS: the Arbitrary Linear Plasma Solver. Journal of Plasma Physics.
• Klein, K. G. (2018). Astrophysical gyrokinetics: turbulence in pressure-anisotropic plasmas at ion scales and beyond. Journal of Plasma Physics.
• Klein, K. G. (2018). Large-scale Control of Kinetic Dissipation in the Solar Wind. The Astrophysical Journal.
• Klein, K. G. (2018). Magnetic Reconnection May Control the Ion-scale Spectral Break of Solar Wind Turbulence. The Astrophysical Journal.
• Klein, K. G. (2018). Majority of Solar Wind Intervals Support Ion-Driven Instabilities. Physical Review Letters.
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  We perform a statistical assessment of solar wind stability at 1 AU against ion sources of free energy using Nyquist's instability criterion. In contrast to typically employed threshold models which consider a single free-energy source, this method includes the effects of proton and He2 + temperature anisotropy with respect to the background magnetic field as well as relative drifts between the proton core, proton beam, and He2 + components on stability. Of 309 randomly selected spectra from the Wind spacecraft, 53.7% are unstable when the ion components are modeled as drifting bi-Maxwellians; only 4.5% of the spectra are unstable to long-wavelength instabilities. A majority of the instabilities occur for spectra where a proton beam is resolved. Nearly all observed instabilities have growth rates γ slower than instrumental and ion-kinetic-scale timescales. Unstable spectra are associated with relatively large He2 + drift speeds and/or a departure of the core proton temperature from isotropy; other parametric dependencies of unstable spectra are also identified.
• Klein, K. G. (2018). Nonlinear energy transfer and current sheet development in localized Alfvén wavepacket collisions in the strong turbulence limit. Journal of Plasma Physics.
• Klein, K. G. (2018). Spatially localized particle energization by Landau damping in current sheets produced by strong Alfvén wave collisions. Journal of Plasma Physics.
• Klein, K. G. (2018). Stochastic proton heating by kinetic-Alfvén-wave turbulence in moderately high- plasmas. Journal of Plasma Physics.
• Klein, K. G. (2018). The Statistical Properties of Solar Wind Temperature Parameters Near 1 au. The Astrophysical Journal Supplement Series.
• Alterman, B., Klein, K., Verscharen, D., Stevens, M., & Kasper, J. (2017). A Deeper Understanding of Stability in the Solar Wind: Applying Nyquist's Instability Criterion to Wind Faraday Cup Data. AGU Fall Meeting Abstracts.
• Bert, C., Kasper, J., Klein, K., & Case, A. (2017). Simple Dependence of Proton Temperature on Solar Wind Speed and Compression in High Alfven Mach Number Solar Wind. AGU Fall Meeting Abstracts.
• Howes, G., Klein, K., & McCubbin, A. (2017). Spatially Localized Particle Energization by Landau Damping in Current Sheets. AGU Fall Meeting Abstracts.
• Kasper, J., Klein, K., Weber, T., Maksimovic, M., Zaslavsky, A., Case, A., Stevens, M., Maruca, B., & Bale, S. (2017). A zone of preferential ion heating extends tens of solar radii from Sun. AGU Fall Meeting Abstracts.
• Klein, K. G. (2017). A Zone of Preferential Ion Heating Extends Tens of Solar Radii from the Sun.
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  The extreme temperatures and nonthermal nature of the solar corona and solar wind arise from an unidentified physical mechanism that preferentially heats certain ion species relative to others. Spectroscopic indicators of unequal temperatures commence within a fraction of a solar radius above the surface of the Sun, but the outer reach of this mechanism has yet to be determined. Here we present an empirical procedure for combining interplanetary solar wind measurements and a modeled energy equation including Coulomb relaxation to solve for the typical outer boundary of this zone of preferential heating. Applied to two decades of observations by the Wind spacecraft, our results are consistent with preferential heating being active in a zone extending from the transition region in the lower corona to an outer boundary 20─40 solar radii from the Sun, producing a steady-state super-mass-proportional α-to-proton temperature ratio of 5.2─5.3. Preferential ion heating continues far beyond the transition region and is important for the evolution of both the outer corona and the solar wind. The outer boundary of this zone is well below the orbits of spacecraft at 1 au and even closer missions such as Helios and MESSENGER, meaning it is likely that no existing mission has directly observed intense preferential heating, just residual signatures. We predict that the Parker Solar Probe will be the first spacecraft with a perihelion sufficiently close to the Sun to pass through the outer boundary, enter the zone of preferential heating, and directly observe the physical mechanism in action.
• Klein, K. G. (2017). A Zone of Preferential Ion Heating Extends Tens of Solar Radii from the Sun. The Astrophysical Journal.
• Klein, K. G. (2017). Applying Nyquist's method for stability determination to solar wind observations. Journal of Geophysical Research: Space Physics.
• Klein, K. G. (2017). Characterizing Fluid and Kinetic Instabilities using Field-Particle Correlations on Single-Point Time Series. Physics of Plasmas.
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  A recently proposed technique correlating electric fields and particle velocity distributions is applied to single-point time series extracted from linearly unstable, electrostatic numerical simulations. The form of the correlation, which measures the transfer of phase-space energy density between the electric field and plasma distributions and had previously been applied to damped electrostatic systems, is modified to include the effects of drifting equilibrium distributions of the type that drive counter-streaming and bump-on-tail instabilities. By using single-point time series, the correlation is ideal for diagnosing dynamics in systems where access to integrated quantities, such as energy, is observationally infeasible. The velocity-space structure of the field-particle correlation is shown to characterize the underlying physical mechanisms driving unstable systems. The use of this correlation in simple systems will assist in its eventual application to turbulent, magnetized plasmas, with the ultimate goal of characterizing the nature of mechanisms that damp turbulent fluctuations in the solar wind.
• Klein, K. G. (2017). Diagnosing collisionless energy transfer using field-particle correlations: Vlasov-Poisson plasmas. Journal of Plasma Physics.
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  Turbulence plays a key role in the conversion of the energy of large-scale fields and flows to plasma heat, impacting the macroscopic evolution of the heliosphere and other astrophysical plasma systems. Although we have long been able to make direct spacecraft measurements of all aspects of the electromagnetic field and plasma fluctuations in near-Earth space, our understanding of the physical mechanisms responsible for the damping of the turbulent fluctuations in heliospheric plasmas remains incomplete. Here we propose an innovative field-particle correlation technique that can be used to measure directly the secular energy transfer from fields to particles associated with collisionless damping of the turbulent fluctuations. Furthermore, this novel procedure yields information about the collisionless energy transfer as a function of particle velocity, providing vital new information that can help to identify the dominant collisionless mechanism governing the damping of the turbulent fluctuations. Kinetic plasma theory is used to devise the appropriate correlation to diagnose Landau damping, and the field-particle correlation technique is thoroughly illustrated using the simplified case of the Landau damping of Langmuir waves in a 1D-1V (one dimension in physical space and one dimension in velocity space) Vlasov-Poisson plasma. Generalizations necessary to apply the field-particle correlation technique to diagnose the collisionless damping of turbulent fluctuations in the solar wind are discussed, highlighting several caveats. This novel field-particle correlation technique is intended to be used as a primary analysis tool for measurements from current, upcoming and proposed spacecraft missions that are focused on the kinetic microphysics of weakly collisional heliospheric plasmas, including the Magnetospheric Multiscale (MMS), Solar Probe Plus, Solar Orbiter and Turbulence Heating ObserveR (THOR) missions.
• Klein, K. G. (2017). Diagnosing collisionless energy transfer using field–particle correlations: gyrokinetic turbulence. Journal of Plasma Physics.
• Klein, K. G. (2017). Nature of Stochastic Ion Heating in the Solar Wind: Testing the Dependence on Plasma Beta and Turbulence Amplitude. The Astrophysical Journal.
• Klein, K. G. (2017). Nature of stochastic ion heating in the solar wind: testing the dependence on plasma beta and turbulence amplitude.
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  The solar wind undergoes significant heating as it propagates away from the Sun; the exact mechanisms responsible for this heating are not yet fully understood. We present for the first time a statistical test for one of the proposed mechanisms, stochastic ion heating. We use the amplitude of magnetic field fluctuations near the proton gyroscale as a proxy for the ratio of gyroscale velocity fluctuations to perpendicular (with respect to the magnetic field) proton thermal speed, defined as $\epsilon_p$. Enhanced proton temperatures are observed when $\epsilon_p$ is larger than a critical value ($\sim 0.019 - 0.025$). This enhancement strongly depends on the proton plasma beta ($\beta_{||p}$); when $\beta_{||p} \ll 1$ only the perpendicular proton temperature $T_{\perp}$ increases, while for $\beta_{||p} \sim 1$ increased parallel and perpendicular proton temperatures are both observed. For $\epsilon_p$ smaller than the critical value and $\beta_{||p} \ll 1$ no enhancement of $T_p$ is observed while for $\beta_{||p} \sim 1$ minor increases in $T_{\parallel}$ are measured. The observed change of proton temperatures across a critical threshold for velocity fluctuations is in agreement with the stochastic ion heating model of \citet{chandran2010perpendicular}. We find that $\epsilon_p > \epsilon_{\rm crit}$ in 76\% of the studied periods implying that stochastic heating may operate most of the time in the solar wind at 1 AU.
• Klein, K. G. (2017). Nonlinear energy transfer and current sheet development in localized Alfv\'en wavepacket collisions in the strong turbulence limit.
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  In space and astrophysical plasmas, turbulence is responsible for transferring energy from large scales driven by violent events or instabilities, to smaller scales where turbulent energy is ultimately converted into plasma heat by dissipative mechanisms. The nonlinear interaction between counterpropagating Alfv\'en waves, denoted Alfv\'en wave collisions, drives this turbulent energy cascade, as recognized by early work with incompressible magnetohydrodynamic (MHD) equations. Recent work employing analytical calculations and nonlinear gyrokinetic simulations of Alfv\'en wave collisions in an idealized periodic initial state have demonstrated the key properties that strong Alfv\'en wave collisions mediate effectively the transfer of energy to smaller perpendicular scales and self-consistently generate current sheets. For the more realistic case of the collision between two initially separated Alfv\'en wavepackets, we use a nonlinear gyrokinetic simulation to show here that these key properties persist: strong Alfv\'en wavepacket collisions indeed facilitate the perpendicular cascade of energy and give rise to current sheets. Furthermore, the evolution shows that nonlinear interactions occur only while the wavepackets overlap, followed by a clean separation of the wavepackets with straight uniform magnetic fields and the cessation of nonlinear evolution in between collisions, even in the gyrokinetic simulation presented here which resolves dispersive and kinetic effects beyond the reach of the MHD theory.
• Klein, K., Kasper, J., Howes, G., Larson, D., Valentini, F., Whittlesey, P., Livi, R., & Case, A. (2017). Numerical Preparations Toward Identifying Heating Mechanisms using Distributions Function Measurements from SWEAP and Parker Solar Probe.. AGU Fall Meeting Abstracts.
• Korreck, K., Klein, K., Maruca, B., & Alterman, B. (2017). Understanding non-equilibrium collisional and expansion effects in the solar wind with Parker Solar Probe. AGU Fall Meeting Abstracts.
• Larson, D., Bale, S., Goetz, K., Livi, R., Whittlesey, P., Kasper, J., Klein, K., & Howes, G. (2017). Preliminary calibration results from a simple Wave/Particle correlator instrument that combines data from SPP/SWEAP and FIELDS. AGU Fall Meeting Abstracts.
• Li, T., Howes, G., & Klein, K. (2017). Nature of Energy Transfer in Kinetic Turbulence Diagnosed with a Field-Particle Correlation Technique. AGU Fall Meeting Abstracts.
• Vech, D., Klein, K., & Kasper, J. (2017). A New Approach to Study Stochastic Heating in the Solar Wind with Implications for Parker Solar Probe and Solar Orbiter. AGU Fall Meeting Abstracts.
• Verniero, J., Howes, G., & Klein, K. (2017). The Physics of Energy Transfer in Localized, Strongly Nonlinear Alfv\'en Wave Collisions. AGU Fall Meeting Abstracts.
• Verscharen, D., Klein, K., Chandran, B., Stevens, M., Salem, C., & Bale, S. (2017). Cutting-edge Kinetic Physics with Parker Solar Probe and Solar Orbiter: The Arbitrary Linear Plasma Solver (ALPS). AGU Fall Meeting Abstracts.
• Klein, K. (2016). Field-Particle Correlations as a Measure of Turbulent Damping in Collisionless Plasmas. AGU Fall Meeting Abstracts, SH12A-04.
• Klein, K. (2016). Identifying and Characterizing Kinetic Instabilities using Solar Wind Observations of Non-Maxwellian Plasmas. AGU Fall Meeting Abstracts, SH51D-2610.
• Klein, K. G. (2016). COLLISIONLESS ISOTROPIZATION OF THE SOLAR-WIND PROTONS BY COMPRESSIVE FLUCTUATIONS AND PLASMA INSTABILITIES. The Astrophysical Journal.
• Klein, K. G. (2016). ENERGY DISSIPATION AND LANDAU DAMPING IN TWO- AND THREE-DIMENSIONAL PLASMA TURBULENCE. The Astrophysical Journal.
• Klein, K. G. (2016). Evolution of The Proton Velocity Distribution due to Stochastic Heating in the Near-Sun Solar Wind. The Astrophysical Journal.
• Klein, K. G. (2016). MEASURING COLLISIONLESS DAMPING IN HELIOSPHERIC PLASMAS USING FIELD–PARTICLE CORRELATIONS. The Astrophysical Journal.
• Tong, Y., TenBarge, J., Klein, K., & Bale, S. (2016). Gyrokinetic simulation of turbulent cascade of slow waves in sub-ion-gyroradius scale in low-beta plasma. AGU Fall Meeting Abstracts, SH21C-2548.
• Vech, D., Kasper, J., Klein, K., Hegedus, A., Stevens, M., Case, A., Szabo, A., & Koval, A. (2016). Study of Velocity and Magnetic Field Fluctuations at Kinetic Scale with the DSCOVR Data. AGU Fall Meeting Abstracts, SH21C-2550.
• Verscharen, D., Chandran, B., Klein, K., & Quataert, E. (2016). Collisionless Isotropization of the Solar-Wind Protons by Compressive Fluctuations and Plasma Instabilities. AGU Fall Meeting Abstracts, SH42A-03.
• Klein, K. G. (2015). A MODIFIED VERSION OF TAYLOR’S HYPOTHESIS FOR SOLAR PROBE PLUS OBSERVATIONS. The Astrophysical Journal.
• Klein, K. G. (2015). On the Conservation of Cross Helicity and Wave Action in Solar-wind Models with Non-WKB Alfvén Wave Reflection. The Astrophysical Journal.
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  The interaction between Alfvén-wave turbulence and the background solar wind affects the cross helicity (\int {d}3x {\boldsymbol{v}}\cdot {\boldsymbol{B}}) in two ways. Non-WKB reflection converts outward-propagating Alfvén waves into inward-propagating Alfvén waves and vice versa, and the turbulence transfers momentum to the background flow. When both effects are accounted for, the total cross helicity is conserved. In the special case that the background density and flow speed are independent of time, the equations of cross-helicity conservation and total-energy conservation can be combined to recover a well-known equation derived by Heinemann and Olbert that has been interpreted as a non-WKB generalization of wave-action conservation. This latter equation (in contrast to cross-helicity and energy conservation) does not hold when the background varies in time.
• Klein, K. G. (2015). Predicted impacts of proton temperature anisotropy on solar wind turbulence. Physics of Plasmas.
• Klein, K., & Chandran, B. (2015). A Role for Stochastic Heating in the Near-Sun Environment. AGU Fall Meeting Abstracts, SH13E-03.
• Dorfman, S., Carter, T., Vincena, S., Sydora, R., Lin, Y., Pribyl, P., Guice, D., Rossi, G., & Klein, K. (2014). Laboratory Observations Consistent with Non-linear Decay of a Kinetic Alfv\'en Wave. AGU Fall Meeting Abstracts, SH54A-08.
• Klein, K. (2014). Diagnostics for Comparing Turbulence in Solar Wind Observations and Numerical Simulations. AGU Fall Meeting Abstracts, SH54A-05.
• Klein, K. G. (2014). PHYSICAL INTERPRETATION OF THE ANGLE-DEPENDENT MAGNETIC HELICITY SPECTRUM IN THE SOLAR WIND: THE NATURE OF TURBULENT FLUCTUATIONS NEAR THE PROTON GYRORADIUS SCALE. ApJ.
• Klein, K. G. (2014). PHYSICAL INTERPRETATION OF THE ANGLE-DEPENDENT MAGNETIC HELICITY SPECTRUM IN THE SOLAR WIND: THE NATURE OF TURBULENT FLUCTUATIONS NEAR THE PROTON GYRORADIUS SCALE. The Astrophysical Journal.
• Klein, K. G. (2014). THE VIOLATION OF THE TAYLOR HYPOTHESIS IN MEASUREMENTS OF SOLAR WIND TURBULENCE. ApJ.
• Klein, K. G. (2014). The Quasilinear Premise for the Modeling of Plasma Turbulence.
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  The quasilinear premise is a hypothesis for the modeling of plasma turbulence in which the turbulent fluctuations are represented by a superposition of randomly-phased linear wave modes, and energy is transferred among these wave modes via nonlinear interactions. We define specifically what constitutes the quasilinear premise, and present a range of theoretical arguments in support of the relevance of linear wave properties even in a strongly turbulent plasma. We review evidence both in support of and in conflict with the quasilinear premise from numerical simulations and measurements of plasma turbulence in the solar wind. Although the question of the validity of the quasilinear premise remains to be settled, we suggest that the evidence largely supports the value of the quasilinear premise in modeling plasma turbulence and that its usefulness may also be judged by the insights gained from such an approach, with the ultimate goal to develop the capability to predict the evolution of any turbulent plasma system, including the spectrum of turbulent fluctuations, their dissipation, and the resulting plasma heating.
• Klein, K. G. (2014). VALIDITY OF THE TAYLOR HYPOTHESIS FOR LINEAR KINETIC WAVES IN THE WEAKLY COLLISIONAL SOLAR WIND. ApJ.
• Klein, K., Chandran, B., & Perez, J. (2014). Predictions for Near Sun Turbulent Spectra from Synthetic Time Series. AGU Fall Meeting Abstracts, SH21B-4119.
• Chen, C., Howes, G., Bonnell, J., Mozer, F., Klein, K., & Bale, S. (2013). Kinetic scale density fluctuations in the solar wind. Solar Wind 13, 1539, 143-146.
• Klein, K. G. (2013). The kinetic plasma physics of solar wind turbulence.
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  As means of investigating the various mechanisms which contribute to the persistence of magnetized turbulence in the solar wind, this dissertation details the development of tools through which turbulence theories can be directly compared to in situ observations. This comparison is achieved though the construction of synthetic spacecraft time series from spectra of randomly phased linear eigenmodes. A broad overview of the current understanding of plasma turbulence through analytic theory, spacecraft observation, and numerical simulation is presented with particular emphasis on previous uses of linear eigenmode characteristics in the literature. An analytic treatment of relevant fluid and kinetic linear waves follows, providing motivation for the choice of three eigenmode characteristics for studying solar wind turbulence in this dissertation. The novel synthetic spacecraft time series method is next detailed and its use in describing magnetized turbulence justified. The three metrics are then individually employed as a means of comparing the turbulence models used to generate synthetic time series with in situ observations. These comparisons provide useful constraints on various proposed mechanisms for sustaining the turbulence cascade and heating the solar wind plasma.
• Klein, K., Howes, G., & TenBarge, J. (2013). How Will the Violation of Taylor's Hypothesis Alter the Turbulent Power Spectra Measured by Solar Probe Plus?. AGU Fall Meeting Abstracts, SH51C-2119.
• Klein, K. G. (2012). INTERPRETING MAGNETIC VARIANCE ANISOTROPY MEASUREMENTS IN THE SOLAR WIND. ApJ.
• Klein, K. G. (2012). THE SLOW-MODE NATURE OF COMPRESSIBLE WAVE POWER IN SOLAR WIND TURBULENCE. ApJ.
• Klein, K. G. (2012). USING SYNTHETIC SPACECRAFT DATA TO INTERPRET COMPRESSIBLE FLUCTUATIONS IN SOLAR WIND TURBULENCE. ApJ.

Proceedings Publications

• Gorman, W., & Klein, K. (2024, feb). Mind the Gap: Effects of Non-local cascades in High-beta Alfvenic Turbulence. In American Astronomical Society Meeting Abstracts, 243.
• Klein, K. (2024, jun). Dissipation and Excitation: The Role of Kinetic-Scale Waves and Instabilities in the Evolution of the Solar Wind. In American Astronomical Society Meeting Abstracts, 244.
• Klein, K., & Broeren, T. (2024, apr). Analysis Techniques for Future Multipoint, Multiscale Observatories. In EGU General Assembly Conference Abstracts.
• Martinovi{\'c}, M., & Klein, K. (2024, apr). Overview of Ion-Driven Instabilities in the Inner Heliosphere. In EGU General Assembly Conference Abstracts.
• Verscharen, D., Micera, A., Innocenti, M. E., Coburn, J., Boella, E., Pierrard, V., Liu, J., Owen, C. J., Nicolaou, G., & Klein, K. G. (2024, apr). Statistical mechanics of the electrons in the solar wind: stability and instability of whistler waves in the inner heliosphere. In EGU General Assembly Conference Abstracts.
• Walters, J., Klein, K., Juno, J., Lichko, E., & Tenbarge, J. (2024, jan). 10-Moment, Multi-Fluid Simulations of Proton Firehose Instabilities with Electron Dynamics. In APS Division of Plasma Physics Meeting Abstracts, 2024.
• Wilson, L., Klein, K., & Tenbarge, J. (2024, jan). Electron velocity distribution functions in the solar wind. In APS Division of Plasma Physics Meeting Abstracts, 2024.
• Klein, K. G. (2023, July). Enabling Discoveries in Heliospheric Science through Laboratory Plasma Experiments. In Bulletin of the American Astronomical Society.
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  Resolving 3D physics occurring on multiple spatial and temporal scales is difficult with spacecraft and computer simulations alone, but can be studied much more easily with laboratory plasma experiments. This white paper proposes increasing funding for both human and physical infrastructure development in existing laboratory plasma facilities....
• Klein, K. G. (2023, July). Firefly: The Case for a Holistic Understanding of the Global Structure and Dynamics of the Sun and the Heliosphere. In Bulletin of the American Astronomical Society.
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  This white paper is on the HMCS Firefly mission concept study. Firefly focuses on the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the deciphering of the solar cycle, the conditions leading to the explosive activity, and the structure and dynamics of the corona as it drives the heliosphere....
• Klein, K. G. (2023, October). Open mission implementation for the HelioSwarm mission. In Data.
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  The HelioSwarm mission (HS) combines data across a multi-spacecraft observatory to probe the dynamics of turbulent near-Earth plasmas (including the pristine solar wind, magnetosheath, and magnetosphere) covering multiple characteristic spatial and temporal scales. In addition to producing ground-breaking science, the HS team intends to be in the forefront of the move to open, reproducible, and reusable processes. In this presentation, we describe our plans for the transparent development of open source data processing and analysis codes for this mission. Consistent with NASA's Transform to Open Science (TOPS) mission, these open tools and software will facilitate open data and high-quality open results. The techniques of open development, drawn in particular from the established open-source software community, are not applied just to satisfy requirements by "checking the box". They are demonstrated engineering tools that enable the creation and maintenance of high quality software and systems in a manner responsive to mission and scientific needs. In particular, we discuss our prioritization of effort and context-sensitive approaches given the wide range of users for mission-maintained software, from flight through ground software to data production and analysis. We also discuss plans for code reuse between data processing and analysis to ensure consistency of results while allowing maximum flexibility in analysis....
• Bowen, T., Squire, J., Chandran, B., Verniero, J., Klein, K., Livi, R., Bale, S., Mcmanus, M., & Mallet, A. (2022, jul). Signatures of Wave-Particle Resonant Interactions Using an Orthonormal Hermite Basis. In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Larson, D., Romeo, O., Mcmanus, M., Rahmati, A., Livi, R., Whittlesey, P., Verniero, J., Stevens, M., Case, A., Kasper, J., Halekas, J., Bale, S., Goetz, K., Pulupa, M., Paulson, K., & Klein, K. (2022, jul). Intercalibration of the PSP SWEAP plasma analyzers and comparison of plasma parameters with FIELDS measurements. In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Salem, C., Bonnell, J., Roytershteyn, V., Franci, L., Verscharen, D., Klein, K., & Chaston, C. (2022, jul). Electric Field Turbulence in the Solar Wind at 1AU from MHD down to Electron Scales: Artemis Observations and Numerical Simulations. In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Stevens, M., Kasper, J., Case, A., Korreck, K., Larson, D., Livi, R., Whittlesey, P., Verniero, J., Niembro, T., Paulson, K., Huang, J., Rahmati, A., Mcmanus, M., Klein, K., & Bale, S. (2022, jul). A survey of proton and alpha particle velocity distributions as measured by the Parker Solar Probe SWEAP experiment. In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Whittlesey, P., Larson, D., Kasper, J., Stevens, M., Case, A., Romeo, O., Halekas, J., Klein, K., Rahmati, A., & Bale, S. (2022, jul). Sunward Strahl in Magnetic Field Reversals: Solar Connectivity and Magnetic Topology during Rapid Switchbacks in Parker Solar Probe Fast Electron Data. In 44th COSPAR Scientific Assembly. Held 16-24 July, 44.
• Afshari, A., Howes, G., Kletzing, C., & Klein, K. (2021, jan). Characterizing velocity space structures of ion cyclotron turbulence in the Earth's magnetosheath plasma. In APS Division of Plasma Physics Meeting Abstracts, 2021.
• Alterman, B. L., Livi, S., Stevens, M., Kasper, J., & Klein, K. (2021, jan). On the limitations of applying reduced free energy parameter spaces to proton beams in the solar wind. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• Klein, K., & Spence, H. (2021, jan). HelioSwarm: The Nature of Turbulence in Space Plasmas. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• Klein, K., Bonnell, J., Larson, D., Dudok, D., Macdowall, R., Bale, S., Stevens, M., Kasper, J., Malaspina, D., Livi, R., Korreck, K., Whittlesey, P., Case, A., Goetz, K., Harvey, P., & Pulupa, M. (2021, jan). Alfvenic Jets, Waves, and Turbulence: Initial Results from Parker Solar Probe's first three orbits. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• Klein}, K., Team, P., Team, P., Team, P., & Team, {. (2021, jan). Parker Solar Probe: Advancing Our Understanding of Plasmas in the Young Solar Wind. In APS Division of Plasma Physics Meeting Abstracts, 2021.
• Lichko, E., & Klein, K. (2021, jan). Effects of distribution structure on predictions of plasma behavior in marginally unstable plasma. In APS Division of Plasma Physics Meeting Abstracts, 2021.
• Martinovi{\'c}, M., Klein, K., Huang, J., Chandran, B., Kasper, J., Lichko, E., Bowen, T., Chen, C., Matteini, L., Stevens, M., Case, A., & Bale, S. (2021, apr). Multiscale Solar Wind Turbulence Properties inside and near Switchbacks measured by Parker Solar Probe. In EGU General Assembly Conference Abstracts.
• Spence}, H., Klein, K., & Team, {. S. (2021, apr). HelioSwarm: The Nature of Turbulence in Space Plasmas. In EGU General Assembly Conference Abstracts.
• Verniero, J., Bonnell, J., Larson, D., Rahmati, A., Livi, R., Klein, K., Sharma, P. P., Whittlesey, P., Bowen, T., & McManus, M. (2021, jan). Evidence of wave-particle interactions observed by Parker Solar Probe. In 43rd COSPAR Scientific Assembly. Held 28 January - 4 February, 43.
• Case, A., Kasper, J., Stevens, M., Korreck, K., Mello, T., Lamirato, T., Larson, D., Whittlesey, P., Livi, R., Horbury, T., Klein, K., Velli, M., Bale, S., Pulupa, M., Malaspina, D., Bonnell, J., Harvey, P., Goetz, K., Wit, T., & MacDowall, R. (2020, jan). Solar Probe Cup \textemdash First Results. In American Astronomical Society Meeting Abstracts \#235, 235.
• Bert, C. M., Kasper, J. C., Klein, K. G., Case, A. W., Maksimovic, M., & Zaslavsky, A. (2019, May). Simple Dependence of Proton Temperature on Solar Wind Speed and Compression in High Alfven Mach Number Solar Wind. In Solar Heliospheric and INterplanetary Environment (SHINE 2019).
• Bookbinder, J., Spence, H., & Klein, K. G. (2019, Jan). Revealing the Multiscale Nature of Turbulence with a Spacecraft Swarm. In American Astronomical Society Meeting Abstracts \#233, 233.
• Chu, T. L., Howes, G., Klein, K., Liu, Y., & TenBarge, J. (2019, May). Collisionless Energy Transfer in Kinetic Turbulence: Field-Particle Correlations in Fourier Space. In Solar Heliospheric and INterplanetary Environment (SHINE 2019).
• Holst, B., Chandran, B., Borovikov, D., Klein, K., Manchester, I. V., & Kasper, J. (2019, May). Magnetohydrodynamic Simulations for the First and Second Parker Solar Probe Encounter. In Solar Heliospheric and INterplanetary Environment (SHINE 2019).
• Horvath, S., Howes, G., & Klein, K. (2019, Jan). Landau Damping Signatures in Realistic and Down-Sampled Simulations of MMS Data: Characterizing the Use of Field-Particle Correlations. In APS Division of Plasma Physics Meeting Abstracts, 2019.
• Howes, G. G., McCubbin, A. J., Horvath, S. A., Montag, P., Verniero, J. L., Klein, K. G., Tenbarge, J. M., Chen, C. H., Schroeder, J. W., & Valentini, F. (2019, Jan). Using Field-Particle Correlations to Diagnose Particle Energization in Turbulence, Magnetic Reconnection, and Shocks. In APS Division of Plasma Physics Meeting Abstracts, 2019.
• Klein, K. G., TenBarge, J., & Spence, H. (2019, Jan). Testing the Utility of a Swarm of Spacecraft to Study Magnetized Turbulence. In American Astronomical Society Meeting Abstracts \#233, 233.
• Klein, K., & Martinovic, M. (2019, Jan). The Ubiquity of Ion-Driven Microinstabilities in the Inner Heliosphere. In APS Division of Plasma Physics Meeting Abstracts, 2019.
• Martinovic, M., Klein, K. G., & Bourouaine, S. (2019, May). Radial evolution of stochastic heating in low-beta solar wind. In Solar Heliospheric and INterplanetary Environment (SHINE 2019).
• McCubbin, A., Howes, G., & Klein, K. (2019, Jan). Characterization of Phase Space Energy Transfer in 2-D Collisionless Magnetic Reconnection using Field-Particle Correlations. In APS Division of Plasma Physics Meeting Abstracts, 2019.
• Salem, C., Bonnell, J., Hanson, E., Chaston, C., Klein, K., Franci, L., Verscharen, D., & Sundkvist, D. (2019, Apr). Understanding Electron-Scale Electric Field Fluctuations in Solar wind Kinetic Turbulence: Artemis Observations. In EGU General Assembly Conference Abstracts.
• Tenbarge, J., Juno, J., Howes, G., Klein, K., & Hakim, A. (2019, Jan). Diagnosing Energy Dissipation in Fully Kinetic Continuum Vlasov-Maxwell Plasmas. In APS Division of Plasma Physics Meeting Abstracts, 2019.
• Vech, D., Klein, K. G., Mallet, A., & Kasper, J. C. (2019, May). Signatures of in situ generated ion-scale coherent structures in the solar wind by magnetic reconnection. In Solar Heliospheric and INterplanetary Environment (SHINE 2019).
• Vech, D., Mallet, A., Klein, K., & Kasper, J. (2019, Apr). Magnetic Reconnection May Control the Ion-scale Spectral Break of Solar Wind Turbulence. In EGU General Assembly Conference Abstracts.
• Verniero, J., Howes, G., Stewart, D., Klein, K., & Larson, D. (2019, May). Particle Arrival Time Correlation for Heliophysics (PATCH): Using discretized distributions for identifying turbulent dissipation mechanisms onboard spacecraft. In Solar Heliospheric and INterplanetary Environment (SHINE 2019).
• Klein, K. G. (2018). Kinetic scale density fluctuations in the solar wind.
• Wilson, L., Stevens, M., Kasper, J., Klein, K., Maruca, B., Bale, S., Bowen, T., Pulupa, M., & Salem, C. (2018, jul). The statistical properties of solar wind temperature parameters near 1 AU. In Solar Heliospheric and INterplanetary Environment (SHINE 2018).
• Howes, G., McCubbin, A., & Klein, K. (2017, jul). Spatially Localized Particle Energization by Landau Damping in Current Sheets. In Solar Heliospheric and INterplanetary Environment (SHINE 2017).

Others

• Klein, K. G. (2024, March). Analysis Techniques for Future Multipoint, Multiscale Observatories. https://doi.org/10.5194/egusphere-egu24-10703
• Klein, K. G. (2023, April). Estimation of the error on the calculation of the pressure-strain term: application in the terrestrial magnetosphere. https://doi.org/10.22541/essoar.168167247.77367409/v1
• Klein, K. G. (2023, July). How Firefly Would Advance Our Understanding of the Solar Wind. Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033 white paper e-id. 053.
  More info

  The proposed Firefly mission would provide nearly continuous 4pi-steradian measurements of the magnetic field on the solar photosphere, as well as in situ measurements of the high-heliolatitude solar wind. These new observational capabilities would lead to significant advances in our understanding of the solar wind....
• Klein, K. G. (2023, July). Unveiling plasma energization and energy transport in the Earth's Magnetospheric System: the need for future coordinated multiscale observations. Decadal Survey for Solar and Space Physics (Heliophysics) 2024-2033 white paper e-id. 337.
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  This WP highlights the importance of plasma scale coupling to study plasma energization and energy transport in the terrestrial Magnetospheric System. The Plasma Observatory is an ESA mission concept to resolve fluid and ion scales. MagCon is a NASA mission concept aiming at studying larger mesoscales. Resolving the couplings among all these scales requires a strong international collaboration....
• Klein, K. G. (2023, May). Generalised Ohm’s Law in the Magnetosheath: How do plasma conditions impact turbulent electric fields?. https://doi.org/10.5194/egusphere-egu23-13782
• Klein, K. G. (2022, March). HelioSwarm: The Nature of Turbulence in Space Plasma. https://doi.org/10.5194/egusphere-egu22-12990
• Klein, K. G. (2021, December). Plasma Parameters from Quasi-Thermal Noise Observed by Parker Solar Probe: A New Model for the Antenna Response. https://doi.org/10.1002/essoar.10509010.1
• Klein, K. G. (2021, March). HelioSwarm: Leveraging Multi-Point, Multi-Scale Spacecraft Observations to Characterize Turbulence. https://doi.org/10.5194/egusphere-egu21-6812
• Klein, K. G. (2021, March). The Near-Sun Streamer Belt Solar Wind: Turbulence and Solar Wind Acceleration. https://doi.org/10.5194/egusphere-egu21-3426
• Klein, K. G. (2013, December). The Kinetic Plasma Physics of Solar Wind Turbulence. http://ir.uiowa.edu/etd/5000/