Ion-Scale Wave Emission and Absorption for Non-Maxwellian Velocity Distributions in the Inner Heliosphere
Klein, K. G., Larson, D., Livi, R., Martinović, M. M., Rahmati, A. et al. (2026). Geophysical Research Letters. DOI · SciX
Using measured ion velocity distributions from Parker Solar Probe and the Arbitrary Linear Plasma Solver (ALPS), we investigate how departures from Maxwellian distributions affect the excitation and damping of ion-scale plasma waves. We demonstrate that realistic velocity-space structure can substantially modify wave–particle energy exchange, underscoring the importance of using observed distributions rather than simplified analytic models to understand plasma heating.
How the Oblique Drift Instability Alters Solar Wind Heating and Constrains the Distribution of Solar Wind Observations
Martinović, Mihailo M., Klein, Kristopher G., Ofman, Leon, Yogesh, Howes, Gregory G. et al. (2026). Physical Review Letters. DOI · SciX
We investigate how relative drifts between solar-wind ion populations drive an oblique plasma instability that redistributes energy, heats ions, and regulates differential flows. This mechanism provides a possible explanation for observed limits on plasma beta and ion drift speeds in the near-Sun solar wind, connecting kinetic instability theory to the plasma conditions measured by Parker Solar Probe.
The dielectric response of plasmas with arbitrary gyrotropic velocity distributions
Klein, K. G., Verscharen, D. (2025). Physics of Plasmas. DOI · SciX
We extend the capabilities of ALPS through an improved analytic continuation method for calculating the response of plasmas with arbitrary gyrotropic velocity distributions. This development enables more accurate treatment of damped plasma waves and provides a framework for investigating how non-Maxwellian structure modifies wave properties, stability, and particle–wave energy exchange.
HelioSwarm: A Multipoint, Multiscale Mission to Characterize Turbulence
Klein, Kristopher G., Spence, Harlan, Alexandrova, Olga, Argall, Matthew, Arzamasskiy, Lev et al. (2023). Space Science Reviews. DOI · SciX
We describe the scientific objectives and measurement strategy of NASA’s HelioSwarm mission, which will use nine spacecraft to investigate the three-dimensional, multiscale structure of plasma turbulence. By providing simultaneous measurements across a range of spatial separations, HelioSwarm will enable investigations of turbulent energy transfer, intermittency, and plasma structures that are inaccessible to traditional single-spacecraft measurements.
The multi-scale nature of the solar wind
Verscharen, Daniel, Klein, Kristopher G., Maruca, Bennett A. (2019). Living Reviews in Solar Physics. DOI · SciX
This review synthesizes the physical processes governing the solar wind, from large-scale expansion and turbulent dynamics to kinetic instabilities and particle heating. We emphasize how processes operating across widely separated spatial and temporal scales interact to shape the observed plasma, providing a broad framework for understanding the solar wind as a laboratory for fundamental plasma physics.
Strong Preferential Ion Heating is Limited to within the Solar Alfvén Surface
Kasper, Justin C., Klein, Kristopher G. (2019). The Astrophysical Journal Letters. DOI · SciX
Using the evolution of the helium-to-proton temperature ratio as a diagnostic of preferential ion heating, we identify a close correspondence between the outer boundary of strong ion heating and the solar Alfvén surface. The result suggests that the most efficient preferential heating occurs within the magnetically dominated region of the solar atmosphere, providing an observational constraint on coronal heating processes and a prediction for Parker Solar Probe measurements.
Evidence for electron Landau damping in space plasma turbulence
Chen, C. H. K., Klein, K. G., Howes, G. G. (2019). Nature Communications. DOI · SciX
We apply the field–particle correlation technique to spacecraft measurements in Earth’s magnetosheath, identifying a velocity-space signature consistent with electron Landau damping. This work provides observational evidence for a specific mechanism of collisionless turbulent energy transfer and demonstrates how measurements of particle distributions can distinguish the physical processes responsible for plasma energization.
ALPS: the Arbitrary Linear Plasma Solver
Verscharen, D., Klein, K. G., Chandran, B. D. G., Stevens, M. L., Salem, C. S. et al. (2018). Journal of Plasma Physics. DOI · SciX
We introduce ALPS, a numerical framework for calculating the linear wave and stability properties of plasmas with arbitrary gyrotropic velocity distributions. By moving beyond the analytic distributions commonly assumed in plasma dispersion calculations, ALPS makes it possible to investigate wave–particle interactions and kinetic instabilities using more realistic representations of measured and simulated plasmas.
Majority of Solar Wind Intervals Support Ion-Driven Instabilities
Klein, K. G., Alterman, B. L., Stevens, M. L., Vech, D., Kasper, J. C. (2018). Physical Review Letters. DOI · SciX
Applying the Nyquist instability criterion to measured ion distributions from the Wind spacecraft, we find that a majority of sampled solar-wind intervals support at least one ion-driven instability. By accounting simultaneously for temperature anisotropies and differential flows among ion populations, this work demonstrates that the solar wind is more broadly susceptible to kinetic instability than suggested by simplified single-parameter instability thresholds.
Diagnosing collisionless energy transfer using field-particle correlations: gyrokinetic turbulence
Klein, Kristopher G., Howes, Gregory G., Tenbarge, Jason M. (2017). Journal of Plasma Physics. DOI · SciX
We demonstrate that field–particle correlations constructed from single-point measurements of electromagnetic gyrokinetic turbulence can identify velocity-space signatures of collisionless energy transfer. This establishes a pathway for diagnosing the kinetic mechanisms responsible for turbulent dissipation using spacecraft measurements, connecting numerical plasma turbulence studies with observational investigations of particle energization.