Papers – Excitation and Dissipation of Kinetic-Scale Fluctuations in Space Plasmas

Peer Reviewed Publications

How the Oblique Drift Instability Alters Solar Wind Heating and Constrains the Distribution of Solar Wind Observations Martinovic et al 2026 PRL

This paper addresses how alpha–proton differential flow and related ion-scale free energy are constrained by the oblique drift instability in the near-Sun solar wind. The central point is that oblique modes can modify the partition of turbulent and wave-mediated heating in ways that are not captured by considering only parallel-propagating or standard anisotropy-driven thresholds. By connecting Parker Solar Probe observations with kinetic stability constraints, the work argues that the observed distribution of solar-wind plasma states is partly shaped by rapid instability-driven relaxation. For the ISSI team theme, it is a useful example of how kinetic-scale wave excitation can both dissipate free energy and act as a selection effect on the VDFs that spacecraft observe.

Ion-Scale Wave Emission and Absorption for Non-Maxwellian Velocity Distributions in the Inner Heliosphere Klein et al 2026 GRL

This study uses measured, non-Maxwellian ion velocity distributions from Parker/SPAN-I to calculate ion-scale wave emission and absorption in the inner heliosphere, rather than replacing the plasma with idealized Maxwellian or bi-Maxwellian fits. The key physics is that fine structure in the observed VDFs changes the resonant wave–particle energy exchange, so the damping or growth of a mode can differ substantially from what a moment-based model predicts. The paper therefore makes a strong methodological case for using resolved distribution functions directly in linear response calculations when evaluating dissipation channels. Within the ISSI context, it is a clear bridge between spacecraft VDF measurements, ALPS-style arbitrary-distribution dispersion analysis, and the interpretation of kinetic-scale fluctuation power.

The Damping and Instability of Ion-acoustic Waves in the Solar Wind: Solar Orbiter Observations Ran et al 2026 ApJ

Ran et al. use Solar Orbiter PAS measurements and a Gaussian-mixture decomposition of proton and alpha-particle VDFs to examine ion-acoustic wave damping and instability with measured distributions. A major result is that fine-scale proton VDF structure can reduce ion-acoustic damping, and in some cases drive the mode unstable even when bi-Maxwellian representations of the same plasma predict strong damping. This is particularly interesting because ion-acoustic waves are often assumed to be heavily damped under solar-wind conditions with comparable ion and electron temperatures. The paper reinforces the broader point that kinetic dissipation estimates based on smooth fitted distributions can miss the sign and magnitude of resonant energy transfer.

Knudsen Number as a Nonthermal Parameter: Possible Origin of Skewness in Space Plasma Distributions Gallo-Méndez et al 2026 ApJ

This paper develops the idea that the Knudsen number can serve as a physically motivated nonthermal parameter for space-plasma velocity distributions. The work is aimed at connecting departures from local thermodynamic equilibrium, especially skewness, to the relative importance of transport over collisional relaxation. That framing is useful because it treats VDF asymmetry not only as a fitted statistical feature but as a consequence of weak collisionality and spatial gradients. For kinetic-scale dissipation studies, the paper helps connect macroscopic transport conditions to the velocity-space structures that can provide free energy for wave growth or modify damping.

Wave–Particle Equilibria with Heavy Ions in Weakly Collisional Space Plasmas Villarroel-Sepúlveda et al 2026 ApJ

This paper studies how heavy-ion velocity distributions evolve under interaction with electromagnetic waves, and how the evolved distributions in turn modify the kinetic dispersion relation. The authors use Boris-orbit test-particle evolution in wave fields, then feed the resulting ion distributions into ALPS to solve the fully kinetic Vlasov–Maxwell response. A central result is that the system evolves toward a wave–particle equilibrium in which resonant interaction and energy transfer are minimized. The work is especially relevant for interpreting minor-ion signatures in weakly collisional plasmas, where nonthermal heavy-ion structure can be both a consequence and a regulator of kinetic wave activity.

Electron-scale Magnetic Holes Generation Driven by Whistler-to-Bernstein Mode Conversion in Fully Kinetic Plasma Turbulence Joaquín Espinoza-Troni et al 2025 ApJ

Espinoza-Troni et al. investigates the generation of electron-scale magnetic holes in fully kinetic simulations of decaying turbulence under magnetosheath-like conditions. The proposed mechanism begins with turbulent velocity shears that create localized electron temperature anisotropy, which then excites oblique whistler waves; as these waves propagate through an inhomogeneous turbulent background, they acquire electrostatic Bernstein-like character. The resulting electrostatic structures produce current filaments and electron vortices that locally depress the magnetic-field amplitude, forming electron-scale magnetic holes. The paper is valuable because it links coherent electron-scale structures directly to turbulent cascade physics and wave-mode conversion, rather than treating magnetic holes as isolated equilibrium structures.

Heat-Fux Instabilities of Regularized Kappa Distributed Strahl Electrons Resolved with ALPS Schröder et al 2025 ApJ

This work applies ALPS to heat-flux instabilities driven by regularized Kappa-distributed strahl electrons, a distribution class that better captures suprathermal tails while retaining well-behaved moments. The paper identifies whistler and firehose heat-flux instability thresholds, and shows that Maxwellian or standard Kappa idealizations can misestimate the growth rates and relative importance of those modes. The ALPS calculation is important because the analytical kinetic response for regularized Kappa distributions is cumbersome, making arbitrary-distribution numerical solvers especially useful. In the broader context of solar-wind electron physics, the study sharpens how strahl and suprathermal structure regulate heat flux through self-generated waves.

The dielectric response of plasmas with arbitrary gyrotropic velocity distributions Klein & Verscharen 2025 PoP

This paper presents and validates an updated formulation of ALPS for calculating the dielectric tensor and normal-mode response of plasmas with arbitrary gyrotropic VDFs. The methodological advance is an improved analytic continuation based on a polynomial representation, designed to handle weakly and moderately damped modes more robustly. The paper demonstrates continuity between bi-Maxwellian and arbitrary-VDF calculations while also showing that detailed VDF structure can change mode polarization, emission, and absorption. For the ISSI team, it provides much of the formal machinery needed to move from measured distribution functions to quantitative kinetic-scale wave–particle energy exchange.

Quasi-linear approach of bi-Kappa distributed electrons with dynamic κ parameter EMEC instability Moya et al 2026 A&A

Moya et al. extend moment-based quasilinear theory for the electromagnetic electron-cyclotron, or whistler, instability by allowing the Kappa parameter itself to evolve dynamically. The key conceptual move is to couple the usual temperature evolution to a kurtosis equation, relaxing the assumption that the suprathermal tail remains fixed while the core relaxes. The results show that instability saturation often decreases κ, corresponding to enhanced suprathermalization, although some low-beta regimes evolve toward a more Maxwellian distribution. This is important because it suggests that quasilinear relaxation of anisotropy does not necessarily move a weakly collisional plasma toward thermal equilibrium; the waves can simultaneously reduce anisotropy and strengthen suprathermal tails.

Total internal reflection of electromagnetic waves in Kappa-distributed plasmas Correa-Gazmuri and Moya, 2025, Physica Scripta

This paper studies electromagnetic-wave total internal reflection in nonmagnetized plasmas described by Kappa distributions. Using kinetic theory, the authors derive how the permittivity, refractive index, and critical angle depend on wave number, κ, temperature, and density. A notable result is that the critical angle depends sensitively on the relative Kappa parameter and temperature of the two media, with a minimum near wavelengths comparable to the inertial length. The paper shows that suprathermal structure can alter how electromagnetic fluctuations transmit, reflect, and transform across plasma regions.

Proton-acoustic wave effects on the relaxation of proton transverse heating in magnetized plasmas Quijada et al 2025, Physica Scripta

Quijada et al. examine the coupled quasilinear evolution of Alfvén-cyclotron waves and ion-acoustic waves in a collisionless proton–electron plasma. The paper emphasizes that although transverse electromagnetic and electrostatic modes are decoupled in linear kinetic theory, resonant wave–particle interactions allow them to exchange energy indirectly through the evolving particle distributions. Their parameter survey shows that Alfvén-cyclotron waves can drive perpendicular proton heating and cyclotron instability at low proton beta, while ion-acoustic waves increasingly absorb fluctuation energy as electron-to-proton temperature ratio rises. The result is a useful regulatory picture: electrostatic ion-acoustic activity can limit the efficiency of cyclotron-driven transverse heating and redirect dissipation into Landau-resonant channels.

Dissertations

This dissertation brings together two themes: kinetic Alfvén wave dispersion in multi-ion plasmas and magnetic-field generation in relativistic astrophysical contexts. The KAW component is directly aligned with the ISSI team’s interest in how composition, especially heavy ions, modifies kinetic-scale wave propagation and polarization. Heavy-ion effects are important because they introduce additional resonant structure and alter the dispersive response in ways that can matter in magnetospheric and solar-wind environments. The dissertation therefore provides a broader theoretical foundation for several of the team’s themes: kinetic Alfvénic turbulence, multi-species dispersion, and the role of non-proton populations in collisionless energy conversion.