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1. Whistler-regulated Magnetohydrodynamics: Transport Equations for Electron Thermal Conduction in the High-β Intracluster Medium of Galaxy Clusters

   https://doi.org/10.3847/1538-4357/ac1ff1  

   Drake, J. F.; Pfrommer, C.; Reynolds, C. S.; Ruszkowski, M.; Swisdak, M.; Einarsson, A.; Thomas, T.; Hassam, A. B.; Roberg-Clark, G. T. (December 2021, The Astrophysical Journal)

   Abstract Transport equations for electron thermal energy in the high- β e intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds V w ∼ v te / β e ≪ v te dominate the turbulent spectrum, with v te the electron thermal speed and β e ≫ 1 the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scattering when L c > L / β e , with L c the classical electron mean free path and L the electron temperature scale length, and (2) in the whistler-dominated regime the electron thermal flux is controlled by both advection at V w and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. Consequences include: (1) the Field length decreases, extending the domain of thermal instability to smaller length scales, (2) the heat flux temperature dependence changes from T e 7 / 2 / L to V w nT e ∼ T e 1 / 2 , (3) the magneto-thermal- and heat-flux-driven buoyancy instabilities are impaired or completely inhibited, and (4) sound waves in the ICM propagate greater distances, as inferred from observations. This description of thermal transport can be used in macroscale ICM models. 

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2. A study of particle acceleration, heating, power deposition, and the damping length of kinetic Alfvén waves in non-Maxwellian coronal plasma

   https://doi.org/10.1051/0004-6361/202452376  

   Ayaz, S; Zank, G P; Khan, I A; Li, G; Rivera, Y J (February 2025, Astronomy & Astrophysics)

   Context.The heating of the solar corona and solar wind, particularly through suprathermal particles and kinetic Alfvén waves (KAWs) within the 0–10 R_Sunrange, has been a subject of great interest for many decades. This study investigates and explores the acceleration and heating of charged particles and the role of KAWs in the solar corona. Aims.We investigate how KAWs transport energy and accelerate and heat the charged particles, focusing on the behavior of perturbed electromagnetic (EM) fields, the Poynting flux vectors, net power transfer through the solar flux loop tubes, resonant particles’ speed, group speed, and the damping length of KAWs. The study examines how these elements are influenced by suprathermal particles (κ) and the electron-to-ion temperature ratios (T_e/T_i). Methods.We used kinetic plasma theory coupled with the Vlasov-Maxwell model to investigate the dynamics of KAWs and particles. We assumed a collisionless, homogeneous, and low-beta electron-ion plasma in which Alfvén waves travel in the kinetic limits; that is,m_e/m_i ≪ β ≪ 1. Furthermore, the plasma incorporates suprathermal high-energy particles, necessitating an appropriate distribution function to accurately describe the system. We adopted the Kappa distribution function as the most suitable choice for our analysis. Results.The results show that the perturbed EM fields are significantly influenced byκand the effect of T_e/T_i. We evaluate both the parallel and perpendicular Poynting fluxes and find that the parallel Poynting flux (S_z) dissipates gradually for lowerκvalues. In contrast, the perpendicular flux (S_x) dissipates quickly over shorter distances. Power deposition in solar flux tubes is significantly influenced byκand T_e/T_i. We find that particles can heat the solar corona over long distances (R_Sun) in the parallel direction and short distances in the perpendicular direction. The group velocity of KAWs increases for lowerκvalues, and the damping length, L_G, is enhanced under lowerκ, suggesting longer energy transport distances (R_Sun). These findings offer a comprehensive understanding of particle-wave interactions in the solar corona and wind, with potential applications for missions such as the Parker Solar Probe, (PSP), and can also apply to other environments where non-Maxwellian particle distributions are frequently observed. 

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3. Wave Generation by Flare-accelerated Ions and Implications for ³ He Acceleration

   https://doi.org/10.3847/1538-4357/ad217f  

   Fitzmaurice, A; Drake, J F; Swisdak, M (March 2024, The Astrophysical Journal)

   Abstract The waves generated by high-energy proton and alpha particles streaming from solar flares into regions of colder plasma are explored using particle-in-cell simulations. Initial distribution functions for the protons and alphas consist of two populations: an energetic, streaming population represented by an anisotropic (T_∥>T_⊥), one-sided kappa function and a cold, Maxwellian background population. The anisotropies and nonzero heat fluxes of these distributions destabilize oblique waves with a range of frequencies below the proton cyclotron frequency. These waves scatter particles out of the tails of the initial distributions along constant-energy surfaces in the wave frame. Overlap of the nonlinear resonance widths allows particles to scatter into near-isotropic distributions by the end of the simulations. The dynamics of³He are explored using test particles. Their temperatures can increase by a factor of nearly 20. Propagation of such waves into regions above and below the flare site can lead to heating and transport of³He into the flare acceleration region. The amount of heated³He that will be driven into the flare site is proportional to the wave energy. Using values from our simulations, we show that the abundance of³He driven into the acceleration region should approach that of⁴He in the corona. Therefore, waves driven by energetic ions produced in flares are a strong candidate to drive the enhancements of³He observed in impulsive flares. 

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4. Particle-in-cell simulations – ion beam instabilities and the generation of Alfvén and whistler waves in low β plasma

   https://doi.org/10.1093/mnras/stad2897  

   Che, H; Benz, A O; Zank, G P (September 2023, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Ion beam-driven instabilities in a collisionless space plasma with low β, i.e. low plasma and magnetic pressure ratio, are investigated using particle-in-cell (PIC) simulations. Specifically, the effects of different ion drift velocities on the development of Buneman and resonant electromagnetic (EM) right-handed (RH) ion beam instabilities are studied. Our simulations reveal that both instabilities can be driven when the ion beam drift exceeds the theoretical thresholds. The Buneman instability, which is weakly triggered initially, dissipates only a small fraction of the kinetic energy of the ion beam while causing significant electron heating, owing to the small electron-ion mass ratio. However, we find that the ion beam-driven Buneman instability is quenched effectively by the resonant EM RH ion beam instability. Instead, the resonant EM RH ion beam instability dominates when the ion drift velocity is larger than the Alfvén speed, leading to the generation of RH Alfvén waves and RH whistler waves. We find that the intensity of Alfvén waves decreases with decrease of ion beam drift velocity, while the intensity of whistler waves increases. Our results provide new insights into the complex interplay between ion beams and plasma instabilities in low β collisionless space plasmas. 

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5. Modeling Hot, Anisotropic Ion Beams in the Solar Wind Motivated by the Parker Solar Probe Observations near Perihelia

   https://doi.org/10.3847/1538-4357/adc812  

   Ofman, Leon; Boardsen, Scott A; Mostafavi, Parisa; Jian, Lan K; Sadykov, Viacheslav M; Klein, Kristopher; Martinović, Mihailo (May 2025, The Astrophysical Journal)

   Abstract Recent observations of the solar wind ions by the SPAN-I instruments on board the Parker Solar Probe (PSP) spacecraft at solar perihelia (Encounters) 4 and closer find ample evidence of complex anisotropic non-Maxwellian velocity distributions that consist of core, beam, and “hammerhead” (i.e., anisotropic beam) populations. The proton core populations are anisotropic, withT_⊥/T_∥ > 1, and the beams have super-Alfvénic speed relative to the core (we provide an example from Encounter 17). Theα-particle population shows similar features to the protons. These unstable velocity distribution functions (VDFs) are associated with enhanced, right-hand (RH) and left-hand (LH) polarized ion-scale kinetic wave activity, detected by the FIELDS instrument. Motivated by PSP observations, we employ nonlinear hybrid models to investigate the evolution of the anisotropic hot-beam VDFs and model the growth and the nonlinear stage of ion kinetic instabilities in several linearly unstable cases. The models are initialized with ion VDFs motivated by the observational parameters. We find rapidly growing (in terms of proton gyroperiods) combined ion-cyclotron and magnetosonic instabilities, which produce LH and RH ion-scale wave spectra, respectively. The modeled ion VDFs in the nonlinear stage of the evolution are qualitatively in agreement with PSP observations of the anisotropic core and “hammerhead” velocity distributions, quantifying the effect of the ion kinetic instabilities on wind plasma heating close to the Sun. We conclude that the wave–particle interactions play an important role in the energy transfer between the magnetic energy (waves) and random particle motion, leading to anisotropic solar wind plasma heating. 

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