Wave-particle equilibria with heavy ions in weakly collisional space plasmas

TL;DR

This paper investigates how wave-particle interactions influence the velocity distributions of heavy ions in weakly collisional space plasmas, leading to steady wave-particle equilibria that minimize energy transfer.

Contribution

It introduces a kinetic framework combining the Boris algorithm and ALPS code to analyze wave-particle dynamics and equilibrium states in two-ion plasmas like the solar wind.

Findings

Heavy ions evolve into steady wave-particle equilibria.

Wave activity modifies ion velocity distributions.

Energy transfer between waves and particles is minimized at equilibrium.

Abstract

Space plasmas are weakly collisional since characteristic time scales related to Coulomb collisions are much larger than those of Larmor gyration or wave--particle interactions. Thus, wave activity is likely to drive some of the non-thermal features that are observed in space plasma velocity distributions, such as temperature anisotropy, beams, and skewness. Therefore, we study how wave--particle interactions shape the velocity distribution functions of minor ions, and how these ions and their statistical properties modify the dispersion relation of electromagnetic waves. To achieve this, we derive the motion of heavy ions in electromagnetic waves using the Boris algorithm. We take the waves to be solutions of the fully kinetic dispersion relation of electromagnetic waves in two-ion component plasmas with parameters representative of the solar wind. We use the Arbitrary Linear Plasma Solver (ALPS) code to derive the linear Vlasov--Maxwell dispersion relation based on the actual distribution of the ions. The test-particles are initially in thermal equilibrium, and their distribution evolves due to interactions with the waves. By solving the dispersion relation using the evolved distributions, we show that the system evolves into a steady wave--particle equilibrium, which is characterized by a minimization of the interaction and energy transfer between wave and particles.