Wave–Particle Equilibria with Heavy Ions in Weakly Collisional Space Plasmas

Abstract Space plasmas are weakly collisional since characteristic timescales 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 nonthermal 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 solar wind. We use the Arbitrary Linear Plasma Solver 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 waves and particles.