Consequences of proton and alpha anisotropies in the solar wind: Hybrid simulations

Abstract

Alfvén fluctuations propagating away from the Sun in the solar corona and solar wind transfer energy via cyclotron resonances to ions of successively larger charge‐to‐mass ratios. This can yield T⟂/T∥ > 1 for each ion species, where the subscripts refer to directions relative to the background magnetic field. If these anisotropies become sufficiently large, they drive electromagnetic ion cyclotron instabilities. This paper describes two‐dimensional hybrid simulations of a collisionless, homogeneous, magnetized plasma to study the consequences of scattering by enhanced field fluctuations from such instabilities. The ions in the simulations consist of majority protons and minority alpha particles with initial bi‐Maxwellian velocity distributions and representative solar wind parameters including a nonzero alpha/proton relative speed. The simulations show that both helium and proton cyclotron instabilities reduce the driving anisotropy, reduce initial differences between the proton and alpha particle anisotropies, and, as a new result, usually reduce initial alpha/proton speeds. These results are somewhat different from theoretical predictions of ion scattering by interaction with outward propagating Alfvén‐cyclotron waves but are consistent with observations from Ulysses.