Ion Scattering in the Solar Wind and Solar Corona: Particle-in-Cell Simulations

Alfv\'en-cyclotron fluctuations at sufficiently short wavelengths and at propagation approximately parallel or antiparallel to a background magnetic field ${\bf B}_o$ in a relatively uniform, collisionless plasma can interact with protons and heavy ions. A cyclotron resonance between such fluctuations and the thermal velocity distribution of an ion species enables strong pitch-angle scattering, typically leading to an increase in the perpendicular (to ${\bf B}_o$) energies of that species. Particle-in-cell simulations in a magnetized, homogeneous, collisionless plasma of electrons, protons, and one very tenuous species of heavy ions are used to study the heavy ion response as a function of the initial magnetic power spectrum, the proton $\beta$, and the heavy-ion/proton relative speed. The goal of these simulations is to obtain better understanding of how Alfv\'en-cyclotron scattering may heat heavy ions in the solar corona. Magnetosonic-whistler fluctuations at ${\bf k} \times {\bf B}_o =$ 0 and sufficiently high $\beta_p$ can also scatter ions; however, this process typically leads to an increase in parallel ion energies. PIC simulations are also used to study proton scattering by such fluctuations; the results suggest that magnetosonic fluctuations may play a role in heating solar wind protons.