Particle‐in‐cell simulations of Alfvén‐cyclotron wave scattering: Proton velocity distributions

Abstract

Alfvén‐cyclotron fluctuations propagating parallel or antiparallel to the background magnetic field Bo help shape solar wind ion velocity distributions fi(v). Alfvén waves may be generated at low, nonresonant frequencies and, by propagation through the inhomogeneous plasma, attain ion cyclotron resonances and thereby scatter the fi(v) to anisotropy. Ion anisotropies of sufficient magnitude lead to the growth of ion cyclotron instabilities; the resulting enhanced Alfvén‐cyclotron fluctuations scatter ions so as to reduce the anisotropy. Here particle‐in‐cell simulations were carried out in a magnetized, homogeneous, collisionless plasma of electrons and one ion species to study the evolution of the fi(v) in response to both of these scattering processes. A simulation with a spectrum of right‐traveling Alfvén‐cyclotron fluctuations imposed at t = 0 leads to non‐Maxwellian ion distributions. The computations show that the pitch angle scattering of left‐traveling (v∥ < 0) ions becomes weaker as v∥ becomes less negative but also that this scattering can transport ions across the condition v∥ = 0, where the subscript denotes the direction parallel to Bo.