Excitation of whistler waves via nonlinear scattering in the ion ring instability

An ion ring velocity distribution is unstable and excites lower hybrid waves. When these waves reach sufficient amplitude, nonlinear scattering of lower hybrid waves can excite whistler and magnetosonic waves and contribute to saturation of the lower hybrid instability. Ring distributions, and subsequent instabilities and waves, are relevant to multiple areas of space plasma physics. We present results from 3D electromagnetic particle-in-cell simulations of the nonlinear evolution of the ion ring instability, including the nonlinear excitation of long-wavelength electromagnetic waves. In the past, there has been some question as to 1) the conversion efficiency from kinetic energy in the ring to electromagnetic waves, and 2) the mechanism responsible for electromagnetic waves (three wave coupling or nonlinear scattering). From these simulations, we account for energy flow from ring to electrostatic waves to electromagnetic waves, including heating of the background plasma and ring via quasilinear and nonlinear stochastic processes. We also show multiple types of evidence from these simulations that nonlinear scattering is responsible for the long-wavelength electromagnetic waves. Finally, we discuss how this analysis affects ring distributions in natural space phenomena.