Whistler Wave Propagation in a Dipole Magnetic Field: Two-Dimensional gcPIC Simulations

Abstract

Magnetospheric whistler waves are of fundamental importance in the formation of radiation belts and the generation of diffuse aurorae. Their propagation has been widely studied using satellite observations and numerical simulations because of their direct impact on the interactions with electrons. Although ray-tracing models have elucidated the propagation paths, wave normal angles (WNAs), and linear growth/damping of whistler waves, their nonlinear evolution, requiring kinetic simulation models, still remains unclear. In this study, we utilize gcPIC simulations to study whistler wave propagation in a dipole magnetic field, and compare the results with ray-tracing simulations. Ray-tracing simulations show that a parallel whistler wave gradually becomes oblique and experiences increasing linear damping during its propagation from the magnetic equator to high latitudes. Particle-in-cell simulations display nearly identical propagation paths and WNAs, but the amplitude evolution shows substantial differences. At lower latitudes, whistler waves will experience extra substantial damping compared with ray-tracing results, which is due to nonlinear Landau and cyclotron resonances. This difference becomes more pronounced when the wave amplitude is larger. Surprisingly, at higher latitudes, whistler waves will experience less damping, attributable to the electron plateau/beam distributions resulting from Landau trapping. Our study demonstrates the crucial role of nonlinear resonances and reshaped electron distributions in modeling the evolution of whistler waves in the Earth's magnetosphere.