Banded chorus generation by an electron shell distribution in an inhomogeneous magnetic field: 1D PIC simulations

Abstract

Although many theoretical models have been proposed over several decades, the origin of banded chorus with a gap in intensity at Ωe/2 (Ωe being the angular electron cyclotron frequency) is still debated. In one of those models, the chorus gap formation is attributed to two anisotropic electron populations separated by an isotropized population (called the parallel plateau) at energy resonant with waves of frequency ∼Ωe/2. Here, we simplify the plateau population as an electron shell distribution in velocity space and investigate its role in the gap formation using a particle-in-cell code in a non-uniform magnetic field. The base plasma is configured to generate rising chorus elements spanning in frequency from below to above Ωe/2. Then, multiple simulations are run with an additional shell distribution with different density values to investigate the gap formation. The simulation results show that even a relatively small fraction of shell is quite effective in arresting the frequency chirping of lower-band chorus at ∼Ωe/2, resulting in a power gap there. Phase space analysis indicates that the resonant current contributed by the phase-trapped shell electrons (forming a phase space hill) can counteract nonlinear chorus growth driven by the phase space hole of energetic, anisotropic electrons. This process occurs in combination with the linear cyclotron damping suggested in earlier studies, and the resulting nonlinear damping may exceed the latter effect.