Nonlinear Wave-Particle Interactions in the Formation of Chorus Wave Subpackets: 2-D Particle-in-Cell Simulation*

We investigate nonlinear wave-particle interactions during the formation of chorus wave subpackets using a 2-D GCPIC simulation in dipole fields. The rising-tone chorus waves are self-consistently excited by energetic electrons near the magnetic equator, and the waves consist of a series of subpackets and become oblique during their propagation. We investigate electron distributions in various phase spaces associated with subpacket formation. It is found that electron holes in the wave phase space, formed due to nonlinear cyclotron resonance, oscillate in size over time during subpacket formation. The associated inhomogeneity factor varies accordingly, leading to different frequency chirping in various phases of the subpackets. Moreover, Landau resonance is found to coexist with cyclotron resonance. This study provides a comprehensive analysis of multidimensional electron distributions involved in subpacket formation, enhancing our understanding of the nonlinear physics in chorus wave evolution.