Generation Mechanisms of Whistler-mode Chorus and Electromagnetic Ion Cyclotron Rising-tone Emissions

Abstract

We give a brief review of the nonlinear wave growth theory of whistler-mode chorus emissions [1, 2]. We describe the nonlinear dynamics of resonant electrons, and the formation of the electromagnetic electron “hole” that results in resonant currents generating rising-tone emissions. We have theoretically derived threshold and optimum wave amplitudes for the nonlinear wave growth of rising-tone emissions. There occurs nonlinear wave growth as an absolute instability above the threshold amplitude near the equator, and the wave grows rapidly along with frequency increase. The nonlinear wave growth saturate around the optimum amplitude, and the wave amplitude damps to the level of the threshold amplitude. The nonlinear wave growth process is repeated, resulting in a rising-tone chorus element with subpacket structures. As the wave packet propagates away from the equator, it undergoes nonlinear convective growth. The profiles of these wave amplitudes as functions of frequencies agree well with those from observations and simulations. As the wave packet propagates away from the equator, it undergoes efficient nonlinear convective growth. The wave normal angle gradually deviate from the parallel direction, and there occurs nonlinear wave damping at half the cyclotron frequency, separating the chorus element into lower-band and upper-band emissions [3].