A "Trap-Release-Amplify" Model of Chorus Waves

TL;DR

The paper introduces the 'Trap-Release-Amplify' (TaRA) model for chorus waves, explaining frequency chirping through nonlinear wave-particle interactions and phase-locking, unifying previous theories and explaining fine structures.

Contribution

The TaRA model provides a comprehensive phenomenological framework for chorus wave chirping, integrating previous results and explaining fine structures and evolution of chorus waves.

Findings

Chorus wave frequency chirping is explained by phase-locking of electrons during wave interactions.

The model predicts chirping rate proportional to wave amplitude, consistent with prior studies.

The TaRA model accounts for fine structures like subpackets and bandwidth in chorus waves.

Abstract

Whistler mode chorus waves are quasi-coherent electromagnetic emissions with frequency chirping. Various models have been proposed to understand the chirping mechanism, which is a long-standing problem in space plasmas. Based on analysis of effective wave growth rate and electron phase space dynamics in a self-consistent particle simulation, we propose here a phenomenological model called the "Trap-Release-Amplify" (TaRA) model for chorus. In this model, phase space structures of correlated electrons are formed by nonlinear wave particle interactions, which mainly occur in the downstream. When released from the wave packet in the upstream, these electrons selectively amplify new emissions which satisfy the phase-locking condition to maximize wave power transfer, leading to frequency chirping. The phase-locking condition at the release point gives a frequency chirping rate that is fully consistent with the one by Helliwell in case of a nonuniform background magnetic field. The nonlinear wave particle interaction part of the TaRA model results in a chirping rate that is proportional to wave amplitude, a conclusion originally reached by Vomvoridis et al. Therefore, the TaRA model unifies two different results from seemingly unrelated studies. Furthermore, the TaRA model naturally explains fine structures of chorus waves, including subpackets and bandwidth, and their evolution through dynamics of phase-trapped electrons. Finally, we suggest that this model could be applied to explain other related phenomena, including frequency chirping of chorus in a uniform background magnetic field and of electromagnetic ion cyclotron waves in the magnetosphere.