地球与行星物理论评 (Sep 2022)
A review of frequency chirping mechanism of chorus waves in the Earth's magnetosphere
Abstract
Whistler mode chorus waves are common and intense electromagnetic waves in the Earth's magnetosphere. They play a crucial role in magnetospheric electron dynamics by accelerating and scattering electrons over a wide energy range. Chorus waves are typically quasi-coherent emissions with frequency chirping. The generation mechanism of frequency chirping of whistler mode chorus has been debated for many years. Some theoretical and numerical models to explain the generation condition of chorus frequency chirping and the detailed nonlinear wave-particle interaction have been proposed. There are two main types of chorus wave generation models in history. The frequency chirping rate of the first model is proportional to the background magnetic field inhomogeneity. The prediction of this model is in good agreement with observation in the Earth's magnetosphere, but it cannot explain the frequency chirping with a uniform background magnetic field. Another model proposes that the frequency chirping rate is proportional to the wave amplitude and can be verified by self-consistent particle-in-cell simulations. However, the forms of the frequency chirping rate from the two theoretical models are very different. This paper will briefly review the observation of rising tone chorus waves, the optimal excitation condition of chorus, nonlinear wave particle interactions, and also the newly proposed "TaRA" (Trap-Release-Amplify) model. In this model, the wave source region near the equator is divided into upstream and downstream based on the wave propagation direction. These two regions play different roles in the wave excitation process. Phase space structures of correlated electrons are formed by nonlinear wave particle interactions, which mainly occur in the downstream of the equator. When released from the wave packet in the upstream, these electrons lead to selective amplification of new emissions which satisfy the phase-locking condition to maximize wave power transfer, resulting in frequency chirping. The similarities and differences between TaRA model and other mainstream models will be discussed. The study of the frequency chirping mechanism of chorus waves can not only explain the characteristics related to chorus fine structures, deepen the understanding of the nonlinear process of wave-particle interactions, but can also be used to explain other related phenomena, such as the frequency chirping of whistler waves in a uniform magnetic field, rising tone electromagnetic ion cyclotron (EMIC) waves in the Earth's magnetosphere, and the frequency chirping structures of chorus and EMIC waves at other planets.
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