Repetitive Emissions of Rising‐Tone Chorus Waves in the Inner Magnetosphere

Quanming Lu,Xinliang Gao,Xueyi Wang,Lunjin Chen,Yu Lin,Shui Wang

doi:10.1029/2021gl094979

Abstract

AbstractChorus waves in the inner magnetospheres of the Earth and other magnetized planets typically occur in the form of discrete and repetitive quasi‐monochromatic emissions with a frequency chirping, which was discovered more than 50 years ago. However, until now there is still no satisfactory explanations for repetitive emissions of chorus waves. In this Letter, chorus emissions excited by energetic electrons with a temperature anisotropy are studied by a one‐dimensional PIC‐ simulation in a dipole magnetic field, and it is demonstrated that a continuous injection of energetic electrons caused by an azimuthal drift is essential for the repetitive emissions of chorus waves. Consistent with satellite observations, both discrete and continuous spectra can be reproduced. An intense injection of energetic electrons will lead to a decrease of the time separation between the chorus elements, and the chorus emissions evolve from a discrete to a continuous spectrum when the injection is sufficiently strong.

Highlights

Chorus waves are electromagnetic emissions that are commonly observed in geospace consisting of ionized gas embedded with a magnetic field, and known for an ensemble of distinct elements with each showing frequency chirping[1,2,3,4,5].Such frequency chirping in every element is often featured with a rising frequency over a short duration of period (~ 0.1 seconds)
Only one distinct element of rising-tone chorus has been observed since the free energy stored in the electron distribution is released in a burst way, and the repetitive feature as typically shown in satellite observations cannot be reproduced[22]. With both a one-dimensional (1-D) δ f PIC simulation and theoretical models, where a continuous injection of energetic electrons with a temperature anisotropy caused by the azimuthal drift is implemented, we demonstrate the generation mechanism of repetitive emissions of rising-tone chorus waves
Theoretical model of repetitive chorus emissions: In order to better understand the repetitive emissions of chorus waves in a dipole magnetic field, we further propose a modification on the theoretical model developed by Omura et al.[19] to account for the effect of electron injection

Summary

Introduction

Chorus waves are electromagnetic emissions that are commonly observed in geospace consisting of ionized gas (plasma) embedded with a magnetic field, and known for an ensemble of distinct elements with each showing frequency chirping[1,2,3,4,5]. Such frequency chirping in every element is often featured with a rising frequency With both a one-dimensional (1-D) δ f PIC simulation and theoretical models, where a continuous injection of energetic electrons with a temperature anisotropy caused by the azimuthal drift is implemented, we demonstrate the generation mechanism of repetitive emissions of rising-tone chorus waves

Results

Conclusions and Discussion

Method