Modelling substorm chorus events in terms of dispersive azimuthal drift

A B Collier,A R W Hughes

doi:10.5194/angeo-22-4311-2004

Abstract

Abstract. The Substorm Chorus Event (SCE) is a radio phenomenon observed on the ground after the onset of the substorm expansion phase. It consists of a band of VLF chorus with rising upper and lower cutoff frequencies. These emissions are thought to result from Doppler-shifted cyclotron resonance between whistler mode waves and energetic electrons which drift into a ground station's field of view from an injection site around midnight. The increasing frequency of the emission envelope has been attributed to the combined effects of energy dispersion due to gradient and curvature drifts, and the modification of resonance conditions and variation of the half-gyrofrequency cutoff resulting from the radial component of the ExB drift. A model is presented which accounts for the observed features of the SCE in terms of the growth rate of whistler mode waves due to anisotropy in the electron distribution. This model provides an explanation for the increasing frequency of the SCE lower cutoff, as well as reproducing the general frequency-time signature of the event. In addition, the results place some restrictions on the injected particle source distribution which might lead to a SCE. Key words. Space plasma physics (Wave-particle interaction) – Magnetospheric physics (Plasma waves and instabilities; Storms and substorms)

Highlights

The substorm chorus event is a recognised VLF signature of the substorm expansion phase (Smith et al, 1996, 1999)
Chorus emissions are commonly observed in the postmidnight sector in association with substorm activity (Tsurutani and Smith, 1974), the Substorm Chorus Event (SCE) is distinctive as it consists of a band of chorus with ascending upper and lower cutoff frequencies
The convection field is undoubtedly of import to the dynamics, an objective of this study is to demonstrate that the inward radial E×B drift is not a prerequisite for SCE occurrence and that energy dispersion is a sufficient mechanism

Summary

Introduction

Within the range of pitch angles present at the observer at a given instant, the larger α correspond to particles arriving from the furthest part of the injection region This is due to the fact that for a given value of W , both W and G(α), and the azimuthal drift velocity, increase with α. This is exemplified by the data plotted tween W⊥ and W for electrons that have the drift rate required to reach the observer at a selection of times after the in Fig. 11 which illustrate that the anisotropy produced by a Maxwellian injection is initially very large but is reduced moment of injection.

Maxwellian source

Lorentzian source

Conclusion