Azimuthal structures of substorm electron injection and their signatures in riometer observations

Abstract

We propose a theoretical model to investigate the effects of the curvature/gradient (c/g) drift and the finite azimuthal extent of the dipolarization region on the electron injection process associated with the substorm dipolarization. We study the azimuthal structure of high‐energy (>30 keV) electron precipitation flux and compare the result with riometer observations. We are able to reproduce three basic archetypes of riometer responses to substorms, namely, the spike, dispersionless injection, and dispersed injection events catalogued in previous observations. The electron injection near the duskward edge of the dipolarization region is most subject to azimuthal c/g drift loss, appearing in riometer observations as the “spike” feature. The “dispersionless injection” response is seen inside the dipolarization region but some distance away from its western border: or, alternatively, when the substorm has a rapid westward expansion, so that the gain and loss of electrons from the duskside and dawnside of a dipolarizing flux tube roughly balance. The “dispersed injection” feature is seen east of the dipolarization region. Our theory successfully explains the statistical differences in terms of magnetic local time location and peak intensity between spikes and injection events. Through the substorm event on 23 May 1998 we demonstrate that our theoretical predictions of riometer responses are very consistent with the observations. We highlight the potential of riometers in resolving the azimuthal extent and evolution of the dipolarization region, which provides a new ground‐based technique of remote sensing the substorm process.