The dawn and dusk electrojet response to substorm onset

E Borälv,P Stauning,P Eglitis,H J Opgenoorth,E Donovan,G Reeves

doi:10.1007/s005850000266

Abstract

Abstract. We have investigated the time delay between substorm onset and related reactions in the dawn and dusk ionospheric electrojets, clearly separated from the nightside located substorm current wedge by several hours in MLT. We looked for substorm onsets occurring over Greenland, where the onset was identified by a LANL satellite and DMI magnetometers located on Greenland. With this setup the MARIA magnetometer network was located at dusk, monitoring the eastward electrojet, and the IMAGE chain at dawn, for the westward jet. In the first few minutes following substorm onset, sudden enhancements of the electrojets were identified by looking for rapid changes in magnetograms. These results show that the speed of information transfer between the region of onset and the dawn and dusk ionosphere is very high. A number of events where the reaction seemed to preceed the onset were explained by either unfavorable instrument locations, preventing proper onset timing, or by the inner magnetosphere's reaction to the Earthward fast flows from the near-Earth neutral line model. Case studies with ionospheric coherent (SuperDARN) and incoherent (EISCAT) radars have been performed to see whether a convection-induced electric field or enhanced conductivity is the main agent for the reactions in the electrojets. The results indicate an imposed electric field enhancement. Key words: Ionosphere (auroral ionosphere; electric fields and currents) - Magnetospheric physics (storms and substorms)

Highlights

The horizontal ionospheric currents are usually divided into the global convection electrojets and theWhile the convection electrojets are always present, the westward directed substorm current wedge is a superposed current system during the expansion phase of the substorm
The satellite employed in this study, 1991-080, is equipped with a synchronous orbit particle analyser (SOPA) instrument (Reeves et al, 1996b), which measures electron energyuxes over the energy range 50 keV to about 26 MeV
In order to be able to use European Incoherent Scatter (EISCAT) data in the dusk sector we present an event where we used another SOPA-carrying Los Alamos National Laboratory (LANL) satellite, 1994-084, which was placed over Russia at a longitude of about 105 east, recording midnight sector substorm activity after 14:00 UT

Summary

Introduction

While the convection electrojets are always present, the westward directed substorm current wedge is a superposed current system during the expansion phase of the substorm. When theows encounter the more dipole-like magnetic ®eld in the near-Earth tail they slow down and divert around the`obstacle'' This produces electric ®elds and vorticalows which in turn can trigger the substorm current disruption (Reeves, 1998, Shiokawa et al, 1998). The loading of energy is visible at ground-based magnetometer stations as a slow gradient change in the magnetograms, indicative of increases of the electrojets due to the enhanced cross-polar cap convection. The aim of this substorm study was to answer these questions by investigating the time delay between a near magnetic midnight onset and any reactions in the dawn and dusk electrojets, clearly separated from the nightside located substorm current wedge. If any responses could be found, their cause would be further investigated with the help of data from coherent and incoherent scatter radars

Instruments and data sets

Observations

Radar studies

Discussion and conclusion