Eastward-expanding auroral surges observed in the post-midnight sector during a multiple-onset substorm

TL;DR

This study reports on eastward-expanding auroral surges observed after multiple small-scale substorm onsets, revealing their properties and potential relation to omega band formation in the post-midnight sector.

Contribution

It provides detailed observations of EEASs and suggests their possible connection to omega band generation, enhancing understanding of auroral dynamics during substorms.

Findings

EEASs occur after multiple small-scale substorm onsets.

EEASs exhibit properties similar to omega bands, including recurrence and scale.

EEASs are associated with magnetic pulsations and auroral structures.

Abstract

We present three eastward-expanding auroral surge (EEAS) events that were observed intermittently at intervals of about 15 min in the post-midnight sector (01:55-02:40 MLT) by all-sky imagers and magnetometers in northern Europe. It was deduced that each surge occurred just after each onset of a multiple-onset substorm, which was small-scale and did not clearly expand westward, because they were observed almost simultaneously with Pi 2 pulsations at the magnetic equator and magnetic bay variations at middle-to-high latitudes associated with the DP-1 current system. The EEASs showed similar properties to omega bands or torches reported in previous studies, such as recurrence intervals of about 15 min, concurrence with magnetic pulsations with amplitudes of several tens of nanotesla, horizontal scales of 300-400 km, and occurrence of a pulsating aurora in a diffuse aurora after the passage of the EEASs. Furthermore, the EEASs showed similar temporal evolution to the omega bands, during which eastward-propagating auroral streamers occurred simultaneously in the poleward region, followed by the formation of north-south-aligned auroras, which eventually connected with the EEASs. Thus, we speculate that EEASs may be related to the generation process of omega bands. (Continued)