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We present three eastward-expanding auroral surge (EEAS) events that wereobserved intermittently at intervals of about 15 min in the post-midnightsector (01:55-02:40 MLT) by all-sky imagers and magnetometers in northernEurope. It was deduced that each surge occurred just after each onset of amultiple-onset substorm, which was small-scale and did not clearly expandwestward, because they were observed almost simultaneously with Pi 2 pulsationsat the magnetic equator and magnetic bay variations at middle-to-high latitudesassociated with the DP-1 current system. The EEASs showed similar properties toomega bands or torches reported in previous studies, such as recurrenceintervals of about 15 min, concurrence with magnetic pulsations with amplitudesof several tens of nanotesla, horizontal scales of 300-400 km, and occurrenceof 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 inthe poleward region, followed by the formation of north-south-aligned auroras,which eventually connected with the EEASs. Thus, we speculate that EEASs may berelated to the generation process of omega bands. (Continued)

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