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

• Published in: Earth, planets, and space planets, and space, 2015-11, Vol.67 (1), p.1-10, Article 182
• Main Authors: Tanaka, Yoshimasa, Ogawa, Yasunobu, Kadokura, Akira, Partamies, Noora, Whiter, Daniel, Enell, Carl-Fredrik, Brändström, Urban, Sergienko, Tima, Gustavsson, Björn, Kozlovsky, Alexander, Miyaoka, Hiroshi, Yoshikawa, Akimasa
• Format: Article
• Language: English
• Subjects: 3. Space science; Auroral streamers; Auroras; Banded structure; Bands; Earth and Environmental Science; Earth Sciences; Eastward-expanding auroral surges; Geofag: 450; Geology; Geophysics/Geodesy; Geosciences: 450; Intervals; Ionospheric equivalent current; Magnetic properties; Magnetic pulsations; Matematikk og Naturvitenskap: 400; Mathematics and natural science: 400; Meteorologi: 453; Meteorology: 453; Nanostructure; Omega bands; Post-midnight sector; Pulsation; Substorm expansion phase; Surges; The 12th International Conference on Substorms; VDP
• ISSN: 1880-5981; 1343-8832; 1880-5981
• DOI: 10.1186/s40623-015-0350-8

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. On the other hand, the EEASs we observed had several properties that were different from those of omega bands, such as greater eastward propagation speed (3–4 km/s), shorter associated magnetic pulsation periods (4–6 min), and a different ionospheric equivalent current direction. The fast eastward propagation speed of the EEASs is consistent with the speed of eastward expansion fronts of the substorm current wedge reported in previous studies. The difference in the ionospheric current between the EEASs and omega bands may be caused by a large temporal variation of the surge structure, compared with the more stable wavy structure of omega bands.