Flux transport, dipolarization, and current sheet evolution during a double-onset substorm

Abstract

[1] We study a substorm with two onsets (at 0220 and 0243 UT) that occurred during a gradual northward interplanetary magnetic field (IMF) turning on 16 February 2008. At these times, Time History of Events and Macroscale Interactions during Substorms (THEMIS) and GOES spacecraft were distributed between 6.6 and 18 RE downtail. Prior to the weak auroral electrojet onset at 0220 UT, a thin current sheet was extended near 10–11 RE. After the onset, Earthward fast flows with dipolarization fronts followed by signatures of magnetic flux pileup were detected in this region. The 0243 UT onset disturbances were more intense and centered at higher latitudes. The reconnection region tailward of 18 RE became activated and reached the lobe flux. We suggest that activations of reconnection Earthward of 18 RE associated with the 0220 UT event led to pileup of flux and redistribution of BZ to form a thin current sheet with small BZ in the midtail region. This made conditions favorable for reconnection tailward of 18 RE involving lobe flux for the 0243 UT onset. The reconfiguration process in the current sheet between the two onsets possibly enabled a relatively strong, high-latitude substorm despite the rather weak IMF driver. The near-Earth dipolarization observed after the 0243 UT onset was accompanied by more localized Earthward flows and flow reversals. Differences in dipolarization signatures could be caused by ambient plasma condition and field configuration between these two events. Our observations of the double-onset substorm suggest that the plasma sheet can be preconditioned by both the IMF driver and internal magnetotail processes.