The January 10, 1997 auroral hot spot, horseshoe aurora and first substorm: A CME loop?

Abstract

The January 10, 1997 interplanetary high-speed stream and the resultant first substorm (∼0332 to 0334 UT onset) is studied. A 47 minute interval of relatively intense southward interplanetary magnetic field (IMF) (BS = 4 to 8 nT) bounded by two tangential discontinuities (TDs) is identified between the interplanetary shock and the magnetic cloud. The two discontinuities arrive at the magnetopause at ∼0219 and ∼0302 UT. The IMF BS event served primarily to transfer solar wind energy to the magnetosphere/magnetotail, as no substorm expansion phase occurred during the event. The eventual energy release was in the form of a large substorm expansion phase which occurred after (∼15–17 min.) a second IMF northward turning (after the end of the BS interval). The interplanetary BS event came after a prolonged northward IMF interval. During the initial part of the BS event, both polar cap Sun-Earth aligned arcs formed (part of a theta aurora) and an auroral hot spot along the main arc took place. Later, during the BS interval, an aurora in the shape of a horseshoe developed at lower (60°) latitudes (an oval with a gap in the noon sector). The dawnside portion of the horseshoe aurora became much brighter than the duskside with increasing time. The dawnside polar cap boundary layer (PCBL) broadband waves were well correlated with low energy ion fluxes (H+, He++, O+) during the event. It is speculated that this IMF BS structure may be an outer loop of the CME coming from the Sun. Another similar loop is identified just adjacent to the cloud.