Relation of substorm pre-onset arc to large-scale field-aligned current distribution

Abstract

Substorm auroral onset often occurs along a pre-existing arc that is associated with an enhanced upward field-aligned current (FAC). To determine the location of the upward FAC of the pre-onset arc relative to the large-scale region 1 (R1) and 2 (R2) FACs, we present coordinated observations of the THEMIS all-sky imagers and CHAMP satellite, where CHAMP passed over relatively stable, pre-onset arcs within the 30 min period before substorm auroral onset. We found that the upward FAC corresponding to the pre-onset arc is latitudinally separated from the upward R1 FAC within the onset MLT range. In contrast, the arc is located near the R1-R2 boundary to the west and east of the onset MLT range. This MLT dependence suggests that an east-west oriented upward FAC sheet forms within the downward R2 FAC as a precursor FAC signature for substorm auroral onset. © 2012. American Geophysical Union. All Rights Reserved.