Figure 1. All-sky images of the onset of an auroral substorm on February 1, 1990, at Poker Flat Research Range. The diffuse “hydrogen arc” is too faint to show and was located from the MSP data. It is obviously equatorward of two electron-induced arcs. The poleward arc develops curls which brighten to initiate onset as the expansion phase develops. The equatorward arc has a higher characteristic energy and indicates the poleward boundary of stable trapping. The H emission expands poleward into the electron aurora on the right-hand side of the last frame (1147:48 UT). North is down, and east is to the right.
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