Two distinct substorm onsets

Abstract

At times, substorms occur in a particular sequence, where a clear growth phase is followed by a first onset at lower latitudes and a second one involving all latitudes between 60° and 70°. The second onset is followed by a clear recovery phase. In the present paper we present nine such sequences in the form of a superposed epoch analysis. Comparison with solar wind data shows that the second onset occurs when the interplanetary magnetic field turns northward. Determination of the change in the open polar cap magnetic flux using the magnetogram inversion technique method indicates that the first onset occurs during an interval of continuous loading of the tail with new open flux merged at the dayside, Hence, despite showing clear ionospheric signatures of substorm expansion, this onset likely involves the reconnection of closed plasma sheet field lines only, while during the second onset and expansion phase, reconnection clearly proceeds from closed plasma sheet to open lobe field lines.