Two distinct substorm onsets

At times, substorms occur in a particular sequence, where a clear growth ph ase is followed by a first onset at lower latitudes and a second one involv ing all latitudes between 60 degrees and 70 degrees. The second onset is fo llowed by a clear recovery phase. In the present paper we present nine such sequences in the form of a superposed epoch analysis. Comparison with sola r wind data shows that the second onset occurs when the interplanetary magn etic field turns northward. Determination of the change in the open polar c ap magnetic flux using the magnetogram inversion technique method indicates that the first onset occurs during an interval of continuous loading of th e tail with new open flux merged at the dayside. Hence, despite showing cle ar 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 close d plasma sheet to open lobe field lines.