At times substorms occur in a particular sequence, where a clear growth phase is followed by a first onset at lower latitudes near Phi similar to 63 degrees, and a second one involving all latitudes between 60-70 degrees. In the present paper we present nine such sequences in a superposed epoch analysis. Changes of Psi, the open magnetic flux in the polar cap, using the MIT2 method, indicate that the first onset occurred during an interval of continuous loading of the tail with new open flux merged at the dayside. Hence, this onset did likely involve the reconnection of closed plasma sheet field lines only, while during the second expansion phase, reconnection clearly proceeded from the closed plasma sheet to open lobe field lines. These data are compared with a substorm scenario based on a global tail stretching instability.