Differences between N-S arc sequences that do and do not lead to substorm expansion onset

A critical, long-standing problem in substorm research is identification of the sequence of events leading to substorm auroral onset. Based on event and statistical analysis of THEMIS all-sky imager data, we have shown that there is a distinct and repeatable sequence of events that is initiated by a poleward boundary intensification (PBI) followed by a north-south (N-S) arc moving equatorward towards the onset latitude leading to substorm auroral onset. We have now investigated differences between N-S arc sequences that do and do not lead to substorm expansion onset. We found that the two types of N-S arcs have similar characteristics and that corresponding plasma sheet flow properties measured by the coordinated THEMIS spacecraft are also similar. There is, however, one difference between the sequences of N-S arc evolution. Each N-S arc leads to small intensification of the growth phase arc, and when the onset-related N-S arc reaches the equatorward portion of the auroral oval, the pre-existing growth phase arc is much brighter than at the times of non-onset related N-S arcs. Assuming that the growth-phase arc is related to enhanced pressures at the inner edge of the plasma sheet, this difference indicates that the near-Earth plasma pressure distribution at the time of plasma sheet fast flows is crucial in substorm triggering. These observations suggest that substorm onset instability is possible only when the pre-existing inner plasma sheet pressure is sufficiently large.