Substorm triggering by new plasma intrusion: THEMIS all-sky imager observations

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 show that there is a distinct and repeatable sequence of events leading to onset, the sequence having similarities to and important differences from previous ideas. The sequence is initiated by a poleward boundary intensification (PBI) and followed by a north-south (N-S) arc moving equatorward toward the onset latitude. Because of the linkage of fast magnetotail flows to PBIs and to N-S auroras, the results indicate that onset is preceded by enhanced earthward plasma flows associated with enhanced reconnection near the pre-existing open-closed field line boundary. The flows carry new plasma from the open field line region to the plasma sheet. The auroral observations indicate that Earthward-transport of the new plasma leads to a near-Earth instability and auroral breakup similar to 5.5 min after PBI formation. Our observations also indicate the importance of region 2 magnetosphere-ionosphere electrodynamic coupling, which may play an important role in the motion of pre-onset auroral forms and determining the local times of onsets. Furthermore, we find motion of the pre-onset auroral forms around the Harang reversal and along the growth phase arc, reflecting a well-developed region 2 current system within the duskside convection cell, and also a high probability of diffuse-appearing aurora occurrence near the onset latitude, indicating high plasma pressure along these inner plasma sheet field lines, which would drive large region 2 currents.