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 ∼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.