Preonset time sequence of auroral substorms: Coordinated observations by allsky imagers, satellites, and radars

[1] Using two conjunction events of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) imagers and spacecraft as well as the Super Dual Auroral Radar Network (SuperDARN) and Poker Flat Incoherent Scatter Radars, we show that longitudinally narrow flow bursts in the nightside polar cap can precede poleward boundary intensifications (PBIs) that are followed by equatorward moving north‐south (N‐S) arcs, including those leading to substorm onset instability within the near‐Earth plasma sheet. The association between the ionospheric flows and PBIs indicates that enhanced flows on open field lines may contribute to parallel potential drop increase, triggering of magnetotail reconnection, and to the earthward flows leading to N‐S arcs and to substorm onset. We also 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, indicating that their corresponding plasma sheet flow properties could also be 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 preexisting 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 pressure gradients 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 preexisting inner plasma sheet pressure is sufficiently large.