Large‐scale aspects and temporal evolution of pulsating aurora

Pulsating aurora is a common phenomenon generally believed to occur mainly in the aftermath of a substorm, where dim long‐period pulsating patches appear. The study determines the temporal and spatial evolution of pulsating events using two THEMIS all‐sky imager stations, at Gillam (66.18 magnetic latitude, 332.78 magnetic longitude, magnetic midnight at 0634 UT) and Fort Smith, (67.38 magnetic latitude, 306.64 magnetic longitude, magnetic midnight at 0806 UT) along roughly the same invariant latitude. Parameters have been calculated from a database of 74 pulsating aurora events from 119 days of good optical data within the period from September 2007 through March 2008 as identified with the Gillam camera. It is shown that the source region of pulsating aurora drifts or expands eastward, away from magnetic midnight, for premidnight onsets and that the spatial evolution is more complicated for postmidnight onsets, which has implications for the source mechanism. The most probable duration of a pulsating aurora event is roughly 1.5 h, while the distribution of possible event durations includes many long (several hours) events. This may suggest that pulsating aurora is not strictly a substorm recovery phase phenomenon but rather a persistent, long‐lived phenomenon that may be temporarily disrupted by auroral substorms. Observations from the Gillam station show that in fact, pulsating aurora is quite common with the occurrence rate increasing to around 60% for morning hours, with 69% of pulsating aurora onsets occurring after substorm breakup.