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Global‐scale ionospheric flow and aurora precursors of auroral substorms: Coordinated SuperDARN and IMAGE/WIC observations

We use global‐scale polar cap flow vector measurements from the Super Dual Auroral Radar Network (SuperDARN) with the concurrent auroral observations from the Wideband Imaging Camera on board Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE/WIC) to study the polar cap flow and auroral precursors during a substorm onset on 26 December 2000. We show, for the first time, close connection between the dayside and nightside polar cap flow enhancements (with the enhanced dayside flow preceding the nightside one by several minutes) and the ensuing poleward boundary intensification (PBI)/streamer, and the later onset, forming a complete preonset sequence for a substorm onset. Our results supplement our previous study by providing further evidence that the dayside polar cap flow disturbance may be the key to initiate the whole process of a certain type of substorm by triggering reconnection somewhere in the tail via applied field (or flow) perturbations on the nightside plasma sheet boundary layer. Our results also indicate that a preexisting double oval structure is likely a favorable precondition for a certain type of substorm to be triggered by polar cap flow disturbance and the associated PBIs/streamers. On the other hand, not all our global‐scale preonset auroral sequences support the recent revised onset scenario proposed by Nishimura et al. ( ) using the all‐sky imagers of the Time History of Events and Macroscale Interactions during Substorms mission. This suggests that the preceding PBI/streamer is not a sufficient condition to trigger a substorm. It may not even be a necessary condition considering the existence of various types of substorm onsets.