Coupling of dipolarization front flow bursts to substorm expansion phase phenomena within the magnetosphere and ionosphere

Recent THEMIS spacecraft observations have shown longitudinally narrow, earthward moving, plasma sheet flow channels with large, abrupt magnetic field dipolarizations that have been referred to as dipolarization fronts. We undertake a detailed study of the subset of these events that had good quality auroral imaging. We find that many occurred during the substorm expansion phase after onset (determined by auroral intensification) and were related to auroral streamers. Both the fronts' relation to streamers and their plasma sheet changes are similar to what has been previously shown for other plasma sheet flow channels and to other flow channels seen during the expansion phases of the events considered here. Surprisingly, the dipolarization fronts examined made major contributions to substorm onset ground and space magnetic signatures. These include substorm near‐Earth dipolarization, which indicates that the current wedge may develop via a series of well‐defined, narrow, post‐onset wedge‐like structures. These also include the auroral zone ground magnetic field, which showed only modest responses to the onsets but abrupt, large responses to post‐onset dipolarization‐front‐related streamers, and midlatitude positive bays, which started near the time of streamer formation. Our observations suggest that traditional magnetic signatures of substorm onset, such as near‐Earth dipolarization, auroral‐zone negative bays, and midlatitude positive bays, may misidentify the expansion phase onset determined by auroral intensification by up to 10s of minutes for some events. It should be interesting in the future to determine the generality of the above relations between dipolarization front signatures and onset signatures. Key Points Large dipolarization fronts occur during substorm expansion phase Dipolarization fronts related to auroral streamers Dipolarization fronts make major contributions to traditional onset signatures