Coupling of Dipolarization Front Flow Bursts to Substorm Expansion Phase Phenomena Within the Magnetosphere and Ionosphere

The THEMIS spacecraft have revealed magnetotail dipolarization fronts propagating Earthward as coherent structures within the plasma sheet and having characteristic signatures that have been predicted by bubble models of flow channels. We address the question of how are these fronts are related to well-known geomagnetic disturbances. The relation between auroral streamers and associated flow channels is well known, but the front associated field and plasma changes are much stronger than typical. We have found that dipolarization fronts are associated with equatorward moving streamers originating after substorm onset. As is well known, streamers form repetitively during the expansion phase, and our results suggest that longitudinally narrow flow bursts accompanied by dipolarization can form several times during an expansion phase. Contrary to expectations, we have found that the largest expansion phase flows and dipolarizations are associated with streamers and associated dipolarization fronts rather than with the substorm onset. We also have found, contrary to expectations, evidence that substorm related mid-latitude positive bays and largest auroral oval magnetic perturbations are delayed by a few minutes from the auroral substorm onset, and instead coincide with streamer initiation. If the above results are found to be generally true, and not just for events with large dipolarization fronts, it would suggest that multiple flow bursts are fundamental element of magnetosphere-ionosphere convection during the substorm expansion phase, and are a major contributor to substorm dipolarizations, auroral zone magnetic bays, mid-latitude positive bays