Tail Processes and Auroral Activity at the Substorm Expansion Onset

In the Geotail data set we have found observations of dipolarizations in the vicinity of the magnetic equator of the near-Earth tail (XGSM~ -10 RE) that have concurrent aurora imager data from Polar UVI and VIS cameras. These events (six in total over the ten years) allow us to compare tail processes and auroral activities. Detailed inspection of the imager data in the course of this study have led us to recognize that, in five of them, auroras developed into large-scale activities in a two-stage manner: an initial auroral brightening and the subsequent "second aurora". When the activity started, the former and the latter can be recognized as a spatially and temporally discrete spot in the images; it is not appropriate to consider the emergence of the second spot as a continuous expansion of the first spot. The second aurora occurred ~ 4 min after the first one for three out of the five events, while the second aurora occurred ~ 10 min afterward for the other two events. All the second auroras are located eastward of the first one, suggesting systematic sequence of the auroral brightening process. During these auroral activities at substorm expansion onsets, the Geotail detected various signatures of plasma processes, fast flows, flow reversals, fast mode waves, and ballooning mode waves. We will discuss a possible framework to synthesize these observed features in the context of magnetospheric substorms.