Stepwise feature of aurora during substorm expansion compared with the near-Earth taildipol arization: Possible types of substorm dynamics

摘要

Previous studies of auroral behaviors have suggested that the substorm expansion is aresult of a chain of processes originating in the magnetotail. We present a detailedstudy on the substorm expansion behavior on 3 November 1998, using the global auroralimages from Polar and the conjugate central plasma sheet data from Geotail in thenear-Earth tail. The closer inspection of the auroral data enables us to characterize thestepwise feature of the auroral evolution, which were found to be spatially and temporallyseparable into two intense auroral activities: the initial brightening of the substorm arc andthe second aurora that took place eastward of the first one with the time delay of 4.5 min.Comparing the near-Earth tail observation of the fast plasma flow, we show that theobserved time lag of 4.5 min can be explained by a systematic chain of plasma processes inthe central plasma sheet, indicating that this time delay can be interpreted as a genericbehavior. The generality of the stepwise evolution within the expected time lag of 2-6 minis also pursued, and is found for three out of six substorms. With respect to the stepwisefeature, our observation of six substorms suggests that there are at least three types ofsubstorm expansion dynamics that are attributable to different chains of tail processes.