Classification of Fast Flows in Central Plasma Sheet: Superposed Epoch Analysis Based on THEMIS Observations

Abstract:

The relationship between fast flows in the central plasma sheet (CPS) and substorm is a key issue of substorm study, and has been extensively studied for several decades. Many studies showed that the fast flows in CPS could link to most substorm phenomena, and suggested that substorm auroral breakups were triggered by fast flow bursts in CPS. Nevertheless, whether the fast flows in CPS are associated with substorm auroral brightening and further development is still under debate. For examples, many observations also showed that the fast flows were not always present during and even before the auroral breakups, and may not have a direct contribution to substorm aurora activation. Here, we performed a comprehensive statistical survey of fast flows in midnight CPS based on THEMIS observations. From superposed epoch analysis, no significant substorm activities are found to be associated with the occurrence of fast flows beyond X = −15 Re. Considering the associations with substorm activities, the fast flows inside of X = −15 Re can be classified into two obvious classes: short duration (< 2.0 min) and long duration (> 4.0 min). Substorm breakups are shown to be more closely correlated to short-duration fast flows. Furthermore, the onset of short-duration fast flows in the dipolarization region (X = −9 to −11 Re) is almost simultaneous with the onset of substorm breakups and dipolarizations. On the other hand, time delays of 2–4 min are both found in the near-Earth region (X = −7 to −9 Re) and in the near-tail region (X = −11 to −15 Re). In comparison, although more magnetic flux is transported toward Earth for long-duration fast flows, no clear substorm breakup is closely associated with them. Compared to the NENL substorm model, the CECL and the CD substorm models are more suitable to interpret these phenomena. The analysis of 2-D ion velocity distribution further shows some differences. For short-duration fast flows, multiple crescent-shaped ion populations are found. However, for long-duration fast flows, there exists only a single crescent-shaped ion population. Such difference may be an important signature for distinguishing these two classes of fast flows.