Spatio-Temporal Development of Fast Plasma Flow in the Plasmasheet

Fast plasma flow in the plasmasheet is an essential element of the so-called outside-in substorm model, which proposes that a fast flow created by near-Earth reconnection around X = - 25 Re causes reduction of the tail current intensity in the near-Earth region (X ~ -10 Re). In terms of this model, the fact that the substorm onset generally takes place in a limited spatial area can be most reasonably explained if the flow proceeds along a narrow channel. However, the spatial development of the fast flow has been an outstanding issue mostly because of limited spatial coverage of satellite observation. In the present study we observationally address this issue by applying the ion sounding technique to fast flow events observed by the Geotail satellite. Since particles coming from different directions have their guiding centers at different locations, we can estimate the local gradient of the energetic particle flux in the flow direction by examining the flux anisotropy. If the advection is the main cause of the temporal variation of the particle flux at the satellite position, we should be able to predict such variations from the estimated spatial gradient and the flow velocity. In this study we will address the temporal development of the flow structure by comparing the actual observation with the temporal flux variation predicted in such a way.