TC-1 observation of ion high-speed flow reversal in the near-Earth plasma sheet during substorm

Based on measurements of FGM and HIA on board TC-1 at its apogee on September 14, 2004, we analyzed the ion high-speed flows in the near-Earth plasma sheet observed during the substorm expansion phase. Strong tailward high-speed flows (V-x similar to -350 km/s) were first seen at about X similar to -13.2 RE in near-Earth magnetotail, one minute later the flows reversed from tailward to earthward. The reversal process occurred quickly after the substorm expansion onset. The near-Earth magnetotail plasma sheet was one of key regions for substorm onset. Our analysis showed that the ion flow reversal from tailward to earthward was likely to be in close relation with the substorm expansion initiation and might play an important role in triggering the substorm expansion onset.