On the retreat of near-Earth neutral line during substorm expansion phase: A THEMIS case study during the 9 January 2008 substorm

The location of magnetic reconnection in the mid-tail during a substorm was studied in many researches. Here we present multi-point THEMIS observations of a reconnection event in the near-Earth magnetotail during substorm. In this event, THEMIS probes stayed in the near-Earth and mid-tail region aligning along the magnetotail. This allows reconnection evolution to be probed simultaneously from about g^-10 to g^-23 down tail. The Hall current related electron streams were observed at the same time by two probes far away from the reconnection site. Before near-Earth reconnection involved the tail lobe magnetic field, the reconnection site was restricted in earthward g ^-23 . When reconnection involved into the tail lobe region, the reconnection site started to retreat gradually.