Observations of an active thin current sheet

摘要

We analyze observations of magnetotail current sheet dynamics during a substorm between 2330 and 2400 UT on 28 August 2005 when Cluster was in the plasma sheet at [?17.2, ?4.49, 0.03] R E (GSM) with the foot points near the IMAGE ground-based network. Observations from the Cluster spacecraft, ground-based magnetometers, and the IMAGE satellite showed that the substorm started in a localized region near midnight, expanding azimuthally. A thin current sheet with a thickness of less than 900 km and current density of about 30 nA/m2 was observed during 5 min around the substorm onset. The thinning of the current sheet was accompanied by tailward plasma flow at a velocity of ?700 km/s and subsequent reversal to earthward flow at V x ≈ 500 km/s coinciding with a B z turning from ?5 to +10 nT. The analysis of magnetic and electric fields behavior and particle distributions reveals signatures of impulsive (with ～1 min timescale) activations of the thin current sheet. These observations were interpreted in the framework of transient reconnection, although the data analysis reveals serious disagreements with the classical 2.5-D X line model.