Tailward and earthward flow onsets observed by Cluster in a thin current sheet

Abstract

From a Cluster survey of the magnetotail during 2001-2007 we select 49 episodes of thin current sheet observations ending with plasma sheet activity onsets. The onsets were defined as flow bursts and/or electric current decrease and/or B_z increase after a period of local quietness (in many cases with signatures of growth phase). Such onsets at 17-20 R_E of radial distance were accompanied mainly by tailward flows with negativeB_z. At 11-17 R_E, earthward flows dominated, except the premidnight sector, where flows of both directions were observed. Tailward velocities were often rather small, within 200-300 km/s. We interpret such tailward flows as reconnection pulses occurring on the closed field lines in the stretched magnetic configuration. Ten activity onsets were not accompanied by plasma flows (faster than 100 km/s). Preonset current density was larger on average in the premidnight and midnight sector in comparison with the postmidnight.