Cluster observations of ?b_z/?x during growth phase magnetotail stretching intervals

摘要

We study a statistics of ?B_z/?x in a thin stretching current sheet (substorm growth phases) observed by Cluster between 8 and 18 R_E downtail. After 2005, spacecraft separation allowed to measure directly this derivative of B_z along the tail axis. The near-tail events (within 14 R_E) exhibited a straight decrease of an initially large positive ?_z/?x to ～1-2 nT/R_E. In the more stretched middle tail, usually the small |?B_z/ ?x|<0.5-1 nT/R_E had no clear trend and fluctuated around zero with time scales 5-15 min. In general, negative ?B_z/?x were ubiquitous. At some onsets, larger negative ?B_z/ ?x<-1 nT/R_E were associated with transient dipolarizations, propagating earthward. There was no clear association of local plasma sheet activity onset with any value of ?B_z/?x. We discuss relation of observations and recent modeling results. Key Points Downtail Bz gradient directly observed for the first timeIn the near tail Bz gradient is always positiveIn the middle tail negative dB/dx are frequent during growth phase