Cluster observations of partial derivativeBz/partial derivativex during growth phase magnetotail stretching intervals

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 B-z/x to approximate 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.