Cluster observations of ∂B_z/∂x during growth phase magnetotail stretching intervals

Abstract

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 ∼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.