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

Abstract

We study a statistics of ∂Bz/∂x in a thin stretching current sheet (substorm growth phases) observed by Cluster between 8 and 18 RE downtail. After 2005, spacecraft separation allowed to measure directly this derivative of Bz along the tail axis. The near-tail events (within 14 RE) exhibited a straight decrease of an initially large positive ∂Bz/∂x to ∼1-2 nT/RE. In the more stretched middle tail, usually the small |∂Bz/ ∂x|<0.5-1 nT/RE had no clear trend and fluctuated around zero with time scales 5-15 min. In general, negative ∂Bz/∂x were ubiquitous. At some onsets, larger negative ∂Bz/ ∂x