Articles | Volume 33, issue 10
Regular paper
26 Oct 2015
Regular paper |  | 26 Oct 2015

A physical explanation for the magnetic decrease ahead of dipolarization fronts

Z. H. Yao, J. Liu, C. J. Owen, C. Forsyth, I. J. Rae, Z. Y. Pu, H. S. Fu, X.-Z. Zhou, Q. Q. Shi, A. M. Du, R. L. Guo, and X. N. Chu

Abstract. Recent studies have shown that the ambient plasma in the near-Earth magnetotail can be compressed by the arrival of a dipolarization front (DF). In this paper we study the variations in the characteristics of currents flowing in this compressed region ahead of the DF, particularly the changes in the cross-tail current, using observations from the THEMIS satellites. Since we do not know whether the changes in the cross-tail current lead to a field-aligned current formation or just form a current loop in the magnetosphere, we thus use redistribution to represent these changes of local current density. We found that (1) the redistribution of the cross-tail current is a common feature preceding DFs; (2) the redistribution of cross-tail current is caused by plasma pressure gradient ahead of the DF and (3) the resultant net current redistributed by a DF is an order of magnitude smaller than the typical total current associated with a moderate substorm current wedge (SCW). Moreover, our results also suggest that the redistributed current ahead of the DF is closed by currents on the DF itself, forming a closed current loop around peaks in plasma pressure, what is traditionally referred to as a banana current.

Short summary
We use THEMIS large data set of dipolarization front events to build a 2-D pressure distribution in XZ plane, and thus derive the current system around the dipolarization front. Our results show that a banana current loop is formed around the dipolarization front. This current is also suggested to be the reason for the magnetic dip observed ahead of the dipolarization front. In addition, the current density is too small to contribute a substorm current wedge.