Formation of intense cross-tail and field-aligned currents in the near-Earth tail during substorms

Abstract:

The basic statistical properties of the near-Earth current sheet from 8 Re to 12 Re were recently revealed by the THEMIS measurements[ Saito, 2015 submitted to JGR]. In this talk, we study the formation of high current densities in the near-Earth tail and their relationship with substorms. Ampere’s law was used to estimate the cross-tail and field-aligned current densities when the locations of two spacecraft were suitable. A typical cross-tail current density was found to be ~2 nA/m^2, while in some cases, the current density increased above 4 nA/m^2. In contrast to the commonly accepted picture, these high current densities appeared in two magnetic configurations: tail-like and dipolar structure. The former configuration is a typical feature for the high cross-tail current density observations during the growth phase of substorm. However, we rule out the notion that the high current density is caused by plasma sheet compression. Instead, we discuss that an alternative process of plasma sheet thinning proposed by Hsieh and Otto [2014], which is caused by an erosion of the magnetic flux in the dayside rather than the loaded magnetic flux in the tail, is relevant. The latter configuration of dipolar structure is a typical feature during the substorm expansion phase. It is found that in the inner-edge of the near-Earth tail (X~-8 RE), cross-tail current density increased during the expansion phase of substorm. At the same time, field-aligned current density also increased, indicating a formation of new current system in this region. These high current densities lasted several to a few tens of minutes after the local dipolarization onset.