High latitude magnetospheric topology and magnetospheric substorm

Abstract. This study is focused on the problem of the localization of substorm expansion onset. In this context, the high latitude topology of transverse magnetospheric currents has been analyzed. This study has included the radial distribution of plasma pressure near noon, obtained using the THEMIS-B satellite data, the daytime compression of magnetic field lines and the existence of magnetic field minima far from the equatorial plane, given by all geomagnetic field models. As a result, the dayside integral transverse currents at the geocentric distances 7–10 R_E has been estimated. It is suggested, that nightside transverse currents at geocentric distances ~7–10 R_E are closed inside the magnetosphere and with dayside transverse currents form surrounding the Earth current system (cut ring current or CRC) which topologically is the high latitude continuation of ordinary ring current. A possibility of localization of substorm expansion onset at the nighside CRC region is analyzed using the experimental evidences that the onset is localized at geocentric distances <10 R_E.