Evidence for spiral magnetic structures at the magnetopause: a case for multiple reconnections

Abstract

We analyze plasma structures within the low latitude boundary layer (LLBL) observed by Interball Tail spacecraft under southward interplanetary magnetic field. There is a variety of ion velocity distributions observed in the LLBL under this condition: (a) D-shaped distributions, (b) ion velocity distributions consisting of two counter-streaming magnetosheath-type, and (c) distributions with three components where one of them has nearly zero velocity parallel to magnetic field (V ∥), while the other two are counter-streaming components. D-shaped ion velocity distributions (a) correspond to magnetosheath plasma injections into reconnected flux tube, which is consistent with a spacecraft location relative to the reconnection site. Simultaneous counter-streaming injections suggest multiple reconnections. Three-component ion velocity distributions and their evolution with decreasing number density in the LLBL indicate that the significant part of it is located on long spiral flux tube islands at the magnetopause, as has been proposed and found to occur in magnetopause simulations. We interpret these distributions as a natural consequence of the formation of spiral magnetic flux tubes consisting of a mixture of alternating segments originating from the magnetosheath and magnetospheric plasmas. We suggest that multiple reconnections play an important role in the formation of the LLBL.