Variations of the flank LLBL thickness as response to the solar wind dynamic pressure and IMF orientation

摘要

Several mechanisms have been discussed as candidates for a formation of the low-latitude boundary layer (LLBL) magnetic reconnection between the magnetospheric and magnetosheath magnetic fields, impulsive penetration of magnetosheath plasma, and viscous/diffusive mixing of plasma populations at the magnetopause. The observed fluctuations of plasma parameters inside the LLBL are attributed either to transient nature of the phenomena forming the layer or to sweeping of deformations of the magnetopause or an inner edge of the LLBL surface along the spacecraft. We are using the Interball-1/Magion-4 satellite pair separated by several thousands of kilometers in order to distinguish between spatial and temporal changes of the LLBL plasma population. Observations of LLBL crossings invoked by sudden changes of upstream conditions show that even during a strongly northward interplanetary magnetic field (IMF), the LLBL is relatively thin and follows the compression of the magnetopause induced by changes of the solar wind dynamic pressure. The thickness of the LLBL increases significantly (from a small fraction of R E to more than 1.4 R E) with increase in upstream density and IMF B Y component. Our results indicate that the dusk LLBL is supplied by high-latitude reconnection in the Southern (Northern) Hemisphere, when IMF B Y is negative (positive) and lies on open field lines. The change of IMF B Y direction leads to brief presence of LLBL plasma outside the magnetopause on magnetosheath field lines. Fluctuations of plasma parameters and magnetic field implicate the presence of surface waves on the inner edge of the LLBL, but no waves were observed on the magnetopause.