Cluster survey of cusp reconnection and its IMF dependence

Abstract

We have surveyed the occurrence of tailward-of-the-cusp reconnection detected by Cluster as a function of the interplanetary magnetic field (IMF) clock angle. The survey covers 3 years (2001-2003) of cusp and magnetopause (MP) crossings and is restricted to periods of relatively stable IMF. Our survey indicates that the reconnection associated plasma flows occur almost exclusively when the IMF has a northward component (or the clock angle is within ~90° of the GSM +z direction). This finding at first seems inconsistent with the component merging model which should allow reconnection to occur for larger than 90° clock angle (or less than 90° in magnetic shear angle). However, it is possible that this is a geophysical effect. When the IMF has a southward component, reconnection equatorward of the cusp could prevent tailward-of-the-cusp reconnection by (1) creating low-shear condition behind the cusp, and (2) preventing the formation of a plasma depletion layer adjacent to dayside MP, resulting in super-Alfvenic magnetosheath flows at high latitude. No reconnection flows can be detected sunward of the X-line in this regime. Finally, the occurrence rate of tailward-of-the-cusp reconnection flows is ~90% when the IMF has a northward component, indicating that simultaneous reconnection in the northern and southern cusps is common during northward IMF.