Cluster survey of cusp reconnection and its IMF dependence -: art. no. L19808

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 similar to90degrees 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 90degrees clock angle (or less than 90degrees 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 similar to90% when the IMF has a northward component, indicating that simultaneous reconnection in the northern and southern cusps is common during northward IMF.