The magnetic field clock angle departure in the Venusian magnetosheath and its response to IMF rotation

Abstract

We investigate the characteristics of interplanetary magnetic field (IMF) draping in the Venusian magnetosheath using both Venus Express (VEX) observations and magnetohydrodynamics simulations. The distributions of magnetosheath field clock angle illustrate the nearly symmetric morphology of draped magnetic field with respect to the solar wind electric field, and the departure of the IMF clock angle is larger at closer distances. Based on VEX data, the sheath field clock angle departures are found to be <45 degrees for 90% of the instances under steady IMF and this parameter can respond almost immediately to the unsteady IMF. We suggest the magnetosheath field just slips around the planet without significant pileup or bending. Our time-dependent simulations indicate that the response time of sheath field to IMF variation is not more than 1 min and it depends on the involved regions of magnetosheath: the timescale in the inner part of magnetosheath adjacent to the induced magnetosphere is longer than that in the outer part. We find this timescale is controlled by the convection velocity in the magnetosheath, emphasizing the magnetohydrodynamic characteristics of the behavior of the sheath field. The finite magnetosheath field clock angle departure and its quick response to IMF variation suggest that the magnetic field clock angle measured within the Venusian magnetosheath can be used as a reasonable proxy for the upstream IMF clock angle.