Hemispheric asymmetry of the vertical ion drifts at dawn observed by DMSP

Based on the measurements from Defense Meteorological Satellite Program (DMSP) F12, F13, and F15 satellites in 1995-2014, we report significant hemispheric asymmetries in vertical ion drift velocity (V-z) at dawn (0500-0700 solar local time) in geomagnetic and geographic coordinates. V-z is distributed in 0300-0900 magnetic local time sector in both the northern (NH) and southern (SH) hemispheres. The north-south asymmetries are persistent no matter under what kind of seasonal, solar activity, and IMF conditions. In the polar cap, downward V-z is stronger in the SH. Such difference shows clear IMF B-Y dependence and is more significant in the local winter and/or under low solar activity conditions. In the geomagnetic coordinates, the auroral zone is dominated by upward V-z in the NH but by downward V-z in the SH statistically. In the geographic coordinates, the geographic longitudinal variation of V-z is more pronounced in the SH. The seasonal variations of high-latitude V-z are different in the NH and SH. The average asymmetric feature of V-z largely depends on the occurrence and magnitude of ion upflow/outflow, which are modulated by the combined effects of the asymmetric magnetic field configuration between the two hemispheres, and the dynamic processes in the tightly coupled ionosphere-thermosphere system, and their (probably nonlinear) interactions with each other.