( 2015) found significant north-south asymmetry in the spatial distributions of subauroral ion drift in the geographic coordinates. They suggested that such hemispheric differences are partly associated with the stronger magnetic flux densities in the near-polar regions of the SH and partly due to the greater offset between the invariant and geographic southern poles. Both observations and simulations by Förster and Cnossen ( 2013) indicated larger magnitude of the cross-polar neutral wind and ion drift in the NH than in the SH. Based on long-term Cluster Electron Drift Instrument satellite measurements over a full solar cycle, Förster and Haaland ( 2015) found persistent asymmetries in the ionospheric convection pattern between the two hemispheres for a given IMF orientation. During disturbed periods, stronger responses in plasma density and drifts were found in the SH than in the NH (Bruinsma et al., 2006). ( 2003) with very high speed flow events occurring only in the SH. Under quiet geomagnetic conditions, strong asymmetric ionospheric convections were reported by Nishitani et al. Previous studies (e.g., Förster & Haaland, 2015 Grocott et al., 2005 Papitashvili & Rich, 2002 Pinnock et al., 1999 Ruohoniemi & Greenwald, 2005 Watanabe et al., 2007 Wilder et al., 2011) have found that the hemispheric asymmetries in the averaged pattern of ionospheric convection strongly depend on the interplanetary magnetic field (IMF) conditions. Such asymmetries have been observed and simulated not only in a statistically averaged sense but also in the dynamic response to external drivers. There are significant differences in both the horizontal and vertical ionospheric plasma drifts between the northern (NH) and southern (SH) hemispheres. 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. The seasonal variations of high-latitude V z are different in the NH and SH. In the geographic coordinates, the geographic longitudinal variation of V z is more pronounced in the SH. 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. 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 polar cap, downward V z is stronger in the SH. The north-south asymmetries are persistent no matter under what kind of seasonal, solar activity, and IMF conditions. V z is distributed in 0300–0900 magnetic local time sector in both the northern (NH) and southern (SH) hemispheres. 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.
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