North-South Asymmetries of High-latitude Plasma Convection and Upper Atmospheric Dynamics
Foerster, Matthias¹; Haaland, Stein²; Cnossen, Ingrid^3
1GFZ German Research Centre for Geosciences, GERMANY; ²Department of Physics and Technology, University of Bergen, NORWAY; ³British Antarctic Survey, Cambridge, UNITED KINGDOM

Recent observations have shown that the ionospheric/thermospheric response to solar wind and IMF dependent processes in the magnetosphere can be very dissimilar in the Northern and Southern Hemisphere. We present statistical studies of both the high-latitude ionospheric convection and the upper thermospheric circulation patterns obtained from almost a decade of measurements starting in 2001 of the electron drift instrument (EDI) on board the Cluster satellites and an accelerometer on board the CHAMP spacecraft, respectively. Using the Coupled Magnetosphere-Ionosphere-Thermosphere (CMIT) model, on the other hand, we simulated a 15-day spring equinox interval of low solar activity with both symmetric dipole and realistic (IGRF) geomagnetic field configurations to prove the importance of the hemispheric differences for the plasma and neutral wind dynamics.

The spatially distributed Cluster/EDI plasma drift measurements were mapped to a common reference level at F-region height (~400 km) in a magnetic latitude/MLT grid by use of the Tsyganenko geomagnetic field model. The CHAMP accelerometer measurements have been analysed to derive the neutral thermospheric wind circulation at high latitudes at about the same height. We obtained both regular upper thermospheric wind vorticity and ionospheric plasma drift pattern according to the various IMF conditions in a statistical average sense. The same procedure of data binning and of sorting for specified IMF directions has been used for both quantities.

A similar data analysis has been performed with the model simulations results of the representative 15-day interval. The survey of both the numerical simulation and the statistical observation results show some prominent asymmetries between the two hemispheres, which are likely due to the different geographic-geomagnetic offset, or even due to different patterns of geomagnetic flux densities. Plasma drift differences can partly be attributed to differing ionospheric conductivities. The forthcoming Swarm satellite mission will provide valuable observations for further detailed analyses of the North-South asymmetries of plasma convection and neutral wind dynamics.