The relationship of thermospheric density anomaly with electron temperature, small-scale FAC, and ion up-flow in the cusp region, as observed by CHAMP and DMSP satellites

We present in a statistical study a comparison of thermospheric mass density enhancements (rho(rel)) with electron temperature (T-e), small-scale field-aligned currents (SSFACs), and vertical ion velocity (V-z) at high latitudes around noon magnetic local time (MLT). Satellite data from CHAMP (CHAllenging Minisatellite Payload) and DMSP (Defense Meteorological Satellite Program) sampling the Northern Hemisphere during the years 2002-2005 are used. In a first step we investigate the distribution of the measured quantities in a magnetic latitude (MLat) versus MLT frame. All considered variables exhibit prominent peak amplitudes in the cusp region. A superposed epoch analysis was performed to examine causal relationship between the quantities. The occurrence of a thermospheric relative mass density anomaly, rho(rel) > 1.2, in the cusp region is defining an event. The location of the density peak is taken as a reference latitude (Delta MLat = 0 degrees). Interestingly, all the considered quantities, SSFACs, T-e, and V-z are co-located with the density anomaly. The amplitudes of the peaks exhibit different characters of seasonal variation. The average relative density enhancement of the more prominent density peaks considered in this study amounts to 1.33 during all seasons. As expected, SSFACs are largest in summer with average amplitudes equal to 2.56 mu Am-2, decaying to 2.00 mu Am-2 in winter. The event related enhancements of T-e and V-z are both largest in winter (Delta T-e = 730 K, V-z = 136ms(-1)) and smallest in summer (Delta T-e = 377 K, V-z = 57ms(-1). Based on the similarity of the seasonal behaviour we suggest a close relationship between these two quantities. A correlation analysis supports a linear relation with a high coefficient greater than or equal to 0.93, irrespective of season. Our preferred explanation is that dayside reconnection fuels Joule heating of the thermosphere causing air upwelling and at the same time heating of the electron gas that pulls up ions along affected flux tubes.