Superposed epoch analyses of thermospheric response to CIRs: Solar cycle and seasonal dependencies

Thermospheric response to Corotating Interaction Regions (CIRs) has been studied previously; however, its solar cycle and seasonal effects have not been fully investigated. Thermospheric mass density at 400 km measured by the CHAMP satellite during 2001-2008 and Sigma O/N2 from the TIMED/GUVI instrument covering a period from 2002 to 2008 are used to investigate the solar cycle and seasonal dependencies of the thermospheric response to CIRs. Our results reveal: (1) solar minimum CIRs compared to solar maximum counterparts have larger solar wind speeds before and after the stream interface. However, solar wind dynamic pressure and merging electric field are slightly larger at solar maximum than solar minimum. (2) CIR-induced variations of Sigma O/N2 are characterized by high latitude depression and low latitude enhancement, a distinction from global enhancement of neutral density at a fixed altitude. These relative thermospheric changes are dependent on solar cycle, with a more pronounced increase in neutral density at all latitudes and a stronger decrease in Sigma O/N2 at high latitude at solar minimum than at solar maximum. (3) A seasonal asymmetry is presented in the relative deviations of thermospheric mass density and composition. On the dayside, the peak increases of neutral density at high latitudes on average are similar to 40% in the summer hemisphere and similar to 26% in the winter hemisphere. Nighttime neutral density changes are more remarkable than that in the same latitudinal bands of daytime and have the same seasonal preference of enhancement as the dayside. At the daytime, Sigma O/N2 at high latitudes suffers more reduction in the summer hemisphere than in the winter hemisphere. At middle latitudes, Sigma O/N2 reduces in the winter hemisphere; nevertheless, it increases slightly in the summer hemisphere.