UT Variation of Geographical Pole Thermospheric Densities and Its Dependence on Solar Wind Sector Polarity and Solar Activity

A superposed epoch analysis is conducted with the thermospheric mass densities near the geographic south and north poles. The densities are derived from the accelerometer onboard the Gravity Recovery and Climate Experiment (GRACE) satellite and are normalized to 400 km. The geographic poles are preferred in this study for multiple reasons. Firstly, the GRACE satellite passes near the poles in every orbit. Secondly, the longitude and local time effects can be neglected at the poles, so we can focus on universal time (UT) variations. More importantly, the south pole passes near the cusp location at about 1600 UT every day. Our results show that the thermospheric densities near the south pole are enhanced by up to 16% of the daily mean at about 1600 UT, which may result from the periodic pass of the cusp zone. It is also found that the UT variation depends on solar wind sector structure and solar activity. Near the south pole, it is likely to observe a more apparent and enhanced UT variation when the earth is embedded in an away (-Bx +By) solar wind sector polarity. The opposite applies to the north pole. In addition, stronger solar activities may enhance the UT change in thermospheric densities near both the south and north poles.