Empirical model of the thermospheric mass density based on CHAMP satellite observations

Abstract

The decadal observations from CHAMP satellite have provided ample information on the Earth's upper thermosphere, reshaping our understandings of the vertical coupling in the atmosphere and near‐Earth space. An empirical model of the thermospheric mass density is constructed from these high‐resolution observations using the multivariable least‐squares fitting method. It describes the density variation with latitude, longitude, height, local time, season, and solar and geomagnetic activity levels within the altitude range of 350–420 km. It represents well prominent thermosphere structures like the equatorial mass density anomaly (EMA) and the wave‐4 longitudinal pattern. Furthermore, the empirical model reveals two distinct features. First, the EMA is found to have a clear altitude dependence, with its crests moving equatorward with increasing altitude. Second, the equinoctial asymmetry is found to strongly depend on solar cycle, with its magnitude and phase being strongly regulated by solar activity levels. The equinoctial density maxima occur significantly after the actual equinox dates toward solar minimum, which may signal growing influence from the lower atmosphere forcing. This empirical model provides an instructive tool in exploring thermospheric density structures and dynamics. It can also be easily incorporated into other models to have a more accurate description of the background thermosphere, for both scientific and practical purposes.