Investigation of longitudinal distribution of thermospheric density by an empirical model derived from the CHAMP data

The CHAMP satellite observes thermospheric mass density around 400km. By applying the least-squares approximate polynomial method to the CHAMP observation data, we constructed an empirical model to reproduce the thermospheric mass density at about 400km altitude. Geographical longitude, latitude, latitude, local time on the CHAMP orbit and so on are indepedendent variable of our model. For example, the model can reproduce a mass density distribution in [longitude,latitude] at the spring equinox and on fixed local time sector. In the present paper, we will show results of an empirical model that reproduces longitudinal dependence of thermospheric density on LT and season. Furthermore, we will refer to physical relations between the ionospheric structure like plasma density and thermospheric density.