Thermospheric density during geomagnetic storm based on EOF analysis

The Empirical Orthogonal Function (EOF) analysis has been applied to study the thermospheric density at 400 km height with accelerometer data from CHAMP satellite during geomagnetic storm from 2002-2006. The global latitudinal distribution of thermospheric density on storm-time is investigated. Additionally, the relations of density and ap index, as well as Dst index are analyzed. The results show that thermospheric density has strong seasonal dependence in latitude variation. Summer hemisphere density is larger than that in winter hemisphere, whereas in spring and autumn density is symmetrical in northern and southern hemisphere. On the dayside of spring and autumn, there exists a structure of double crests at low latitude which is known as Equatorial Mass density Anomaly (EMA). Moreover, at mid- and high-latitude, density decreases as the latitudes change from low to high. On the night side, density distribution has a profile of parabola with the minimum amplitude near equator. The characteristic of density latitudinal distribution is steady in several days, which leads to similar distribution curves of different storms occurring at close times. Furthermore, the variation of density latitudinal distribution from pre-storm to storm time is small. By correlation analysis it is found that the change of density lags behind ap index by 2 similar to 6 hours and before Dst index 0 similar to 1 hour. In addition, thermospheric density response to geomagnetic storm on the dayside is faster than on the night side. Density has close correlation with ap index and Dst index.