Principal modes of thermospheric density variability: Empirical orthogonal function analysis of CHAMP 2001-2008 data

In this paper we characterize the dominant modes of global thermosphere density variability as empirical orthogonal functions (EOFs) using densities obtained from the accelerometer experiment on board the CHAMP satellite during 2001-2008. We determine the significance of different types of thermospheric density variability to the overall density variation and also examine the drivers of these primary modes of variability. From a sequential nonlinear regression analysis of the density observations along satellite trajectories, we obtain a set of EOFs in magnetic latitude and magnetic local time coordinates and their orbit time-dependent amplitudes. EOF1 includes a strong global mean component and takes the form of the diurnal variation. It correlates highly with the daily F10.7 index. It underscores that solar EUV is by far the strongest driver of the overall thermospheric density variability. Additionally, the primary mode is modulated semiannually, and its magnitude decreases with declining solar activity. EOF2 has a hemispherically asymmetric structure and represents the summer-to-winter annual density variation. Density responses to geomagnetic forcing are primarily manifested in three different modes: EOF1 that represents the global mean response, EOF3 that has a pronounced magnetic local time-dependent feature, and EOF4, whose main features are high-latitude density increases. These modes have very different response time scales with respect to changes in solar wind parameters. In addition, EOF3 and EOF4 contain signatures of local time variability that are possibly connected with the effects of a solar terminator wave and high-order solar tides that propagate upward from the lower atmosphere and/or with local plasma-neutral interactions in the F region.