The thermospheric response to geomagnetic storms

Abstract. The dynamics of the upper atmosphere is strongly controlled by the intensity of the solar EUV radiation. This dependence is rather well described by models of the atmosphere, such as MSIS. On magnetically active days there are other sources of energy to the upper atmosphere, such as electric currents, which can surmount the effect of solar radiation. Due to high spatial and temporal variability of the geomagnetic input, the characterization of the thermospheric response is still quite vague. This situation is partly due to the lack of sufficient data. The low-altitude, polar orbiting CHAMP satellite provides with its high-sensitive accelerometer continuous measurements of thermospheric density and winds since August 2000. With the help of this unique data set, covering now half a solar cycle, the various aspects of the thermospheric dynamics can be studied in detail. We present recent findings of thermospheric dynamics based on CHAMP observations. For example, the degree of Joule heating depends not only on the current intensity, but also to a large extent on the dissipation efficiency. This has regional, local time and seasonal dependences. Only when taking these aspects into account, the storm-time thermospheric response can be interpreted correctly. For a few major storms in 2003 we review upper atmospheric features derived from CHAMP air drag and magnetic field measurements, which are compared to model predictions as well