The satellite CHAMP with its sensitive accelerometer on board provides the opportunity to investigate the thermospheric dynamics in great detail. On its near-polar, low-Earth orbit (about 400 km) it is well suited to map the air density and winds along the track. In this talk we concentrate on the low latitude region. In a multi-years statistical study we investigate how the air density in a latitude band of ±30° about the equator on the day and on the night side responds to external forcing. The prime driver of thermospheric density is the solar flux. For magnetically quiet days a high degree of correlation (cc>0.9) is obtained when comparing the mass density with the solar flux index, 0.5 (F10.7 + F10.7_mean81), from the previous day. The slope of the regression line is, however, steeper by a factor of 2 on the dayside than on the night side. This ratio is independent of season and solar flux level. Magnetic activity is another driver. In this case the energy input takes place at auroral latitudes. Density bulges created at high latitudes in both hemispheres propagate subsequently towards the equator. We observe a delayed response on the night side compared to the dayside, which implies a later energy input in that time sector. We furthermore investigate which magnetic activity index is best suited to characterize the air density increase. In addition it is checked whether a preconditioning of the thermosphere by solar flux has an influence on the amplitude of the density anomaly caused by magnetospheric forcing. The presented results can be used as constrains for the improvement of atmospheric models.

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Lühr, H.; Müller, S. (2008): The different response of the thermospheric mass density on the day and night side to solar flux and magnetospheric forcing. 37th COSPAR Scientific Assembly (Montreal, Canada 2008).