• thermosphere;
  • solar EUV flux

[1] Long-term changes in the thermospheric composition (O/N2 column density ratio) and exospheric temperature (Texo) have been examined. The data are obtained from TIMED/GUVI disk and limb measurements over half of a solar cycle from 2002 to 2007. The data indicate a positive correlation between O/N2 and solar EUV flux (Qeuv), despite large variability in O/N2 due to geomagnetic activity and season and local time effects. A function fitting provides quantitative specification of the O/N2 dependence on Qeuv. The magnitude of changes in O/N2 due to solar EUV variation between 2002 and 2007 is 40% of the mean (0.5) O/N2 over the same period and is of the same order to the changes in O/N2 due to nonsolar EUV effects (geomagnetic activity, season, and local time). Similar results are also obtained for the Texo dependence on Qeuv. The O/N2 dependence on Qeuv is due to thermal expansion or contraction that alters the reference height of the fixed N2 column density (1017 cm−2). This differs from the O/N2 depletion associated with high Texo during storm time where upward wind plays an important role.