Solar cycle effects on the upper atmosphere - Implications for satellite drag

High thermospheric temperatures are maintained against thermal diffusion primarily by the absorption of extreme ultraviolet (EUV) radiation by atmospheric constituents (mainly, atomic oxygen). Because the solar flux of EUV varies strongly over the 11-yr solar cycle, thermospheric temperatures likewise vary strongly over the solar cycle. In an average solar cycle, solar EUV and thermospheric temperature increase by about a factor of two from solar cycle minimum to solar cycle maximum. Because density is mainly an integrated effect of temperature, the change in density over the solar cycle is amplified with respect to the temperature change. Changes in temperature cause changes in composition and these changes act to mitigate the direct thermal effects. The net effect of temperature and composition is to give order-of-magnitude changes in density over the solar cycle. The atmospheric drag on satellites is proportional to atmospheric density. The effects on satellite lifetimes are profound. A fairly typical satellite initially at 500 km would have a lifetime of about 30 years under typical solar cycle minimum conditions and only about 3 years under solar maximum conditions. Large increases in the rate of change of orbital period from solar minimum to solar maximum are possible. The fractional increase in the highly reactive species atomic oxygen can exceed two orders of magnitude.