Solar rotation effects on CHAMP neutral and plasma densities at solar maximum and solar minimum

Thermospheric neutral and plasma densities derived from the Challenging Minisatellite Payload (CHAMP) accelerometer measurements are analyzed for the years 2003 and 2007. Statistical analysis in the time and frequency domains of the neutral and plasma densities, and the TIMED-SEE solar irradiances reveal a very different ionospheric response to solar radiation based on the phase of the solar cycle. We found that during solar maximum Sun's radiation dominates the dynamics of the ionospheric neutral density, whereas during solar minimum internal processes control its variability. We show that the solar radio flux (F10.7), widely used as a proxy for extreme ultraviolet (EUV) radiation, correlates well with neutral density only when solar forcing is high, whereas the Ap index captures more variability when solar radiation is low. Additionally, we found that at solar maximum the 133.5 nm solar EUV irradiance correlates with neutral density better than all other solar indices.