Global characteristics of the correlation and time lag between solar and ionospheric parameters in the 27-day period

The 27-day variations of topside ionosphere are investigated using the in situ electron density measurements from the CHAMP planar Langmuir probe and GRACE K-band ranging system. As the two satellite systems orbit at the altitudes of ∼370 km and ∼480 km, respectively, the satellite data sets are greatly valuable for examining the electron density variations in the vicinity of F2-peak. In a 27-day period, the electron density measurements from the satellites are in good agreements with the solar flux, except during the solar minimum period. The time delays are mostly 1–2 day and represent the hemispherical asymmetry. The globally-estimated spatial patterns of the correlation between solar flux and in situ satellite measurements show poor correlations in the (magnetic) equatorial region, which are not found from the ground measurements of vertically-integrated electron content. We suggest that the most plausible cause for the poor correlation is the vertical movement of ionization due to atmospheric dynamic process that is not controlled by the solar extreme ultraviolet radiation.

► The correlation between solar flux and electron density is relatively poor in the equatorial region.
► The time delays are mostly 1–2 day and represent the hemispherical asymmetry.
► The SOHO/SEM is more highly correlated with the electron density than F10.7 is.