The factors affecting noontime ionospheric NmF2 equinoctial asymmetry over mid-latitude regions

Mid-latitude semiannual noontime NmF₂ peaks were analyzed at four North Hemisphere (Boulder, Rome, Wakkanai, Juliusruh) and two South Hemisphere (Hobart, Port Stanley) stations. Aeronomic parameters responsible for the observed NmF₂ variations were obtained solving an inverse problem of aeronomy with the original THERION method. The NmF₂ autumnal peak on average is larger than the vernal one in both Hemispheres under solar minimum. The observed NmF₂ difference in the two peaks is attributed to the difference in thermospheric parameters not related to solar and geomagnetic activity. The vernal peak may occur in the course of three months in both Hemispheres while the occurrence of the autumnal peak is confined by two months. The abundance of atomic oxygen [O] plays the leading role in the difference between NmF₂ in the two peaks. A two-hump NmF₂ variation with a trough in December-January (Northern Hemisphere) is a manifestation of low [O] concentration in December/January relative to October/November values rather than the solar zenith angle effect. The empirical (based on observations) MSISE00 model indicates the global increase of the total atomic oxygen abundance during equinoxes which cannot be attributed to any redistribution of [O] in the thermosphere as we have the absolute global scale [O] increase. The downward transfer of [O] by eddy diffusion is the process which can globally control the amount of [O] in the thermosphere. Anyway it is not seen any other way to explain the global increase of the total amount of atomic oxygen during equinoxes.