Causes of the mid-latitude NmF2 winter anomaly at solar maximum

A detailed numerical study is made of the physical and chemical processes, which determine the electron density and temperature measured by the Millstone Hill radar during the geomagnetically quiet periods of 3 June 1979 and 6 January 1980 at solar maximum. It is shown that inclusion of vibrationally excited N2(v>0) and O2(v>0) in the loss rate of O+(4S) ions brings the measured and modeled electron densities and temperatures into reasonable agreement while the effects of N2(v>0) and O2(v>0) on the ion temperature caused by changes in the electron densities and temperatures are negligible at F region and topside ionosphere altitudes. A one-dimensional theoretical model of the middle-latitude ionosphere and plasmasphere is applied to study three distinct mechanisms of the NmF2 winter anomaly formation. In winter the O density increases and the N2 and O2 densities decrease in comparison with those in summer. These seasonal changes in [O], [N2], and [O2] determine a part of the studied NmF2 winter anomaly through variations in the loss rate of O+(4S) ions due to the reactions of O+(4S) ions with vibrationally unexcited molecular nitrogen and oxygen, and variations in the production rates of O+(4S) ions due to atomic oxygen ionization by solar EUV fluxes and photoelectrons. In the second mechanism, the increases in the vibrational temperatures of N2 and O2 in summer compared to winter (due to concomitant increases in the neutral and electron temperatures) and the increases in the vibrationally excited N2 and O2 number densities due to the winter/summer variations in [N2] and [O2] increase the O+(4S) loss rate. The third mechanism of the NmF2 winter anomaly is connected to electronically excited atomic oxygen ions, which constitute a large source, Qex, of O+(4S) ions through chemical reactions. Neutral composition and electron density seasonal variations result in seasonal variations of Qex providing a part of the studied NmF2 winter anomaly.