Dependence of the ratio [NO+] / [O2+] in the ionospheric E-region on solar activity

A connection between the ion concentration ratio [NO+]/[O2+] in the mid-latitude E-region and solar activity is considered. There is no evidence for such a connection in the empirical ionospheric IRI model. Nevertheless, we demonstrate that contrary to the model approximations there is a strong dependence of [NO+]/[O2+] on the solar activity index F10.7 at heights of the lower thermosphere. Probable causes for the discrepancy between measurement data and the model are discussed.