| Summary: | It has been previously demonstrated that a two-ion (O + and H + ) 8-moment time-dependent fluid model was able to reproduce correctly the ionospheric structure in the altitude range probed by the EISCAT-VHF radar. In the present study, the model is extended down to the E-region where molecular ion chemistry (NO + and O + 2 , essentially) prevails over transport; EISCAT-UHF observations confirmed previous theoretical predictions that during events of intense E × B induced convection drifts, molecular ions (mainly NO + ) predominate over O + ions up to altitudes of 300 km. In addition to this extension of the model down to the E-region, the ionization and heating resulting from both solar insolation and particle precipitation is now taken into account in a consistent manner through a complete kinetic transport code. The effects of E × B induced convection drifts on the E- and F-region are presented: the balance between O + and NO + ions is drastically affected; the electric field acts to deplete the O + ion concentration. The [NO + ]/[O + ] transition altitude varies from 190 km to 320 km as the perpendicular electric field increases from 0 to 100 mV m -1 . An interesting additional by-product of the model is that it also predicts the presence of a noticeable fraction of N + ions in the topside ionosphere in good agreement with Retarding Ion Mass Spectrometer measurements onboard Dynamic Explorer.
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