SUNRISE EFFECTS IN THE EQUATORIAL F-LAYER

In the sunrise period, under certain conditions, a cavity of a low ele ctron density arises in the equatorial ionosphere at altitudes between the peaks of the night F-layer and the morning F-layer. The dependenc e of the cavity parameters on the initial conditions, solar activity, season, photochemistry, geomagnetic disturbance, diffusion, and vertic al drift of the ionospheric plasma is studied theoretically. It is sho wn that temporal variations of the electron density, as well as of the cavity parameters, depend mainly on the initial profile parameters, i onizing solar radiation flux, ion formation rate, diffusion, and geoma gnetic activity. It is concluded that reduced diffusion can be one of the major physical mechanisms which can assist the preservation, at la rge heights in the pre-sunrise F-layer, of the electron density at the day-time level. (C) 1997 Elsevier Science Ltd.