Abstract
Regular variations of the F2-layer in the quiet ionosphere are studied using the universal variation of the F2-layer critical frequency, i.e. the dependence of the instantaneous mean-longitudinal value of foF2 at a fixed latitude on universal time (UT). Algorithms for the universal variation calculation have been developed and are used to process the ionospheric vertical sounding data for 1957-1990. It is shown that the foF2-universal variation forms are essentially different for solar minima and maxima years. In the solar minima years, the foF2-universal variation is similar to the known universal variation of the atmospheric electric field near the Earth's surface which has a maximum at 19 UT. There is no foF2-universal variation seasonal change in solar minimum years. The foF2-universal variation form changes for solar maximum years have a minimum of foF2, at 16-17 UT, and there are small seasonal changes in the foF2-universal variation. Such behaviour of the foF2-universal variation suggests that under the quiet conditions in solar minimum years the electric field penetrates from the lower atmospheric layers up to the F2 layer of the ionosphere.
