UNIVERSAL VARIATION OF THE F-2-LAYER CRITICAL FREQUENCY AND SOLAR-ACTIVITY

Regular variations of the F-2-layer in the quiet ionosphere are studie d using the universal variation of the F-2-layer critical frequency, i .e. the dependence of the instantaneous mean-longitudinal value of foF (2) at a fixed latitude on universal time (UT). Algorithms for the uni versal variation calculation have been developed and are used to proce ss the ionospheric vertical sounding data for 1957-1990. It is shown t hat the foF(2)-universal variation forms are essentially different for solar minima and maxima years. In the solar minima years, the foF(2)- universal variation is similar to the known universal variation of the atmospheric electric field near the Earth's surface which has a maxim um at 19 UT. There is no foF(2)-universal variation seasonal change in solar minimum years. The foF(2)-universal variation form changes for solar maximum years have a minimum of foF(2), at 16-17 UT, and there a re small seasonal changes in the foF(2)-universal variation. Such beha viour of the foF(2)-universal variation suggests that under the quiet conditions in solar minimum years the electric field penetrates from t he lower atmospheric layers up to the F-2 layer of the ionosphere.