Universal variation of the F₂-layer critical frequency and solar activity

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 es-sentially 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.