Dependence of the midlatitude F-Spread probability on solar and geomagnetic activity

The dependences of the monthly mean P and running annual mean P-yr probabilities and quasi-biennial oscillations DeltaP(yr) of the probabilities of the F-spread on solar and geomagnetic activities have been analyzed using the data of Moscow station for 1975-1985. It has been established that an increase in solar activity leads to a shift of the P maximum toward morning hours during all seasons. The shift value is 2-3 h at the transition from low to high solar activity. An increase in solar activity leads to a decrease in P, P-yr, and DeltaP(yr) Under equal other conditions, this dependence is best pronounced in pre-midnight hours. The dependence of P on solar activity is significant only in winter and late autumn. Nevertheless, the P-yr and DeltaP(yr) dependences on solar activity are significant at pre-midnight hours. Geomagnetic activity increase leads to an increase in P, P-yr, and DeltaP(yr), this dependence being best pronounced after midnight under equal other conditions. At these hours the P-yr and DeltaP(yr) dependences on geomagnetic activity are significant. On the whole, the solar activity contribution to the P-yr variation is more important than the contribution of geomagnetic activity. The opposite tendency is typical of DeltaP(yr) A qualitative interpretation of these F-spread regularities is presented.