Connection of seasonal changes of nighttime middle-latitude F scattering with solar and magnetic activity

Seasonal changes of frequency midnight F scattering taken from the data of the Moscow station are analyzed. It is shown that the probability P of occurence of F scattering is determined mainly by two factors: the latitudinal gradient Ne below hmF2, inversely proportional to the exospheric temperature T∞ and the frequency of substorm recurrence. The first factor dominates at a low solar activity, providing for the main P maximum in winter and the additional one in summer in correspondence with the changes in T∞-1. The second factor prevails at a high solar activity