Regularities of the midlatitude spread F occurrence probability during sunrises and sunsets

The regularities of a change in the spread F occurrence probability (P) during the sunrises and sunsets have been analyzed based on data of Moscow station for low (1975, 1976, and 1985) and high (1979-198 1) solar activity. It has been obtained that additional (morning and evening) P maximums are observed only in winter before the sunrise and after the sunset over this station, when the magnetically conjugate ionosphere is sunlit. The appearance of these maximums is apparently related to heating of the ionosphere by photoelectrons from the conjugate hemisphere. It has been established that the evening and morning boundaries of the spread F observation interval almost coincide with the instants of sunset and sunrise over this station at low solar activity. At high solar activity, the spread F observation interval shifts toward increased local times. In such a case, the evening, boundary of this interval shifts in winter, and the morning boundary is displaced in summer and during equinoxes. As a result, the morning boundary of this interval is sunlit in the ionosphere. The established regularities are qualitatively interpreted.