Abstract
According to frequent five-minute vertical sounding of the ionosphere in Almaty (76°55′ E, 43°15′ N) conducted in 2000–2014, the rate of occurrence of nighttime enhancements of the electron concentration at the F 2-layer maximum is analyzed, the distributions of enhancement durations are obtained, and the parameters of several very large enhancements recorded at the same time in Irkutsk and Almaty are compared. During the analyzed period, 2272 observation sessions were carried out. In 1430 sessions NmF 2 enhancement was observed. The high probabilities of enhancement formation (up to 90%) in January, February, November and December are distribution features that are identical for high and low solar activity. In addition, a rapid decrease in the probability from February to March and smooth increase from September to December occurs in the same manner. High solar activity is characterized by a distinct maximum frequency in the summer months, whereas low activity is characterized by a minimum frequency. The seasonal dependence of distributions of enhancement durations is shown: durations are distributed over a wide range in the autumn–winter season and in a narrow range in the spring–summer season.