The aim of the paper is to study the dependence of the occurrence probability of the sporadic Es layer on solar activity at different values of the Es critical frequency. The paper uses data from ionosondes at Irkutsk (52.3° N, 104.3° E), Kaliningrad (54.36° N, 20.12° E), and Novosibirsk (54.84° N, 83.23° E). A new method for correlation analysis of the relationship between the Es occurrence probability and solar activity has been developed. The method involves calculating the correlation coefficients R between annual average values of the solar activity index F10.7 and annual average occurrence probabilities of sporadic Es layer, whose critical frequency foEs is in the range fi÷fi+1. As a result of applying the new method, we have found that the correlation coefficients are negative for the foEs ranges of 1-2 and 2-3 MHz and positive for 4-5 MHz for all three stations; however, the results for foEs>5 MHz differ for different stations. The obtained results are interpreted on the basis of three factors of solar activity influence: 1) increase in photoionization; 2) weakening of the zonal component of the prevailing wind; 3) increase in absorption of radio waves in the underlying layers of the ionosphere.
