Solar activity proxies and the E layer

Analysis of the quality of description of the dependence of the E-layer critical frequency foE on solar activity by five solar activity proxies (F, F, MgII, Ly-α, and Rz) is performed. It is emphasized that interest to the problem of description of ionospheric parameters behavior by various SA proxies increased sharply during the recent years, and a brief review of the recent publications on that problem is given. The behavior with solar activity of the foE monthly medians based on observations at three European vertical sounding stations Juliusruh, Slough, and Rome in a period of 1957–1980 for two months of each season (winter, equinox, and summer) is considered. As a measure of the quality of that description, the determination coefficient R according to Fisher F-test is used, and the method used earlier for similar analysis of the foF2 data is applied. It is found that three proxies (F, MgII, and Ly-α) provide similar quality of the description (almost coinciding values of R in the majority of cases above 0.9) for all considered situations. The F and Rz proxies describe the foE behavior worse than these three proxies, the Rz proxy being the worst.