Solar Activity Index for the Critical Frequency of the E Layer

— The index P = ( F 1 + F 81 )/2 is the optimal solar activity index for the critical frequency of the E layer, foE , where F 1 and F 81 are the flux of radio emission from the Sun at a wavelength of 10.7 cm on a given day and the 81-day average value of this flux centered on a given day. Therefore, to calculate F 81 on a given day, knowledge of F 1 is needed not only on this and previous days, but also 40 days in advance. Instead of index F 81 , in problems on short-term forecasting of this index, it is possible to use F (27, 81), the weighted average solar activity index with a characteristic time of 27 days for the current and previous 80 days. Therefore, to calculate F (27, 81), knowledge of F 1 on this and previous days suffices. This paper presents the first estimates of the effectiveness of such a replacement for foE . For this, changes in the accuracy of calculating foE were analyzed when index P is replaced by P * = ( F 1 + F (27, 81))/2 in empirical models constructed from foE data of ionospheric stations in the daytime at middle and subauroral latitudes for 1959–1995. It turns out that the P and P * indices are almost equivalent for calculating foE based on the empirical models constructed at these latitudes: the difference in the coefficients of variation for foE does not exceed 0.3% in each season at different solar cycle phases. Therefore, P * can be recommended for use in short-term foE forecasting problems, since it is based on indices F 1 for the current and previous days, as opposed to index P , which requires a forecast 40 days in advance to calculate F 1 .