Solar Activity Index for the Critical Frequency of the E Layer

The index P = (F1 + F81)/2 is the optimal solar activity index for the critical frequency of the E layer, foE, where F1 and F81 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 F81 on a given day, knowledge of F1 is needed not only on this and previous days, but also 40 days in advance. Instead of index F81, 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 F1 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 * = (F1 + 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