Effective Solar Activity Index for the Median of the Critical Frequency of the F2 Layer

By comparing the moving 12-month averaged values of the F107 solar radiation flux at a wavelength of 10.7 cm and ionospheric solar activity index T, we established the effective solar activity index T-F, which is defined as a second-degree polynomial from F107 with coefficients independent of time; it is intended for use in problems of forecasting the median of the critical frequency of the F2 layer, foF2, using well-known ionospheric models instead of T and F107 indices or international sunspot numbers Ri. T-F is applicable for any solar activity level and the entire F107 measurement period. By definition, T-F is independent of geomagnetic activity. The T index may depend on geomagnetic activity due to the dependence of the median of foF2 on this activity. It was found that the moving 12-month averaged index of geomagnetic activity Ap(12) and the difference Delta T = T - T-F decreased over time in 1954-2019, but for Delta T, this trend was likely qualitative. As a result, a linear dependence of Delta T on Ap(12) is virtually absent for this interval: their correlation coefficient was less than 0.1.