Solar Activity Index for the Long-Term Prediction of the F2 Layer Critical Frequency

Abstract

Based on a comparison of the 12-month moving averages of the solar activity indices with the ionospheric index of solar activity IG12 for the period of 1954–2014, we estimated the relative accuracies of the solar indices as indicators of solar activity for the medians of the F2 layer critical frequency for a month. These solar indices are the previous (Rz12) and new (Ri12) versions of the relative sunspot number, as well as the 10.7 cm solar radio flux F12 scaled to Rz12, (Rf12) correction for low solar activity. The interval of 1954–2014 spans solar cycles 19–23 and incomplete cycle 24. It was found that, on the whole, the index Ri12 is more accurate than Rz12 and that the indices RF12 and Rf12 are more accurate than Rz12 and Ri12. The accuracies of the indices RF12 and Rf12 for cycles 19–20 coincide. For cycles 21–24, the index Rf12 is more accurate than RF12, and this advantage of Rf12 is especially distinct in cycles 23–24. The index Rf12 differs from RF12 only by the introduction of a new additional correction for low solar activity. This analytical correction was obtained from the condition of the minimum average deviation of Rf12 from IG12, which makes the index Rf12 advantageous as the most adequate indicator of the solar activity for the foF2 median among the solar indices.