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 R_F12 and Rf₁₂ are more accurate than Rz₁₂ and Ri₁₂. The accuracies of the indices R_F12 and Rf₁₂ for cycles 19–20 coincide. For cycles 21–24, the index Rf₁₂ is more accurate than R_F12, and this advantage of Rf₁₂ is especially distinct in cycles 23–24. The index Rf₁₂ differs from R_F12 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.