Trends in Ionospheric Solar Activity Indices

The article presents the first results of identifying trends in annual average ionospheric indices ΔIG12 and ΔT12, which are obtained after excluding from IG12 and T12 the dependence of these indices on solar activity indices. In this case, solar activity indices are F10 and F30—solar radio emission fluxes at 10.7 and 30 cm. It was found that for the interval of 1957–2023, all analyzed linear trends are negative, i.e., quantities ΔIG12 and ΔT12 decrease over time, and these trends are significant. In absolute value, they are maximum for ΔIG12, taking into account the IG12 dependence on F1012, and minimum for ΔT12, taking into account the T12 dependence on F3012. Account for the nonlinearity of trends shows that, e.g., after 2010, they intensified. Relations are presented that make it possible, based on data from trends of the ionospheric indices (ΔIG12 or ΔT12), to judge the nature of the Δ foF2 trend over a specific point. For this, using the IRI model for foF2, a coefficient was obtained that gives the relationship between the trends of the ionospheric index and Δ foF2 over this point. Comparison with experimental data at mid-latitudes revealed that trends of the ionospheric indices make it possible to correctly determine the sign of the Δ foF2 trend and the general tendency for this trend change, but the calculated value of the trend over a specific point may differ markedly from the experimental data.