Revisiting sunspot number as an extreme ultraviolet (EUV) proxy for ionospheric F2 critical frequency

This study reconsiders sunspot number (Sn) as a solar extreme ultraviolet (EUV) proxy for modeling the ionospheric F2 layer's critical frequency (foF2) over the period 1960–2023. We compare the performance of Sn with F10.7 and F30 solar radio fluxes, focusing on their ability to model the Ionospheric Global index (IG). Our results reveal that while F30 has shown a better correlation in recent solar cycles, Sn is the most stable and reliable over the entire dataset, obtaining the highest correlation. In addition, if we remove the saturation effects from considering a maximum value of Sn, the correlation increases, outperforming all other proxies and correctly predicting the long-term trend estimated by general circulation models.