The best solar activity proxy for long-term ionospheric investigations

Abstract To select the best solar activity proxy is important for long-term trend or climatological studies of ionospheric parameters as well as for modeling. We deal with solar proxies for foF2 and foE analyses. The yearly average and monthly median values of foF2 and yearly values of foE of selected European stations with long and high-quality data series are used. Four solar proxies are utilized: F10.7, F30, Mg II and He II. Mg II and F30 are found to be the best solar proxies for both yearly and monthly foF2 values as well as for years of deep solar minima (2008–09, 2018–19). On the other hand, F10.7 is found to be the best solar proxy for yearly values of foE. The variability of yearly values of foF2 and foE is almost fully described by solar proxies and this relation is highly linear. Different solar proxies applied might result in somewhat different long-term trends of both foF2 and foE. All the results were derived for the European area.