LAUSR

To study ionospheric climate, to model the ionosphere (e.g., the International Reference Ionosphere—IRI) and to investigate its long‐term changes and trends, solar activity proxies/indices have been used, because long and homogeneous data series of solar ionizing flux are not available. To identify the optimum solar activity proxies, we use yearly average foF2 data of 11 ionospheric stations from middle and low/equatorial latitudes of four continents over 1976–2014 and six solar activity proxies, F10.7, sunspot numbers, F30, Mg II, He II, and solar H Lyman‐α flux. Mg II and F30 are found to be the best solar proxies for variability of foF2 at middle latitudes, not the usually used F10.7 or sunspot numbers. At equatorial latitudes the situation seems to be different with likely He II as the optimum solar proxy but all low/equatorial results are very preliminary. Solar activity describes 99% of the total variance of yearly foF2 and the foF2 dependence on solar proxies is highly linear at middle latitudes. The dependence of foF2 on F10.7 and sunspot numbers is significantly steeper in 1996–2014 than in 1976–1995, whereas for F30 both intervals provide the same dependence. We recommend for investigating the midlatitude yearly values of foF2 the solar proxy F30 followed by Mg II as the second one, not traditional F10.7 or sunspot numbers.