F30 is the most suitable solar activity proxies for foF2 at middle latitudes

To study ionospheric climate and to study its long-term changes and trends we need solar activity proxies, because long and homogeneous data series of solar ionizing flux are not available. Also models like IRI are based on solar proxies. To select the optimum solar activity proxies, we use yearly average foF2 data of six ionospheric stations from middle latitudes of four continents over 1976-2014 and six solar activity proxies, F10.7, sunspot numbers, F30, Mg II, He II and solar Lyman-α flux. The highest percentage of total variance of yearly foF2 is described equally by F30 and Mg II. However, when we divide period 1976-2014 into two parts, 1976-1995 and 1996-2014, F30 is the only proxy which reveals the same dependence of foF2 on solar proxy. Moreover, F30 is available since March 1957 whereas Mg II only since November 1978. Thus for long-term studies of yearly foF2 at middle latitudes F30 is the most suitable solar activity proxy.

Change of the dependence of foF2 on solar activity proxies from 1976-1995 to 1996-2014 appears to be of solar origin; it is related to changes of interdependences among solar proxies between the first and second periods.