To model ionospheric climate and to study its long-term changes we need solar activity proxies, because long and homogeneous data series of solar ionizing flux are not available. Here we use solar activity proxies/indices F10.7, sunspot numbers, F30, Mg II, He II and solar Lyman-α flux, and yearly average foF2 values from six midlatitude ionospheric stations from four continents (Juliusruh, Pruhonice, Roma, Boulder, Kokubunji and Canberra) over 1976-2014. Main results are as follows: (1) Relationships among solar activity indices/proxies differ between solar cycles 21 and 22 (represented by years 1976-1995) on the one hand, and cycles 23 and 24 (represented by years 1996-2014) on the other hand. (2) Consequently the relationships between midlatitude foF2 and solar activity proxies also differ except for F30. (3) The variability of yearly values of foF2 at middle latitudes appears to be described best by solar activity proxies F30 and Mg II.