A global or planetary index value for the ionization density of the F2 layer can be found by suitably averaging the diurnal critical frequency values as observed at different stations along all parallels of latitude and longitude. Monthly planetary valaues of F2 layer ionization show marked maxima around the equinoxes. It is shown that the sesonal trends of the EUV index F10.7 and the geomagnetic index Ap, when combined in the ratio of 4:1, reproduce faithfully the observed seasonal trend of planetary F2 layer ionization. Detailed analysis shows that the seasonal enhancement of the planetary F2 ionization is not limited to the planetary scale but is also applicable to the averaged diurnal ionization along any pair of conjugate dip latitudes when taken together. ¿ American Geophysical Union 1992