Seasonal variation of world-wideF2 ionization for noon and midnight hours

From a study of F2 ionization for each month during the two years 1946 and 1947, it is found that for the noon hours there is a definite dip at the geomagnetic equator with a maximum on each side similar to the curves published by Appleton and Liang for the equinoctial months. Further, there is an indication of a second maximum at about 50° geomagnetic latitude. The midnight curves show only one maximum, which closely follows the solar declination. It is suggested that the midnight ionization provides a better criterion for the study of F2 seasonal variation than noon-hour values.