Origin of annual and subannual periodicities in F region ionization

The monthly median values of maximum electron density NmF2 in the F region are harmonically analyzed for a number of high-latitude and auroral zone stations for each hour of the day during 1958 and 1964. In the high-activity year 1958 the annual maximum appears in December around 70°N (geographic latitude) for a few hours around noon. The December excess diminishes gradually toward the south, though the duration over which it lasts increases toward the dip equator. By 70°S a slight June enhancement is seen around noon. During the remaining hours of the day a December phase is seen around 70°S, having a large amplitude which decreases toward the north, reaching a slight June excess by 70°N. A 6-monthly component with equinoctial maximums exists at all stations for all hours of the day, the amplitude being somewhat larger in the south than it is in the north. In the low-sunspot year 1964 the December phase of the annual component is present in the north with low amplitude for a few hours around noon. The same phase prevails in the south. This is followed in local time by a large-amplitude June phase in the north and a December phase in the south. The 6-monthly component is predominant during daytime only and has equinoctial maximums. The current theories are shown to be inadequate to explain these results.