On the Long-Period Variations in the F2Region of the Ionosphere

The long-period variations in the F
2
region are discussed with data based on observations of the ionosphere for the last 42 months since May, 1934, by the Naval Technical Research Department, Tokyo. The variations in the critical frequency of the penetration are presented in the form of monthly averages of the noon values. The mtean values of the hourly critical frequencies of the penetration for a month were calculated in order to measure the approximate value of the energy absorbed by the F
2
region. It is shown that these mean values change with the zenith angle of the sun, and they seem to have similar characteristics in both hemispheres. The rate of the seasonal variation in these values, moreover, is much greater in the southern hemisphere than in the northern hemisphere. From this it is presumed that there are two kinds of variations in the energy absorbed by the F
2
region, the one seasonal and the other annual, the effect of these variations being differential in the northern hemisphere and additional in the southern hemisphere. The anomalous variation in the critical frequency of the F
2
region may be explained by this presumption with the aid of the hypothesis of thermal expansion. A graph of the seasonal variations in the minimum virtual height is given, in which it is shown that this variation seems to prove the foregoing presumptions.