Abstract
A study has been made of the diurnal variations of F2 critical frequencies and virtual heights as a function of phase of sunspot cycle, season, and geographic and geomagnetic position. The main results are derived from ten stations that are well observed at both sunspot maximum and minimum. The chief characteristics of ƒ 0F2 may be described in terms of (a) the sunspot minimum anomaly, (b) the sunrise anomaly, and (c) the diurnal range anomaly. The features of these anomalies are evaluated quantitatively and then, as far as possible, the anomalies are extracted from the data to determine the normal curves. The world distributions of indices representing the anomalies are studied. The variations of the F2 virtual height are not related in any very significant manner to either the sunspot cycle or the ƒ 0F2 diurnal curves. Some attempt at explanation of the anomalies has been made but no complete theory is available. Some effects are thought to be thermal or tidal oscillations, and evidence is given for the conclusion that F2 electrons disappear by a decay law that is independent of gas pressure.
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