The seasonal anomaly in the behaviour of the F2-layer critical frequency

Abstract

An analysis of several years of monthly median noon critical-frequency data from more than 100 ground-based ionosondes has yielded a morphological picture of the ionosphere suggesting that the seasonal anomaly should be regarded in a new light. The number of months separating the observed biannual maxima of critical frequency is shown to vary markedly with the latitude of the observatory and also through the solar cycle. The midwinter criticalfrequencies are greater than the summer ones when the separation of the biannual maxima is least; this occurs in local winter at higher middle latitudes during sunspot-maximum conditions.