The F2 layer seasonal anomaly

The midday F2 layer critical frequencies are greater in winter than in summer. This phenomenon is particularly pronounced at the middle latitude stations of the northern hemisphere and this feature is referred to as the seasonal anomaly. The existence of this anomaly is known for a long time from the bottomside ionosonde measurements. Recent data obtained from Alouette I and other topside sounders have also confirmed the existence of this anomaly. Several different explanations of this anomaly have been put forward. At the same time arguments have been advanced disfavouring some of these explanations. In this paper the seasonal anomaly has been examined in the light of the neutral wind-plasma interaction and the composition changes. The computational results are given which show a preference for the composition changes as the cause of the seasonal anomaly