The winter anomaly in radio-wave absorption

Progress towards resolving the winter anomaly in radio-wave absorption is reviewed with particular reference to work carried out by U.C.W. Aberystwyth. The anomaly, a local winter effect, is a D-region phenomenon showing strong latitude dependence. In the long term the effect shows a solar cycle variation but no similar short-term solar control is evident. Three types of winter absorption day are recognised and are reconciled with increased enhancement of D-region electron concentrations. Certainly in the extreme case, and probably in all cases, the enhanced electron concentrations are due to increases in concentration of the minor constituent nitric oxide. Transport from auroral to middle latitudes in winter is inferred with it is suggested, considerable modification of the D-region chemistry at these times