On the ‘winter anomaly’ in the midlatitudeDregion

Two causes are often proposed for the ‘winter anomaly,’ i.e. for the excess winter absorption of high-frequency waves reflected from the ionosphere at midlatitudes. It has been suggested on the one hand that meteorological effects might be responsible, but no detailed proof is available at present. The alternative explanation as being the precipitation of energetic charged particles was often rejected because of such features of the winter anomaly as its apparent lack of correlation with geomagnetic activity. However, in this note, actual measurements of precipitated electrons are shown to yield good correlation with the ionospheric effects. Furthermore, it is shown that the ‘anomalous’ absorption is correlated at magnetically conjugate stations, as one would expect if precipitated particles were responsible. It is therefore demonstrated quantitatively that the winter anomaly can be explained at least partly, as being due to precipitated electrons.