Seasonal variations in the parameters of the effects of noon absorption recovery during PCA

During many PCAs at auroral zone stations in the daytime, a smooth decrease in the absorption, called the noon recovery effect, is observed. The manifestation of this effect in PCA events registered at various seasons of the year (in spring, summer, and fall under a completely illuminated ionosphere and in winter under a completely (or partially) dark ionosphere) is analyzed in this paper. In the summer PCA, the amplitude of the noon recovery (the strongest decrease in the absorption in the hours close to the noon) and the frequency of occurrence of this effect are lower than in equinox conditions. A decrease in temperature and the enhanced content of water vapor in the upper mesosphere in summer as compared to the equinox seasons are the main factors governing the small amplitude of the noon recovery in summer PCA. Using experimental and calculated data, the presence of the noon recovery effect in winter PCA is found for the first time. In winter PCA, the absorption variation during noon recovery has a complicated shape: a smooth decrease in the daytime hours and a short-term (pulse-type) increase in the absorption in the local noon. This increase is caused by the fact that the ionosphere over auroral stations in the winter season is illuminated for a short time interval (around the local noon), which causes a sharp increase in the absorption. The amplitude of the noon recovery in winter PCA is higher than in equinox and summer PCA.