Maps for the winter anomaly intensity in O/N₂ ratio at different levels of solar activity (a)–(c). White domains correspond to the regions, for which the winter/summer ratio is less than 1. Panels (d)–(f) show the longitudinal variations of the winter anomaly intensity in the O/N₂ ratio, averaged in the 40–60° geographic latitude bands in the Northern (blue) and Southern (red) Hemispheres.