The Night-time Winter Anomaly (NWA) effect is shown to be a regular phenomenon at the mid-latitude ionosphere in the American and Asian longitude sectors under low solar activity conditions. Total electron content, vertical sounding data as well as topside sounder measurements onboard Alouette indicate a significant higher night-time ionization level in winter than in summer under certain conditions which are discussed in more detail. It is shown that the NWA effect is closely related to the asymmetry of the geographic-geomagnetic relationships in the American and in the Asian longitude sectors, respectively. In the former sector NWA occurs in the northern hemisphere, in the latter in the southern hemisphere. Since the sea-sonal variation of the night-time ionization is similar at conjugated hemispheres, interhemispheric coupling processes are assumed to generate the NWA effect. This idea is consistent with numerical calculations, modelling the system ionosphere-plasmasphere-ionosphere along magnetic flux tubes in the American longitude sector. The model calculations show that interhemispheric fluxes react very sensitive to dynamical forces as f.i. meridional winds in both ionospheres. The derived field-aligned plasma fluxes as well as the resulting higher tube content in December than in June are consistent with the observations.