ABOUT THE NATURE OF THE NIGHTTIME WINTER ANOMALY EFFECT (NWA) IN THE F-REGION OF THE IONOSPHERE

The Night-time Winter Anomaly (NWA) effect is shown to be a regular ph enomenon at the mid-latitude ionosphere in the American and Asian long itude sectors under low solar activity conditions. Total electron cont ent, vertical sounding data as well as topside sounder measurements on board Alouette indicate a significant higher night-time ionization lev el in winter than in summer under certain conditions which are discuss ed in more detail. It is shown that the NWA effect is closely related to the asymmetry of the geographic-geomagnetic relationships in the Am erican and in the Asian longitude sectors, respectively. In the former sector NWA occurs in the northern hemisphere, in the latter in the so uthern hemisphere. Since the seasonal variation of the night-time ioni zation is similar at conjugated hemispheres, interhemispheric coupling processes are assumed to generate the NWA effect. This idea is consis tent with numerical calculations, modelling the system ionosphere-plas masphere-ionosphere along magnetic flux tubes in the American longitud e sector. The model calculations show that interhemispheric fluxes rea ct very sensitive to dynamical forces as f.i. meridional winds in both ionospheres. The derived field-aligned plasma fluxes as well as the r esulting higher tube content in December than in June are consistent w ith the observations.