15-19Some typical cases of ionospheric disturbances associated with geomagnetic storms in different seasons are studied for dip-conjugate low and middle latitude places located along nearly the same longitude. The results show initial changes in NmF2 similar to those in the magnetic field at all places, but later, the changes in the F2-ionization seem to be governed by the heating of the upper atmosphere and the accompanied changes in the neutral wind pattern and the gas composition. The anomalous increase of F2 electron density in winter at midlatitude during the main phase and after, of the storm is traced to the change in the meridional wind system and to the neutral wind transport from the summer hemisphere to the winter hemisphere. Satellite observations of the day-to-day and disturbed F-region also confirm changes in the gas composition in good correlation with the changes in NmF2 or total electron content. As against the instantaneous effects of the storm in the F2-ionization, there is an after-effect of the storm in the D-region at midlatitudes but practically no such storm-effects in the low-latitude D-region. It is believed that the post-storm low energy electrons precipitate into the lower ionosphere from the magnetospheric slot and cause the delayed increase in the D-region ionization at midlatitudes. The D-region effects are inferred from the absorption of hf radio waves
