WORLDWIDE DISTRIBUTION OF POSITIVE IONOSPHERIC STORMS

Worldwide foF2 data from 42 ionosonde stations are analyzed for the period 15-18 June 1965, to study worldwide occurrences of positive ionospheric storms. The start of ionospheric storms in this period takes place in the polar region as negative storms, then the regions of negative storms spread equatorward along the geomagnetic latitude. Results of the present analysis show that there appear two types of positive storms during this period. The positive storms of the first type commence to appear in equatorward fronts of the spreading negative regions in both hemispheres. These two regions of positive storms then move equatorward in synchronism with the negative regions and merge near the magnetic equator to create the equatorial positive region. It is concluded from these characteristics that the positive storms of the first type originate from disturbances of the thermosphere. The positive storms of the second type are the evening enhancements which appear in the evening sector near the plasmapause of the American meridian in this case. According to the progress of the storm, the region of maximum evening enhancements shows equatorward drift in just the same manner as the motion of the plasmapause during the storm. Conjugate effects are also observed in the occurrences of the evening enhancements. These characteristics in occurrences indicate that the positive storms of the second type are caused by the effects of the magnetospheric processes. Among mechanisms which include the magnetospheric effects, plasma flux from the protonosphere seems most plausible as the cause of the evening enhancements from the fact that enhancements begin after the sunset in the winter hemisphere of the American meridian. © 1979.