AFTEREFFECTS OF ISOLATED MAGNETOSPHERIC SUBSTORM ACTIVITY ON MID-LATITUDE IONOSPHERE - LOCALIZED DEPRESSIONS IN F-LAYER ELECTRON-DENSITIES

Ground-based ionosonde data from a global network of about 100 stations have been used to study ionospheric disturbances at mid-latitudes associated with isolated magnetospheric substorm activity. The results of four case studies during solstice periods are presented. In each case, pronounced (about 10 to 30%) depressions in f//0F//2 developed in a limited longitude sector in the summer hemisphere and lasted for about 1 day. It is suggested that these depressions are caused by changes in thermospheric composition and that the primary heat source for the thermospheric disturbance is Joule heating in the dayside polar cusp region.