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 here. In each case, pronounced (~10-30%) depressions in F0F2 developed in a limited longitude sector in the summer hemispher and lasted for about 1 day. The region of depressed F0F2 appears as a tongue extending from the auroral zone down to about 20¿ geomagnetic latitude and corotating with the earth. This region is found to have been on the dayside of the earth while the substorm activity was in progress. It is suggested that these depressions are caused by changes in the thermospheric composition and that the primary heat source for the thermospheric disturbances is Joule heating in the dayside polar cusp region.