Geomagnetic response of F2-layer at sunrise

The mechanisms responsible for the geomagnetic disturbances in the F2 layer at sunrise are investigated, on the basis of ionospheric soundings carried out at the Sofia Ionospheric Observatory in Bulgaria. The frequency of the sounding measurements was 7.67 MHz. The roles of general thermospheric circulation and meridional winds due to Joule heating in the formation of the disturbances is evaluated. It is shown that about 50 percent of the geomagnetic disturbances occurring at sunrise are unrelated to geomagnetic storm activity.