Geomagnetic activity threshold expressed in ap¿index units for F2--layer negative storms onset at middle latitudes exhibits seasonal (winter/summer) variation with summer thresholds being larger than winter ones. That is, for a given magnitude of the ionospheric disturbance, the associated magnetic activity must be larger in summer than in winter. There are also relative minima (or plateau) in the threshold annual variation during equinoctial periods. An analysis has shown that seasonal effect is due to seasonal difference in the thermosphere (O/N2 ratio) reaction to geomagnetic disturbances, while the equinoctial plateau in the threshold annual variations may be related to the thermospheric circulation and atomic oxygen abundance changes during the equinoctial transition periods.