We analyze simultaneous measurements by the incoherent scatter radars at Saint-Santin (45° latitude) and Chatanika (65° latitude) during a major magnetic storm in April 1978 to examine several disturbance mechanisms operating on the mid-latitude ionosphere during periods of strong magnetospheric and auroral activity. The first type of mechanism, the extension of magnetospheric convection electric fields to mid-latitudes, is illustrated by two large localized departures of the E×B plasma drifts over Saint-Santin that appeared in conjunction with the two storm negative phases. In both cases they were associated with large electric fields in the afternoon sector, within the eastward electrojet region, over Chatanika. The first event can be very clearly interpreted as a global short term enhancement of magnetospheric convection electric fields, since simultaneous plasma flux intensifications were identified over the polar cap, in the afternoon eastward electrojet, and at 45° latitude. The IMF and ring current signatures of these two events confirm our previous findings: large southward values of BƵ and large values of the rate of energy injection into the ring current system, are associated with the extension of magnetospheric convection electric fields to midlatitudes. In addition, a marked equatorward shift of the auroral oval was observed during both events. During the night of the first event, the mid-latitude drift perturbation magnitudes remained moderate (100 m/s) and did not seem to induce any significant ionospheric perturbation. The joint increase in ion and electron temperatures by 100 to 200° K above the quiet day reference level, which preserved the thermal equilibrium of the plasma, can be simply interpreted in terms of the storm-induced global heating of the thermospheric gas. Conversely, on the night of the second event, when drift magnitudes reached several hundreds of m/s, large-amplitude fluctuations in the F layer height and density, and very large increases (by more than 500° K) in the ion and electron temperatures were observed. Whereas collisional heating can account for a large part of the ion temperature increase, an additional energy source, heating preferentially the electron gas, is needed to explain the maintenance of the electron temperature 200° K above the ion temperature observed from the onset of the Dst decrease onward during that night. The identification of this source suffers from the lack of additional data, in particular photometric ones, in the Saint-Santin sector. However, the unusually large fields observed at Saint-Santin and the equatorward shift of the auroral zone observed at Chatanika in the early afternoon suggest that the plasmapause may have moved close to Saint-Santin and that the enhanced electron temperatures observed there might have been produced by a SAR arc.