In this paper we have examined the fluid equations relevant to the ionosphere above 400 km in a region of horizontal gradients in plasma density and electron temperature such as those detected near the ionospheric projection of the plasmapause. The equations are unstable to growth of the temperature gradient drift mode. Electric field fluctuations have been observed in this region of the ionosphere by two independent electric field experiments flown on separate satellites. In one case (Ogo 6) simultaneous measurements of density and electron temperature were made. By using these data the growth rate of the temperature gradient drift mode is calculated and plotted as a function of distance along the trajectory and is shown to peak in the region where the irregularities were detected. Thus we conclude that the temperature gradient drift mode may contribute to the growth of irregularities at the equatorward edge of the ionospheric plasma trough.