Solar wind energy is transferred to and from the closed field line region of the magnetosphere by transport across the boundary between open and closed field lines (the separatrix). The rate of this transfer is measured by the reconnection electric field, which is tangential to the separatrix. Although it is not possible to measure directly the reconnection electric field in the magnetosphere, the electric field in the ionosphere can readily be used to calculate the magnetic reconnection rate. This paper describes a technique for using the Sondrestrom incoherent scatter radar in Greenland to estimate this rate from measurements of the plasma velocity in the separatrix reference frame. The ionospheric plasma drift and the separatrix location and velocity are determined from the radar observations, and the separatrix orientation is inferred from all-sky images. This technique has been applied to obtain measurements of the reconnection electric field in the midnight sector, with 3- to 5-min time resolution during the night of January 14-15, 1989. The reconnection electric field was found to be less than 15 mV/m during periods of local polar cap expansion, which corresponded to substorm recovery phases, and to be 30 to 40 mV/m during times of polar cap contraction, which corresponded to substorm expansive phases. During a short interval, the measurements showed the separatrix moving equatorward faster than the plasma, which implies that there was plasma transfer from closed to open field lines, rather than the usual nightside transfer from open to closed field lines. ¿American Geophysical Union 1991