The tangential electric field at the magnetopause in the magnetopause frame of reference has been estimated for 205 magnetopause crossings of the ISEE 1 satellite within 2 hours of local noon during 1977--1981. At most of these crossings, time variations of the electric field caused its fluctuations to be large compared with its mean value. This implies both that variations of the field (due, for example, to time varying reconnection) may be of fundamental significance, and that statistical analyses of a large number of crossings are required to determine the average dependence of the tangential electric field at the magnetopause on the magnetosheath magnetic field. A generalized least squares method, which takes into account uncertainties in both the electric and magnetic field measurements, has been developed and applied to the data. The results indicate that, on the average, reconnection occurs at the subsolar magnetopause; that the average reconnection flow speed of magnetosheath plasma toward the magnetopause is 20+14-6 km/s (which is about 15% of the local Alfv¿n speed); and that the dependence of the average tangential electric field at the magnetopause on the magnetosheath north-south magnetic field is sufficient to account for a major part of the magnetospheric potential and convection. ¿American Geophysical Union 1990