The IMAGE extreme ultraviolet (EUV) imager obtained global images of the plasmasphere erosion of 10 July 2000, in which the nightside plasmapause moved inward by about 2 RE during 5--8 UT. We use plasmapause motion to infer the electric (E) field component tangent to the moving plasmapause; in general we lack knowledge of the perpendicular E-field component. In the midnight-to-dawn quadrant where the plasmapause shape is very nearly circular, the tangential E-field component is equal to the azimuthal electric field Eϕ. Peak westward E-fields of 0.6--1.3 mV/m were found at the plasmapause between L ≈ 4--6, an inner magnetospheric E-field that was 25% of the solar wind E-field. The MLT-concentration of the inferred E-field suggests that convective flows may produce partial indentation of the nightside plasmapause (especially in the midnight-to-dawn sector) that widens as the edges of the indentation propagate azimuthally.