We report rocket-borne ion convection velocity and ground-based optical measurements in an early evening, equatorward drifting, NW--SE oriented auroral arc. Our measurements indicate a detailed correspondence between the normal component of the ion convection velocity (the component normal to the auroral arc) and the drift motion of the auroral arc. The two velocities were identical on a time scale of minutes, and correlated strongly with each other in smaller-scale temporal variations. The correspondence implies a near-zero tangential electric field in an auroral arc, in the frame of reference of the arc. Our observations have serious consequences on several auroral theories.