Observations of a premidnight auroral arc were made with high temporal and spatial resolution using the Chatanika incoherent scatter radar. The arc was a quiet east-west aligned arc, about 40 km wide, that drifted slowly through the radar beam without appreciably changing in shape or luminosity. The distribution of ionsopheric electric fields and currents associated with the arc were computed from the ion density and line of sight velocity. The electric field was 30 mv/m outside the arc and decreased to a few mv/m within the arc. The horizontal potential drop was only ~0.6 kV across the arc. By deconvolving the electron density profiles, differential energy spectra and velocity distriubtions were obtained within the arc. From these the accelerating potential parallel to the magnetic field lines was estimated by several techniques. One technique makes use of an empirical relationship between the parallel potential drop and the total electron current. The accelerating potential reached 7 kV at the arc center. The parallel potential drop within the arc is combined with ionospheric electric field measurements to reconstruct the equipotential contours above the arc.