On the evening of 14 November, 1985, the Sondrestrom incoherent scatter radar made measurements of ionization and drifts associated with a sun-aligned polar cap arc near local midnight. All-sky photographs at 630.0 nm showed that the arc, which appeared to move westward across the Sondrestrom meridian, was connected to the auroral oval. The radar measured height-integrated Hall and Pedersen conductances in the arc of 2 to 3 mhos. The plasma drift within the arc was anti-sunward with velocities greater than 1 km/sec, indicating a westward electric field across the arc of about 60 mV/m. The radar data imply a model current system that includes a northward Hall current within the sun-aligned arc and a westward electrojet in the auroral oval to the south. The model currents produce ground magnetic perturbations that are consistent with those measured by stations in the Greenland magnetometer chain; in particular the sun-aligned arc is detectable by a negative deflection in the D component of the magnetometers. Divergence of the currents at the edges of the auroral forms yield field-aligned currents that are upward on the west edge of the arc and downward on the east edge. An upward current is required at the point where the polar cap arc joins the auroral oval because of the gradient in the conductivity and electric field at that location. This upward current implies the presence of enhanced electron precipitation which can account for the observed brightening of polar cap arcs at the point where they intersect the auroral oval. ¿American Geophysical Union 1987