A bright and long-lived Sun-aligned auroral arc was observed over the north magnetic pole from 1120 to 1220 UT, December 14, 1990 at Eureka, Canada. It was monitored through its full lifetime with high resolution with an all-sky imager and a meridian scanning spectrometer. Its measured off-zenigh 5577-¿ intensity reached 12 kR. The recorded electron flux obtained by a DMSP F9 overpass at 1124:15 UT was 56 ergs/(cm2 s sr), yielding an inferred 5577-¿ intensity in that region of the arc of at least 80 kR. The arc drifted slowly in the dusk to dawn direction during its early stages, then reversed its drift direction during later stages. The characteristics of the polar arc (its intensity, duration, and small-scale dynamics) were very similar to those of auroral oval arcs and strongly suggest that the polar arc occurred on closed magnetic field lines. Emission on the duskside of the arc further suggest that the region of closed magnetic field lines extended from the evening sector to the magnetic pole. The inference is that the interplanetary magnetic field was most likely Bz northward and By positive during the period of appearance of the arc. ¿ American Geophysical Union 1994