We present ground-based observations of an isolated, daytime auroral event that was detected above Svalbard by means of an all-sky TV camera and a multichannel, meridian scanning photometer. This 5-min event occurred near magnetic noon, poleward of a stable cusp aurora. It emitted ~20 kR of 557.7-nm light, with little if any admixture of 560.0-nm radiation. The structure rotated rapidly then underwent a sudden breakup. The spectral distribution of emitted light indicates that electrons responsible for this auroral structure were accelerated though several kilovolts between the magnetosheath and the ionosphere. Such a potential structure requires that the precipitating, arc electrons constitute a field-aligned current out of the ionosphere. Current continuity and Ohm's law require radial electric fields within the arc. From the structure's rotational speed, we estimate that the accelerated electrons carried an upward field-aligned current of order 100 &mgr;A/m2. Satellite observations of polar cap convection patterns suggest that the interplanetary magnetic field had turned northward prior to the event. The auroral structure may be explained as resulting from a transitory magnetic merging of interplanetary and lobe magnetic field lines at and/or a penetration of plasma across the high-latitude magnetopause. ¿ American Geophysical Union 1993