Electric fields, magnetic fields, and plasmas measured on the Polar satellite are studied to determine the altitude extent of the upward field-aligned current portion of the auroral acceleration region and the physical processes that populate it with parallel electric fields. This region extends upward to a geocentric altitude of 3 or 3.5 Earth radii at premidnight local times during the spring and fall. In a model for this region a necessary but not sufficient condition for the appearance of parallel electric fields is an upward field-aligned current. Parallel electric fields will exist where the plasma density is insufficient to carry this imposed field-aligned current through the converging magnetic field. These parallel fields are associated with (1) high-altitude electron acceleration required to carry the imposed current through the low-density region; (2) a low-altitude sheath required to inject compensating positive charge into the region and to restrict entry of secondary electrons from below; and (3) the midaltitude maintenance of local charge neutrality. The parallel electric field in the low-altitude sheath is directly measured to be as large as several hundred mV m-1, while the higher-altitude fields appear to have magnitudes of a few mV m-1. The low-altitude sheath is corrugated in a manner that does not depend on striations of the field-aligned current but that may depend on density variations. ¿ 2001 American Geophysical Union