Origin and geometry of upward parallel electric fields in the auroral acceleration region

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(l) 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.