This is the first paper that combines observations of the electric and magnetic fields, particles, wave emissions and UV images taken at intermediate altitude (two Earth radii) above the acceleration region of a 14 MLT aurora. The observations were made by the Swedish Viking satellite. The aurora appeared in the region of large-scale reversal of the perpendicular electric field. Strong fluctuations in the electric and magnetic field indicated that the aurora consisted of many arcs of varying latitudinal extent. Spin-averaged data show that an upward current sheet was connected with the observed UV aurora, with an ionospheric current density of 4 &mgr;A/m2 in the center of the sheet and 7 &mgr;A/m2 near the equatorward edge. The field-aligned potential drop V∥ was determined from the mean energy of the upward ion beams and the width of the electron loss cone. V∥ increased from several hundred volts near the equatorward edge of the current sheet to at least 3 kV in the center. Auroral kilometric radiation (AKR) and the widened electron loss cone indicated that the top of the acceleration region with V∥≥3 kV was located near one Earth radius. The critical field-aligned potential drop, V∥c, which is strong enough to accelerate the bulk of the electrons into the loss cone, was derived from the characteristic electron energy T and the ratio of the magnetic field in the ionosphere to that at the top of the acceleration region, Bi/BV.

The source plasma parameters were typical for magnetosheath plasma with a characteristic electron energy of about 200 eV. This means that in the center of the current sheet V∥c was 1.4 kV, and thus the observed field-aligned potential drop, V∥≥3 kV, was greater than V∥c, indicating a saturation current. At the edges V∥c was higher than the measured potential drop, indicating a linear relationship with current density. There was a density depletion in the region of saturation current, and upward ion conics were observed with energies above V∥c. The electron source density determined from (1) the particle data and (2) the local plasma frequency agrees fairly well with that calculated from the current density observed by the magnetometer, the characteristic electron energy, and the altitude of the top of the acceleration region. This fact provides further evidence fore the existence of a saturation current. Comparisons with earlier work indicate that the observed values for T, Bi/BV, and V∥ are not exceptions but rather typical for the postnoon sector. We therefore conclude that a saturation current may be a common phenomenon for intense postnoon aurora. Similarities with the current-voltage relationship for strong laboratory double layers are discussed. ¿ Copyright 1990 by the American Geophysical Union