Auroral electron beams: electric currents and energy sources

The equipotential structures and electron beams seen during rocket flight 18:152 are analyzed. The equipotentials which lead to beam formation can be produced by either a maximum in the high-altitude potential or a minimum in the low-altitude potential near a closed U-shaped structure. Cold ionospheric electrons can enter the acceleration region near a cusplike feature that separates closed and open electric equipotentials. The analysis suggests that some midnight auroras may derive energy from the cross-tail electric field. Birkeland currents distort magnetic field lines so that plasma flow becomes sheared and the electric field divergence increases near the earth. The observed closed and open equipotential structures develop at still lower altitudes as these electric fields interact with the ionosphere. Finally, horizontal ionospheric electric currents are derived for the flight. Sudden changes in horizontal currents at the edge of an active arc are much larger than can be explained by Birkeland currents. The observed effects can be produced by substantial distortions of horizontal current flow near the rocket