Origins of enhanced field‐aligned current at the edge of an auroral arc

Abstract

Rocket and radar observations of auroral arcs indicate that the upward field‐aligned currents associated with such arcs sometimes are larger at their edges than within the arcs. Brüning and Goertz (1985) reported one such set of observations, made by instruments on the Porcupine F4 rocket. Porcupine F4 measured a fourfold increase in the magnitude of the upward field‐aligned current at the edge of an auroral arc over its value in the arc's center, while the energy of the peak in the electron differential flux decreased at the edge of the arc and the electron distribution function remained nearly isotropic in pitch angle throughout the arc. One mechanism which has been proposed to explain the creation of auroral arcs is the adiabatic acceleration of magnetospheric electrons through a field‐aligned electrostatic potential. The upward field‐aligned current which results is a function of the density and parallel and perpendicular temperatures of the magnetospheric electron population as well as the magnitude and altitude of the electrostatic potential drop. If the arc traversed by Porcupine F4 was created in this way, the increase in upward current at the edge of the arc cannot be explained solely by an increase in the altitude of the top of the field‐aligned potential drop. Rather, an increase in the density, a decrease in the perpendicular temperature of the magnetospheric electron population at the edge of the arc, or both are needed to explain the observed increase in upward field‐aligned current.