Characteristics of parallel electric fields in the downward current region of the aurora

Abstract

Direct measurements of parallel electric fields suggest that they are, in part, self-consistently supported as strong double layers in the auroral downward current region. The observed parallel electric fields have amplitudes reaching nearly 1 V/m and are confined to a thin layer of approximately 10 Debye lengths. The structures are moving at roughly the ion acoustic speed in the direction of the accelerated electrons, i.e., anti-earthward. On the high-potential side of the parallel electric field there is a clear signature of an accelerated electron beam which rapidly plateaus within a few hundred Debye lengths from the parallel electric field. Strong wave turbulence is observed in the vicinity of the plateaued electron distribution. Fast solitary waves, identified as a signature of electron phase-space holes, are seen farther away from the parallel electric field on the high-potential side. The observed ion distributions also indicate the presence of the parallel electric field. On the low-potential side of the double layer an ion beam is observed moving in the opposite direction of the electron beam and ion conics appear to be trapped between their mirror point and the moving double layer.