Ion distribution effects of turbulence on a kinetic auroral arc model

Abstract

We extend an earlier plasma‐kinetic model of an inverted V auroral arc structure to include, in a phenomenological way, the effects of electrostatic turbulence with k∥/k⊥ ≪ 1. In the absence of turbulence, a parallel potential drop is supported by magnetic mirror forces and charge quasi‐neutrality, with energetic auroral ions penetrating to low altitudes; relative to the electrons, the ions' pitch angle distribution is skewed toward smaller pitch angles. The electrons energized by the potential drop form a current that excites electrostatic turbulence; we consider the specific case of the ion cyclotron mode. The turbulence heats the ions in T⊥ only, thus tending to reduce the differential pitch angle anisotropy between electrons and ions, which in turn reduces the potential drop—an effect of opposite sign to that associated with anomalous resistivity. In equilibrium the plasma is marginally stable, with growth rates and diffusion constants some 2 orders of magnitude below naive estimates. The conventional anomalous resistivity contribution to the potential drop is very small, because anomalous‐resistivity processes are far too dissipative to be powered by auroral particles; this is why growth rates and diffusion constants are so small. Under certain circumstances equilibrium may be impossible and relaxation oscillations set in; the time scale for such pulsating auroras is the ion transit time of 6–10 s.