Velocity filter mechanism for ion bowl distributions (bimodal conics)

Abstract

Klumpar et al. (1984) have recently observed ion distributions in the high‐altitude auroral region which are conical at high energies and field‐aligned at lower energies. In velocity space these distributions appear “bowl‐shaped.” These observed distributions were interpreted as being due to two‐stage acceleration involving transverse ion heating and upward field‐aligned electric field acceleration. Here we show that qualitatively similar bowl‐shaped distributions can also be formed by transverse heating in a region of finite horizontal extent, followed by essentially adiabatic convective flow to the observation location. The latter stage contributes a velocity‐filtering effect which produces some characteristics of the distributions observed. For the published distributions of Klumpar et al. the evidence favors the parallel electric field acceleration as the dominant mechanism for the second stage. However, in general, it may be possible to use multiple ion species observations to distinguish these two mechanisms for ion bowl distributions.