‘Alfvénic shocks’ and low‐altitude auroral acceleration

Abstract

Electrostatic shocks (ESs) and small‐scale, field‐aligned currents (FACs) are known to be associated with low‐altitude auroral acceleration. We show that this phenomenon can be consistently described by including the transverse wave‐guide for shear Alfvén waves (TAWG) and the density dip in the Alfvén wave model. The TAWG is the density depletion in a low β plasma at h ≤ 2 RE. Paired, anti‐parallel FACs are found to be its eigenmodes. The ne‐dip in the centre, caused by a steep gradient of the electric field, promotes formation of the strong double layer (SDL). This in turn yields a narrow field‐aligned electron beam at the edge of an arc or the black aurora, depending upon whether it is the region of the upward or downward Birkeland current, respectively.