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 Alfv¿en waves (TAWG) and the density dip in the Alfv¿en wave model. The TAWG is the density depletion in a low &bgr; plasma at h≤2RE. 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. ¿ American Geophysical Union 1995