The role of alfvén waves in the formation of auroral parallel electric fields

The visual aurora is a manifestation of a system of field-aligned currents that couple the magnetosphere and ionosphere. Changes in these currents are mediated by the propagation of shear mode Alfvén waves along auroral field lines. The development of parallel electric fields on these field lines is considered in terms of the propagation characteristics of Alfvén waves. Auroral parallel electric fields occur in low-density regions where the ratio Bln maximizes. On the scale of individual auroral arcs, the electron inertial effect becomes critically important, and inhomogeneities in the Alfvén speed both parallel and perpendicular to the field must be considered to develop a complete theory of the formation of auroral arcs. Mechanisms such as nonlinear effects, ionospheric feedback or phase mixing at horizontal gradients in the Alfvén speed can act to produce the narrow scales observed in auroral arcs.