MECHANISM BY WHICH MERGING AT X-LINES CAUSES DISCRETE AURORAL ARCS

A mechanism is presented by which X lines cause auroral arcs. The mechanism should apply to most existing steady state merging models if the fluid is an electron-ion plasma. Hall electric fields are produced in the plane perpendicular to the X line in order to equalize the flow of electrons and ions through the diffusion region despite their differences in inertia. It is suggested that these electric fields map along magnetic field lines to the auroral acceleration region at much lower altitudes. Since mapping along field lines occurs by oblique Alfven waves, stationary in the convective flow, there are accompanying Birkeland currents. The currents and fields provide the upper boundary condition for the auroral electron acceleration region believed to exist above discrete arcs. A solution of a flow problem, similar to the merging situation, by Sonnerup and Wang is used to show that the voltage is sufficient for auroral arcs and that momentum transfer between the diffusion and exterior regions is consistent with the production of "paired electrostatic shocks" for a rigorous solution. An integration along a path through the diffusion region for the general case shows that auroral-sized voltages are expected in both collisional and collisionless cases. Finally it is shown that the ion-inertial scale length is appropriate for the diffusion region in the collisionless case, and maps to the ionosphere as approximately 10 km.