ELECTRON FLOW AND VISUAL AURORA AT THE HARANG DISCONTINUITY

Several examples are presented of the ionospheric electron drift patterns and of the aurora near the Harang discontinuity. The drift patterns were detected with the Scandinavian Twin Auroral Radar Experiment, and the auroral features were determined by using all-sky camera data. The behavior of the discontinuity is found to be strongly dependent on the level of magnetic activity as measured by the K//p index. With increasing magnetic activity the Harang discontinuity tends to surge rapidly equatorward with mean speeds ranging from 150 to 600 m/s. The equatorward surges are terminated by the sudden activation of an arc in the discontinuity, followed by a poleward expansion. Following these substorms the Harang discontinuity disappears from the field of view. In general, auroral arcs are located in or poleward of the discontinuity, even though active arcs at times appear south of the discontinuity. Diffuse aurora have been observed only equatorward of the discontinuity.