On the orientation of auroral arcs

Abstract

The results of an investigation of the diurnal variation in the direction of quiet auroral arcs at Kiruna are presented. Other investigations—largely of recent years—of the diurnal variation in the orientation of auroral arcs and bands are reviewed and the observational results are compared with Alfvén's theory. Regularities in the average direction of arcs and bands at a given time are discussed. It has been found that near the auroral zones the diurnal variation curve is similar to the theoretical curve of Alfvén. Of special interest is a “discontinuity” observed in the mornings at Kiruna. From a review of numerous recent investigations it is concluded that over the central polar cap the auroral geometric pattern seems to be similar to the pattern of Chapman and Vestine's SD current system. In or near the auroral zones, the average arc direction at a given time seems to be determined by the projections of circles in the geomagnetic equatorial plane on to the earth's surface along the geomagnetic field lines.