Polar arcs: New results from Viking UV images

The magnetic field topology associated with high‐latitude polar arcs has been an interesting problem since the advent of satellite auroral imagery. These large‐scale polar arcs suggest that the topology of the magnetosphere is modified by either bifurcation of closed field lines into the magnetotail lobe or else expansion of closed field lines up to quite near the magnetic pole. The former view recognizes the generally low level of emission of both sides of such high‐latitude features. Recently, new insight has been gained by examining Viking UV images and in situ plasma measurements for cases where polar arcs could be quantitatively analyzed. A statistical analysis of the UV emissions in the vicinity of polar arcs has indicated that the regions on opposite sides of the arcs are consistently asymmetric, not occasionally asymmetric as previously reported in the literature. Simultaneous particle observations confirm the presence of precipitating electrons throughout the region of enhanced emission which have characteristics consistent with a closed field line source for the particles. Further, our results show for the first time that the extent of emission associated with these large‐scale polar arcs extends hundreds of kilometers poleward (i.e., in the direction of the presumed polar cap).