POLAR ARCS - NEW RESULTS FROM VIKING UV IMAGES

The magnetic field topology associate-d 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 genera lly 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 qu antitatively analyzed. A statistical analysis of the UV emissions in t he vicinity of polar arcs has indicated that the regions on opposite s ides of the arcs are consistently asymmetric, not occasionally asymmet ric as previously reported in the literature. Simultaneous particle ob servations 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 result s 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).