SATELLITE-OBSERVATIONS OF POLAR ARCS

The increased probability of observing polar arcs during periods of northward IMF has tended to obscure their significance in terms of magnetospheric topology because of the presumed 'inactive' state of the magnetosphere. However, satellite imaging has shown that these high latitude features are quite dynamic both in their intensity and spatial variations. The overall morphology of the high latitude aurora has been described by a variety of imaginative terms, but its primary optical characteristic is of a polar arc(s) extending between the dayside and nightside auroral distribution on one or both of the dawn/dusk sides of the high latitude region. This large scale morphology is controlled by the azimuth angle of the IMF and the predominant configuration is one wherein the region between the polar arc and the normal auroral distribution is filled with low intensity diffuse emission. Simultaneous particle and electric field measurements show this region exhibits a closed field line character with predominantly sunward flowing plasma. These large scale polar arcs are connected (in either a diffuse or discrete fashion) to the nightside auroral distribution with essentially equal probabilities, but exhibit a clear peak near 12 MLT. This dayside connection is commonly associated with isolated high latitude features poleward of the normal auroral distribution which probably represent processes occurring on the front surface of the magnetotail poleward of the cusp. The existence of polar arcs is not always controlled by substorm activity: polar arcs can maintain their form and position well past expansion phase suggesting that they represent a fundamental boundary in the magnetosphere which is not modified by even large substorms.