A continuous view of the dawn-dusk polar cap

Continuous monitoring of airglow and auroral emissions over Eureka (89 degr ees magnetic latitude) through each winter has shown that the dusk and dawn boundaries of the polar cap can be routinely seen when the IMF B-z is nort hward. When this condition prevails the auroral oval shrinks poleward, and the polar boundaries are clearly and continuously seen in photometric dusk- dawn meridian scans of OI 630 nm emission brightness. This fact is illustra ted with two 24-hr days of records on December 19, 1996 and January 19, 199 8. These examples show, and others similarly, that polar arcs emerge from t he dusk or dawn flanks of the auroral oval (depending on the sign of B-y) a nd thus occur on closed field lines. In a few cases polar arcs emerge from the midnight or noon sector of the oval and extend along the sun-earth line to cross the Eureka meridian, bifurcating the polar cap. This new techniqu e for monitoring the polar cap dusk-dawn extent and all auroral activity wi thin promises to be valuable for diagnostic studies of both polar auroral a ctivity and magnetospheric topology during solar wind changes and major sun -earth events.