Extremely high latitude auroras

Over 1600 Defense Meteorological Satellite Program north polar images from the winters 1972–1973, 1974–1975 were studied for the occurrence of auroral arcs at corrected geomagnetic latitudes greater than 80°. Auroras at these latitudes are designated extremely high latitude auroras and are categorized by the local time sectors in which they occur and their relationship to discrete oval arcs. A data base of ∼300 cases of cap auroras was formed for correlation studies with geomagnetic activity indices and solar wind parameters. By far the greatest number of extremely high latitude auroras occur in the morning sector and appear to be expansions of the system of morning oval arcs poleward. The remaining categories include evening arcs expanding into the cap; single, sun‐aligned arcs; and midnight sector oval auroras expanding into the cap. Except for the last category the auroras occur for similar magnetospheric conditions: moderate values of Kp and Dst , but comparatively low values of AE . They also occur preferentially for high solar wind speed and for positive B z . While far less clear, their location in the cap shows a dependence on B y . Morning (evening) arcs occur with greater frequency for negative (positive) B y .