The dynamical morphology of the aurora polaris

Simultaneous changes of auroral form, brightness, and motion over the whole polar region are studied, using IGY all-sky camera records from widely distributed stations in eastern Siberia, Alaska, Canada, northern United States (south of the Canadian border), and Greenland. During very quiet periods, and sometimes for brief intervals during magnetic storms, the aurora may consist of faint, diffuse, quiet arcs extending along the whole of the auroral zone that is in darkness. The onset of a polar magnetic substorm suddenly changes this quiet state of the aurora, especially on the morning part of the auroral zone. The aurora becomes active all along the zone, but in different ways at different local times, that is, at different positions relative to the midnight meridian. Active auroral displays last for a few hours, 1 to 3, for example. Great auroral displays consist of an intermittent succession of such active periods. Large-scale dynamical features of the auroras, such as the breakup, formation of large loops, and drift motions, are discussed.