Dynamic morphology of auroras

Abstract

Simultaneous changes of auroral forms, brightness, and motions over the whole polar region are studied, using IGY all-sky camera records from widely distributed stations in eastern Siberia, Alaska, Canada and the northern United States. It is found that the auroral system centered in the midnight sector in the auroral zone repeatedly undergoes an expansion and subsequent contraction; during the maximum stage of the activity, the whole auroral system extends over a substantial portion of the darkened polar region. Such extensive auroral activity as a whole may be regarded as a single event, and is described in terms of the auroral substorm. The substorm has two characteristic phases, an expansive phase and a recovery phase. Characteristic auroral displays over the entire polar region during the substorm are described in detail. The basic physical processes involved for the auroral substorm are also discussed. Geomagnetic disturbances associated with the auroral substorm are also described in detail in terms of the polar magnetic substorm, and it is shown that both the auroral substorm and the polar magnetic substorm are different aspects of the manifestation of a large-scale plasma motion in the magnetosphere. The distribution of the aurora for different degrees of the geomagnetic activity is also discussed in terms of the auroral belt. It is shown that the center line of the auroral belt moves greatly with respect to its average location (namely the auroral zone), depending on the degree of the magnetic activity.