The expansive phase of magnetospheric substorms: 1. Development of the auroral electrojets and auroral arc configuration during a substorm

A detailed description of the development of the auroral and magnetic signatures of a magnetospheric substorm is presented in this paper. The isolated nature of the event makes it ideal to demonstrate the character and uniqueness of the phases of a substorm, and it is found that the various perturbation patterns associated with each substorm phase dominate in different regions of the high-latitude ionosphere. It is shown that the ground-based signatures of the growth of isolated substorms are confined to the polar cap and to the evening sector and that these signatures are the same as those noted for the more obvious expansive phase in the average auroral zone. The expansive phase involves the growth of the auroral electrojet through a series of steplike poleward jumps of the northern border of the current system. The substorm continues to develop through a series of distortions and intensifications of the poleward border of the electrojet, each involving the generation of a bright auroral loop or surge. The major portion of the ionospheric current flow is apparently unaffected by the northern border activity of the auroral arcs; thus it is suggested that the energy feeding the substorm is dissipated by different mechanisms active in different regions of the night side magnetosphere.