Explosive poleward advance of auroral activity in auroral substorms

The explosive activity is the most important feature of auroral substorms. The explosive feature is manifested best by a sudden poleward advance (about 500 km) of auroral activity in the midnight sector during the expansion phase. The explosive activity is, in general, caused by a sudden unloading accumulated energy. Thus, a sequence of processes leading to the explosive activity is considered for isolated substorms by synthesizing available data. During the growth phase, the power is accumulated in the inner magnetosphere at about 6 Re by enhanced solar wind-magnetosphere dynamo and the cross-tail current; it causes inflation of the magnetosphere as a result of the energy accumulation. When the accumulated energy reaches about 1016 J, the cross-tail current at 6 Re becomes unstable and is disrupted. The unloading of the energy results in deflation, which generates an earthward electric field, which produces a new current system, ionizing the ionosphere and causing the initial brightening an auroral arc. The subsequent poleward advancing auroral activity produces a wide ionization belt, allowing the disrupted cross-tail current to flow into the ionosphere (the auroral electrojet, the power dissipation). Two possible processes of the poleward advance are suggested.