A new model for auroral breakup during substorms

A model for substorm breakup is developed, based on (1) the relaxation of stretched (closed) dipolar field lines, and (2) the formation of an incipient current wedge within a single arc structure. It is argued that the establishment of a coupled current structure within a single arc leads to a quasistable system, i.e. the prebreakup regime. Perturbation of the prebreakup structure leads to an instability criterion. It is found, consistent with observations, that the narrower auroral arcs at lower L shells undergo the most explosive poleward expansion. According to this model, the precise location at which breakup occurs depends on the O⁺ density in the plasma sheet, the level of magnetic activity (K_p), and the intensity of the substorm westward electrojet in the ionosphere. An enhancement of any of these features will cause breakup to occur at lower L shells. Comparison of the proposed model with the Heppner-Maynard polar-cap potential model indicates that breakup is restricted to the west of the Harang discontinuity, consistent with observations from the Viking satellite