Auroral breakup during substorm onsets

Summary form only given, as follows. The authors have developed a model for auroral breakup that consists of two coupled circuits, one east-west, the other north-south, that connect the ionosphere and the magnetosphere. They found that narrow arcs formed at lower L shells were the most unstable to poleward expansion. They have now made the model more realistic by defining the magnetospheric electric field and current as produced by the dipolar collapse of the stretched magnetic field lines. Preliminary results indicate that the collapse of a moderately stretched field configuration over the period of a couple of minutes could induce a westward magnetospheric electric field of 2-10 mV/m at geosynchronous orbit. These values are consistent with those needed by the model for the formation of a breakup arc near geosynchronous orbit. The stability criteria of the authors´ breakup model suggest that the background convection electric field as mapped to the equatorial plane controls the location of breakup. They expect breakup to occur south of the Harang discontinuity near local midnight, which is consistent with observations.<>