Dynamics of substorm breakup

The brightening auroral arc at substorm breakup is assumed to be due to a sudden increase in the magnetic field-aligned potential drop above the arc. The dynamic effects of this sudden increase is analyzed in terms of a model consisting of two coupled circuits. One circuit contains an upward current sheet above the arc and a downward current sheet further equatorward. The other circuit contains the westward electrojet within the arc and the current wedge from the ring current or cross-tail current. It is shown that inductive fields appear as a response to the sudden change in potential drop. These fields are consistent with tail field collapse towards a more dipolar field. Quantitative estimates give reasonable values for the rate of change of the currents, including the electrojet and the electric fields in the ionosphere and magnetosphere. No neutral line formation is necessary to trigger this instability