In a companion paper, we present observational evidence that the stable growth-phase auroral arc that intensifies at substorm expansion phase onset often forms on magnetic field lines that map to within ~1 to 2 Re of synchronous. The equatorial plasma pressure is 1 to 10 nPa in this region, which can give a cross-tail current >0.1 A/m. In this paper, we propose that the arc is formed by a perpendicular magnetospheric-current divergence that results from a strong dawn-to-dusk directed pressure gradient in the vicinity of magnetic midnight. We estimate that the current divergence is sufficiently strong that a >1 kV field-aligned potential drop is required to maintain ionospheric-current continuity. We suggests that the azimuthal pressure gradient results from proton drifts in the vicinity of synchronous orbit that are directed nearly parallel to the cross-tail electric field. ¿ American Geophysical Union 1992