Formation of the stable auroral arc that intensifies at substorm onset

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 suggest 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