Effects of Birkeland current limitation on high‐latitude convection patterns

Abstract

It is shown how the high‐latitude convection pattern may be modified by substorm‐enhanced polarization electric fields. These are generated whenever the flow of those Birkeland currents which are associated with ionospheric conductivity gradients is limited. Such Birkeland currents are fed mainly by the enhanced Pedersen current in the evening and morning sectors of the auroral oval and by the enhanced Hall current around local midnight. As the current limitation increases, the ionospheric potential, represented here by a symmetric two‐cell pattern, will rotate clockwise and deform, as will the associated Birkeland current distribution, which will attain roughly a symmetric shape relative to the noonmidnight magnetic meridian. The resulting patterns are shown to agree well with observations. A pronounced westward intrusion of the equipotential contours occurs in the auroral oval and may be associated with the westward traveling surge. This feature does not, however, require any assumed longitudinal conductivity gradients. Rather, it falls out naturally when the closure of the enhanced Pedersen current is also taken into account.