A model of the bifurcated current sheet

Abstract

Recent Geotail and Cluster observations revealed that thin current sheets in the near‐Earth tail may have a bifurcated structure. In some cases the electrons are found to dominate the current. We present a generalization of the Harris current sheet equilibrium, which reproduces these features. The model ion distribution contains an additional element, viz. the quasi‐adiabatic invariant of the ion motion across the sheet, and assumes ion temperature anisotropy outside the sheet. Bifurcated current sheets appear in the case of small ion anisotropy with T ⟂ i > T ∥ i (pancake distributions). In the opposite case (cigar distributions) the model describes a single‐peaked ion‐dominated current sheet embedded in a thicker Harris sheet, which is more relevant to the outflow region of the collisionless reconnection pattern in the deHoffman‐Teller frame.