Structure and length scales of rotational discontinuities

Abstract

We have carried out a series of one‐dimensional hybrid (kinetic ions, fluid electrons) simulations, using a nondynamic method to form rotational discontinuities (RDs) at an angle of θBn = 60° between the normal direction and the upstream magnetic field. Ion kinetic effects are found to be important concerning the stability of the current layer, its thickness and scaling, and its dependence on initial conditions. When the ions are initialized with transverse velocities derived from Hall MHD, RDs with both senses of rotation α1 = ±180° can be stable down to widths of a few c/ωpi, (ion inertial lengths). This initialization gives reliable results except in the case of thin electron‐sense RDs with a rotation angle α1 = −180°, which have a pronounced depression of the transverse ion velocity, and develop more easily starting from a nonrotational transition of the ion flow. At a plasma beta of order unity, there is a minimum width of a few c/ωpi for transitions of both senses of rotation. Thin RDs show a significant upstream heat flux and temperature anisotropies. Their small‐scale structure is embedded in a larger envelope of the order of 100 c/ωpi. In a cold plasma, ion‐sense RDs with α1 = 180° have a minimum width which scales with the ion inertial length. In a warm plasma, the thickness increases with the square root of the ion beta due to the finite ion Larmor radius. There appear to be no stationary structures for rotation angles |α1| significantly larger than 180°. However, the associated breakup time increases with the initial thickness, eventually approaching the stable MHD limit for transitions very wide compared to the ion Larmor radius. The two senses of rotation α1 = ±270° show completely different mechanisms for disintegration. Electron‐sense rotations appear to be composed of a stable, large amplitude α1 = −360° solitary wave at the upstream edge and an ion sense α1 = 90° rotation; a slight difference in speed separates the α1 = −270° total rotation over time. Ion‐sense RDs with α1 = 270° are metastable: their current structure disintegrates explosively after an initial stable interval (many 100 Ωci−1 for wide transitions, where Ωci is the ion cyclotron frequency), when internal ion heating has increased the average gyroradius above the width of the transition.