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 &thgr;Bn=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 sense of rotation &agr;1=¿180¿ can be stable down to widths of a few c/&ohgr;pi (ion inertial lengths). This initialization gives reliable results except in the case of thin electron-sense RDs with a rotation angel &agr;1=-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 plasma beta of order unity, there is a minimum width of a few c/&ohgr;pi 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/&ohgr;pi. In a cold plasma, ion-sense RDs with &agr;1=180¿ have a minimum width which scales with the ion inertial length. In a warm plasma, the thickness increase 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 ‖&agr;1‖ 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 &agr;1=¿270¿ show completely different mechanisms for disintegration. Electron-sense rotations appear to be composed of a stable, large amplitude &agr;1=-360¿ solitary wave at the upstream edge and an ion sense &agr;1=90¿ rotation; a slight difference in speed separates the &agr;1=-270¿ total rotation over time. Ion-sense RDs with &agr;1=270¿ are metastable: their current structure distintegrates explosively after an initial stable interval (many 100 &OHgr;ci-1 for wide transitions, where &OHgr;ci is the ion cyclotron frequency), when internal ion heating has increased the average gyroradius above the width of the transition. ¿American Geophysical Union 1993