On the stability of rotational discontinuities

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Abstract
The stability of symmetric rotational discontinuities in which the magnetic field rotates by 180° is investigated by means of a one‐dimensional self‐consistent hybrid code. Rotational discontinuities with an angle Θ > 45° between the discontinuity normal direction and the upstream magnetic field are found to be relatively stable. The discontinuity normal is in the x direction and the initial magnetic field has a finite y component only in the transition region. In the case of the ion (lefthanded) sense of rotation of the tangential magnetic field, the transition region does not broaden with time. In the case of the electron (right‐handed) sense of rotation, a damped wavetrain builds up in the By component downstream of the rotational discontinuity and the discontinuity broadens with time. Rotational discontinuities with smaller angles, Θ, are unstable. Examples for a rotational discontinuity with Θ = 30° and the electron sense of rotation as well as a rotational discontinuity with Θ = 15° and the ion sense of rotation show that these discontinuities disintegrate into waves. These waves travel approximately with Alfvén velocity in the upstream direction and are therefore phase standing in the simulation system. The magnetic hodograms of these disintegrated discontinuities are S‐shaped. The upstream portion of the hodogram is always right‐handed; the downstream portion is always left‐handed.