THE STABILITY OF DIFFERENTIALLY ROTATING, WEAKLY MAGNETIZED STELLAR RADIATIVE ZONES

We continue our study of a powerful weak-field MHD instability. Consequences of the instability for pressure-supported systems are discussed, with emphasis upon stellar radiative zones. It is suggested that the instability, rather than hydromagnetic waves, is responsible for establishing uniform rotation in weakly magnetized stellar radiative zones. When the Brunt-Vaisala frequency much exceeds the rotation frequency, only displacements in spherical shells are unstable. Thus the transport induced by the instability is tangential and leads to little radial mixing. If angular velocity is initially constant on cylinders, the effect of the instability is to transport angular momentum from the poles to the equator. If angular momentum is locally deposited by thermalization of the instability, this will probably have the effect of establishing solid-body rotation in radiative zones. If angular momentum is not deposited locally, it will tend to accumulate in the mid-plane of the star, with uncertain consequences. It may contribute to an anisotropic wind. A review of the instability in discs is also presented.