The Stability of Magnetized Rotating Plasmas with Superthermal Fields

During the last decade it has become evident that the magnetorotationalinstability is at the heart of the enhanced angular momentum transport inweakly magnetized accretion disks around neutron stars and black holes. In thispaper, we investigate the local linear stability of differentially rotating,magnetized flows and the evolution of the magnetorotational instability beyondthe weak-field limit. We show that, when superthermal toroidal fields areconsidered, the effects of both compressibility and magnetic tension forces,which are related to the curvature of toroidal field lines, should be takenfully into account. We demonstrate that the presence of a strong toroidalcomponent in the magnetic field plays a non-trivial role. When strong fieldsare considered, the strength of the toroidal magnetic field not only modifiesthe growth rates of the unstable modes but also determines which modes aresubject to instabilities. We find that, for rotating configurations withKeplerian laws, the magnetorotational instability is stabilized at lowwavenumbers for toroidal Alfven speeds exceeding the geometric mean of thesound speed and the rotational speed. We discuss the significance of ourfindings for the stability of cold, magnetically dominated, rotating fluids andargue that, for these systems, the curvature of toroidal field lines cannot beneglected even when short wavelength perturbations are considered. We alsocomment on the implications of our results for the validity of shearing boxsimulations in which superthermal toroidal fields are generated.