On the global stability of magnetized accretion discs — III. Non-axisymmetric modes

We investigate the global stability of a differentially rotating, ideal MHD fluid shell to linear, non-axisymmetric perturbations. This system, which approximates an accretion disc near its mid-plane, is known to be unstable to both axisymmetric and non-axisymmetric local perturbations. We find two distinct classes of globally unstable modes. One of these is a magnetic analogue of the Papaloizou-Pringle (PP) instability of thick hydrodynamic …