A POWERFUL LOCAL SHEAR INSTABILITY IN WEAKLY MAGNETIZED DISKS .4. NONAXISYMMETRIC PERTURBATIONS

In this paper, we continue our study of a powerful axisymmetric MHD instability recently put forward by the authors as the underlying cause of anomalous transport in accretion disks. The theory of local non-axisymmetric perturbations in weakly magnetized disks is presented. Such disturbances are of interest for several reasons, most notably in the context of a dynamo magnetic field amplification scheme. The most volatile disturbances are those associated with the presence of a poloidal field, which grow by tens of orders of magnitude with e-folding times measured in fractions of an orbital period. We also examine the stability of a purely azimuthal field configuration and find that nonaxisymmetric instability is present, but with a growth time measured in tens of orbital periods. In general, the most rapid growth occurs for very small radial and azimuthal wavenumbers, leading to coherent magnetic field structure in planes parallel to the disk. We suggest that this instability will prove to be a key ingredient for the generation of magnetic fields in disks.