Magnetohydrodynamic Instabilities in Shearing, Rotating, Stratified Winds and Disks

Abstract

We investigate shear and buoyancy instabilities in radially stratified, magnetized, cylindrical flows, for application to magnetocentrifugally driven winds-such as those from protostars-and to magnetized accretion disks. Our motivation is to characterize the susceptibility of cold MHD disk winds to growing internal perturbations and to understand the relation of wind instabilities to known accretion disk instabilities. Using four different linear analysis techniques, we identify and study nine principal types of unstable or overstable disturbances, providing numerical and analytic solutions for growth rates for a wide range of parameters. When magnetic fields are predominantly toroidal, as in protostellar winds far from their source, we find the system is susceptible to growth of five different kinds of perturbations: axisymmetric fundamental and toroidal resonance modes, axisymmetric and nonaxisymmetric toroidal buoyancy modes, and nonaxisymmetric magnetorotational modes. Winds having a sufficiently steep field gradient (dlnB/dlnR <~1.