The stabilizing effect of viscosity on magnetic wind flows from accretion discs

摘要

Magnetically channelled winds are believed to be a feature of most accretion discs. It has been shown that such flows can remove significant amounts of angular momentum from the disc and make a major contribution to driving the inflow. For a suitable range of poloidal magnetic field bending, only a small fraction of the disc mass is lost in the wind flow, so most material reaches the inner region of the disc. However, discs driven purely by such a process are prone to a field-bending instability which can lead to runaway mass loss. It is shown here that a small amount of disc viscosity can quench such an instability and allow steady disc-wind models to be constructed. The effects of perturbations to the coupling between the radial and vertical structures are allowed for, with the thermal balance having particular relevance. Runaway increases in field bending are prevented by increases in the disc temperature and magnetic diffusivity mainly caused by viscous dissipation.