Accretion Disc Magnetic Winds

Abstract

There is observational evidence that accretion discs in interacting binaries have wind flows emanating from their surfaces, and that the flow tends to become collimated parallel to the disc rotation axis. It is known that a differentially rotating, turbulent disc can have a large-scale magnetic field generated by dynamo action. Magnetically channelled winds are effective at removing angular momentum from accretion discs, provided that the initial flow is well sub-Alfvenic and the poloidal magnetic field has a suitable geometry with a sufficient inclination to the vertical at the disc surface.

Firstly, the wind launching and field source problems are considered, and wind structure calculations are reviewed. A detailed analysis of the disc-wind system is then presented, incorporating a dynamo generated magnetic field, with solutions for the radial and vertical structures of the disc and for the sub-Alfvénic wind region. The removal of angular momentum by the wind outflow can make a major contribution to driving the disc inflow, together with viscosity. A significant amount of mass can be lost from the inner region of the disc, due to enhanced wind mass fluxes. Disc models having inflows driven purely by magnetic winds tend to be subject to a field bending instability, but this can be quenched by allowing for a temperature dependent turbulent viscosity.