Accretion Disk Magnetohydrodynamics and the Origin of Jets

Abstract

Accretion disks are believed to be present in active galactic nuclei, binary X-ray sources, cataclysmic variables and protostars. Disks in all four types of source have been associated with collimated jets or bipolar outflows. It is argued that magnetic extraction of disk angular momentum provides a natural method for launching these jets and that poloidal field that passes through the disk is more likely to be effective than dynamo-generated field. Electromagnetic and hydromagnetic models for jet production are reviewed, emphasising their shortcomings and the dependence on physical conditions in the disk. Magnetic torques may also extract rotational energy from a central black hole. It is conjectured that, in practice, a black hole magnetosphere will be magnetically dominated and practically force-free and that no extra restrictions are imposed on its structure by the application of relativistic MHD theory. It is also argued that, despite recent concerns, the hole will be able to communicate its angular velocity to the magnetosphere and so drive a unipolar inductor.