In this paper we review the possibilities formagnetohydrodynamic processes to handle the angular momentum transportin accretion disks. Traditionally the angular momentum transport hasbeen considered to be the result of turbulent viscosity in the disk,although the Keplerian flow in accretion disks is linearly stable towardshydrodynamic perturbations. It is on the other hand linearly unstableto some magnetohydrodynamic (MHD) instabilities.The most important instabilities are the Parker and Balbus-Hawleyinstabilities that are related to the magnetic buoyancy and the shearflow, respectively. We discuss these instabilities not only in thetraditional MHD framework, but also in the context of slender fluxtubes, that reduce the complexity of the problem while keeping most ofthe stability properties of the complete problem. In the non-linearregime the instabilities produce turbulence. Recent numericalsimulations describe the generation of magnetic fields by a dynamo inthe resulting turbulent flow. Eventually such a dynamo may generate aglobal magnetic field in the disk. The relation of the MHD-turbulenceto observations of accretion disks is still obscure. It is commonlybelieved that magnetic fields can be highly efficient in transportingthe angular momentum, but emission lines, short-time scale variabilityand non-thermal radiation, which a stellar astronomer would take assigns of magnetic variability, are more commonly observed during periodsof low accretion rates.
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