THE STANDARD-ACCRETION DISK DYNAMO

We consider the classic MHD turbulent dynamo operating in a thin, Kepl erian accretion disk powered by turbulent viscous stress. The characte r of the turbulence is encapsulated into two constant parameters: the Shakura-Sunyaev dimensionless viscosity, alpha (ss), and the Coriolis number, Omega = 2 tau(corr)Omega. The dependence of the total electro motive force on alpha(ss) and Omega is derived and the dynamo-generat ed magnetic field is calculated in both kinematic and nonlinear regime s for a variety of different conditions in the halo. The calculations revealed that the dynamo number, D, increases with Omega, but decreas es with alpha(ss). The value of the critical dynamo number, D-crit, de pends on the magnetic diffusivity of the halo. In general, the smaller the halo diffusivity, the easier it is to generate the magnetic field in the disk. The nonlinear models are calculated without taking into account the back-reaction of magnetic field on the structure of the di sk. Such an approach is consistent providing that the magnetic Mach nu mber, M(mag), is Smaller than unity. We show that M(mag), is determine d primarily by the value of Omega, and that the consistency condition M(mag) < 1 requires, in some cases, turbulence with relatively large Omega. In such a regime the dynamo number, the magnitude of the equil ibrated large-scale magnetic field, as well as the ratio of the poloid al and the toroidal magnetic field strength, depend mostly on the valu e of alpha(ss). The generated field has a quadrupolar symmetry with re spect to the equator, is mostly confined within the disk's density sca le-height, and is concentrated in the radially inner part of the disk. The obtained solutions are very regular, they lack any radial reversa ls, and no oscillatory or chaotic behavior has been found.