The standard-accretion disk dynamo.

Details The standard-accretion disk dynamo. The standard-accretion disk dynamo.

Jun 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995a%26a...298..934r&link_type=abstract

Astronomy and Astrophysics, v.298, p.934

Astronomy and Astrophysics

Astrophysics

24

Accretion Disks, Magnetohydrodynamics, Turbulence, Novae, Cataclysmic Variables

Scientific paper

We consider the classic MHD turbulent dynamo operating in a thin, Keplerian accretion disk powered by turbulent viscous stress. The character of the turbulence is encapsulated into two constant parameters: the Shakura-Sunyaev dimensionless viscosity, α_SS_, and the Coriolis number, {OMEGA}^*^=2τ_corrOMEGA. The dependence of the total electromotive force on α_SS_ and {OMEGA}^*^ is derived and the dynamo-generated magnetic field is calculated in both kinematic and nonlinear regimes for a variety of different conditions in the halo. The calculations revealed that the dynamo number, D, increases with {OMEGA}^*^, but decreases with α_SS_. The value of the critical dynamo number, D_crit_, depends 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 disk. Such an approach is consistent providing that the magnetic Mach number, M_mag_, is smaller than unity. We show that M_mag_ is determined 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 equilibrated large-scale magnetic field, as well as the ratio of the poloidal and the toroidal magnetic field strength, depend mostly on the value of α_SS_. The generated field has a quadrupolar symmetry with respect to the equator, is mostly confined within the disk's density scale-height, and is concentrated in the radially inner part of the disk. The obtained solutions are very regular, they lack any radial reversals, and no oscillatory or chaotic behavior has been found.

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