Diffusion coefficient of a passive contaminant in a local MHD model of a turbulent accretion disc

Details Diffusion coefficient of a passive contaminant in a local MHD model of a turbulent accretion disc Diffusion coefficient of a passive contaminant in a local MHD model of a turbulent accretion disc

2005-01-21

arxiv.org/abs/astro-ph/0501474v1

Mon.Not.Roy.Astron.Soc. 358 (2005) 1055

Astronomy and Astrophysics

Astrophysics

7 pages, 4 figures, to be published in MNRAS

Scientific paper

10.1111/j.1365-2966.2005.08850.x

We calculate the radial diffusion coefficient for a passive contaminant in an accretion disc which is turbulent due to the action of the magnetorotational instability. Numerical MHD simulations are used to follow the evolution of a local patch of the disc using the shearing box formalism. A separate continuity equation for the mass fraction of contaminant is integrated along with the MHD system, and radial profiles of this fraction are obtained as a function of time. Solutions of a linear diffusion equation are fitted to the numerical measured profiles of the contaminant, treating the diffusion coefficient D as the fitting parameter. At early times, the value of D is found to vary, however once the contaminant is spread over scales comparable to the box size, it saturates at a steady value. The ratio of D to the transport coefficient of angular momentum due to shear stress is small. If D can be used as a proxy for the turbulent magnetic diffusivity, the effective magnetic Prandtl number P_eff=\nu/D (where \nu is the coefficient of "efective viscosity" associated with shear stress) would be large.

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