Vertical structure of stationary accretion disks with a large-scale magnetic field

Abstract

In earlier works we pointed out that the disk's surface layers are non-turbulent and thus highly conducting (or non-diffusive) because the hydrodynamic and/or magnetorotational instabilities are suppressed high in the disk where the magnetic and radiation pressures are larger than the plasma thermal pressure. Here, we calculate the vertical profiles of the stationary accretion flows (with radial and azimuthal components), and the profiles of the large-scale, magnetic field taking into account the turbulent viscosity and diffusivity and the fact that the turbulence vanishes at the surface of the disk. Also, here we require that the radial accretion speed be zero at the disk's surface and we assume that the ratio of the turbulent viscosity to the turbulent magnetic diffusivity is of order unity. Thus, at the disk's surface there are three boundary conditions. As a result, for a fixed dimensionless viscosity α-value, we find that there is a definite relation between the ratio of the accretion power going into magnetic disk winds to the viscous power dissipation and the midplane plasma-β, which is the ratio of the plasma to magnetic pressure in the disk. For a specific disk model with of order unity we find that the critical value required for a stationary solution is βc ≈ 2.4r/(αh), where h is the disk's half thickness. For weaker magnetic fields, β > βc, we argue that the poloidal field will advect outward while for β < βc it will advect inward. Alternatively, if the disk wind is negligible (), there are stationary solutions with β ≫ βc. © 2012. The American Astronomical Society. All rights reserved.