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| Titel |
Advection/diffusion of large scale magnetic field in accretion disks |
| VerfasserIn |
R. V. E. Lovelace, G. S. Bisnovatyi-Kogan, D. M. Rothstein |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 16, no. 1 ; Nr. 16, no. 1 (2009-02-13), S.77-81 |
| Datensatznummer |
250013089
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| Publikation (Nr.) |
 copernicus.org/npg-16-77-2009.pdf |
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| Zusammenfassung |
| Activity of the nuclei of galaxies and stellar mass systems involving disk
accretion to black holes is thought to be due to (1) a small-scale turbulent
magnetic field in the disk (due to the magneto-rotational instability or MRI)
which gives a large viscosity enhancing accretion, and (2) a large-scale
magnetic field which gives rise to matter outflows and/or electromagnetic
jets from the disk which also enhances accretion. An important problem with
this picture is that the enhanced viscosity is accompanied by an enhanced
magnetic diffusivity which acts to prevent the build up of a significant
large-scale field. Recent work has pointed out that the disk's surface layers
are non-turbulent and thus highly conducting (or non-diffusive) because the
MRI is suppressed high in the disk where the magnetic and radiation pressures
are larger than the thermal pressure. Here, we calculate the vertical (z)
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 due to the MRI and the fact
that the turbulence vanishes at the surface of the disk. We derive a
sixth-order differential equation for the radial flow velocity vr(z) which
depends mainly on the midplane thermal to magnetic pressure ratio β>1
and the Prandtl number of the turbulence P=viscosity/diffusivity.
Boundary conditions at the disk surface take into account a possible magnetic
wind or jet and allow for a surface current in the highly conducting surface
layer. The stationary solutions we find indicate that a weak (β>1)
large-scale field does not diffuse away as suggested by earlier work. |
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