Ion viscosity mediated by tangled magnetic fields in Accretion flows onto black holes

Details Ion viscosity mediated by tangled magnetic fields in Accretion flows onto black holes Ion viscosity mediated by tangled magnetic fields in Accretion flows onto black holes

Dec 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995aas...187.3105s&link_type=abstract

American Astronomical Society, 187th AAS Meeting, #31.05; Bulletin of the American Astronomical Society, Vol. 27, p.1328

Astronomy and Astrophysics

Astronomy

Scientific paper

We examine the viscosity mechanism due to momentum transfer between ions in the presence of tangled magnetic fields in accretion flows onto black holes. We do not attempt to present a self-consistent description of the tangled magnetic field; we merely assume its existence and take its coherence length lambda_ {coh} (which is the maximum length a given magnetic field line can be expected to be straight, on average) to be a free parameter. We arrive at a formulation for a hybrid viscosity in which the tangled magnetic field plays the role of an intermediary in momentum transfer between ions. It does so by affecting the mean free path of the ions; the effective mean free path of the ions in our formulation lies in between the usual ion-ion mean free path lambda_ {ii} and lambda_ {coh}. Our calculations are relevant in the scenario of accretion disks around BLACK HOLES, which are expected to have near equipartition tangled magnetic fields embedded in them. We apply this viscosity mechanism to a steady state model of a two temperature quasi-Keplerian accretion disk. The values of the USUAL alpha parameter arising from this mechanism are FOUND to range from =~ 0.01 to =~ 0.5. We find the viscosity to be influenced both by the degree to which the magnetic fields are tangled (represented by xi = lambda_ {coh}/r) and the relative accretion rate dot {M}/dot {M}Eddington. Our results suggest the viability of quasi-spherical accretion for near and super-Eddington accretion rates.

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