Session 31 - Black Holes.
Oral session, Monday, January 15
Corte Real, Hilton

## [31.05] Ion viscosity mediated by tangled magnetic fields in Accretion flows onto black holes

P. Subramanian, P. A. Becker, M. Kafatos (George Mason U.)

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 \simeq 0.01 to \simeq 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 \dotM/\dotM_Eddington. Our results suggest the viability of quasi-spherical accretion for near and super-Eddington accretion rates.