Astronomy and Astrophysics – Astronomy
Scientific paper
Mar 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008head...10.1406p&link_type=abstract
American Astronomical Society, HEAD meeting #10, #14.06
Astronomy and Astrophysics
Astronomy
Scientific paper
Over the last decade, magnetohydrodynamic (MHD) turbulence driven by the magnetorotational instability (MRI) has emerged as the most promising candidate to enable angular momentum transport in black hole accretion disks. The development of three-dimensional MHD numerical codes has allowed us to study this process in detail, motivating numerical estimations of the Shakura-Sunyaev alpha parameter. It is now clear that the alpha parameter is not constant and that depends on the strength and geometry of the magnetic field, the size of the simulation domain, and the resolution. Until recently, however, it had not been possible to disentangle numerical from physical dependencies in a clear way. I will discuss recent work that lead to understanding how the characteristics of the turbulent stresses responsible for angular momentum transport depend on the various physical (pressure, magnetic field, etc.) and numerical (box size and resolution) factors. Based on these results, I will explain the reason behind the small values of the effective alpha viscosity reported in the past. I will also show that, in order for MRI-driven turbulence to account for the large value of the effective alpha viscosity inferred observationally, the disk must be threaded by a significant vertical magnetic field and the turbulent magnetic energy must be in near equipartition with the thermal energy. I will discuss the physical and observational implications for accretion disk models in which the angular momentum transport is mediated by MRI-driven turbulence.
Chan Ching-Kit
Pessah Martin E.
Psaltis Dimitrios
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