Accretion Disks and Dynamos: Toward a Unified Mean Field Theory

Abstract

Conversion of gravitational energy into radiation in accretion discs and the origin of large scale magnetic fields in astrophysical rotators have often been distinct topics of research. In semi-analytic work on both problems it has been useful to presume large scale symmetries, necessarily resulting in mean field theories. MHD turbulence makes the underlying systems locally asymmetric and nonlinear. Synergy between theory and simulations should aim for the development of practical mean field models that capture essential physics and can be used for observational modeling. Mean field dynamo (MFD) theory and alpha-viscosity accretion theory exemplify such ongoing pursuits. 21st century MFD theory has more nonlinear predictive power compared to 20th century MFD theory, whereas accretion theory is still in a 20th century state. In fact, insights from MFD theory are applicable to accretion theory and the two are artificially separated pieces of what should be a single theory. I discuss pieces of progress that provide clues toward a unified theory. A key concept is that large scale magnetic fields can be sustained via local or global magnetic helicity fluxes or via relaxation of small scale magnetic fluctuations, without the kinetic helicity driver of 20th century textbooks. These concepts may help explain the formation of large scale fields that supply non-local angular momentum transport via coronae and jets in a unified theory of accretion and dynamos. In diagnosing the role of helicities and helicity fluxes in disk simulations, each disk hemisphere should be studied separately to avoid being misled by cancelation that occurs as a result of reflection asymmetry. The fraction of helical field energy in disks is expected to be small compared to the total field in each hemisphere as a result of shear, but can still be essential for large scale dynamo action.

For the Proceedings of the Third International Conference and Advanced School "Turbulent Mixing and Beyond," TMB-2011 held on 21 - 28 August 2011 at the Abdus Salam International Centre for Theoretical Physics, Trieste, http://users.ictp.it/~tmb/index2011.html Italy, To Appear in Physica Scripta (corrected small items to match version in print)

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Cited by in corpus (16)

• Super-Eddington Accretion Disks around Supermassive black Holes
• Dynamo theories
• The influence of accretion disk thickness on the large-scale magnetic dynamo
• Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow
• Nonmodal growth of the magnetorotational instability
• Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?
• Dynamo generated magnetic configurations in accretion discs and the nature of quasi-periodic oscillations in accreting binary systems
• Global Three-Dimensional Radiation Magnetohydrodynamic Simulations of Accretion onto a Stellar Mass Black Hole at Sub- and Near-critical Accretion Rates
• AGN Feeding and Feedback in M84: From Kiloparsec Scales to the Bondi Radius
• Retrograde versus prograde models of accreting black holes
• Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability
• On characterizing nonlocality and anisotropy for the magnetorotational instability
• A heuristic model for MRI turbulent stresses in Hall MHD
• Dynamics of magnetic flux tubes and IR-variability of Young Stellar Objects
• Helical and nonhelical large-scale dynamos in thin accretion discs
• Generalized quenching of large-scale magnetic dynamos in anisotropic flows