Vertical Structure and Coronal Power of Accretion Disks Powered by MRI Turbulence

Abstract

In this paper we consider two outstanding intertwined problems in modern high-energy astrophysics: (1) the vertical thermal structure of an optically thick accretion disk heated by the dissipation of magnetohydrodynamic (MHD) turbulence driven by the magneto-rotational instability (MRI), and (2) determining the fraction of the accretion power released in the corona above the disk. For simplicity, we consider a gas-pressure-dominated disk and assume a constant opacity. We argue that the local turbulent dissipation rate due to the disruption of MRI channel flows by secondary parasitic instabilities should be uniform across most of the disk, almost up to the disk photosphere. We then obtain a self-consistent analytical solution for the vertical thermal structure of the disk, governed by the balance between the heating by MRI turbulence and the cooling by radiative diffusion. Next, we argue that the coronal power fraction is determined by the competition between the Parker instability, viewed as a parasitic instability feeding off of MRI channel flows, and other parasitic instabilities. We show that the Parker instability inevitably becomes important near the disk surface, leading to a certain lower limit on the coronal power. While most of the analysis in this paper focuses on the case of a disk threaded by an externally imposed vertical magnetic field, we also discuss the zero-net-flux case, in which the magnetic field is produced by the MRI dynamo itself, and show that most of our arguments and conclusions should be valid in this case as well.

31 pages

References in corpus (13)

• Local Study of Accretion Disks with a Strong Vertical Magnetic Field: Magnetorotational Instability and Disk Outflow
• MHD simulations of the magnetorotational instability in a shearing box with zero net flux. I. The issue of convergence
• Angular Momentum Transport in Accretion Disks: Scaling Laws in MRI-driven Turbulence
• Local outflows from turbulent accretion disks
• Advection of Magnetic Fields in Accretion Disks: Not So Difficult After All
• Emergent Mesoscale Phenomena in Magnetized Accretion Disc Turbulence
• Advection/Diffusion of Large-Scale B-Field in Accretion Disks
• On Self-Sustained Dynamo Cycles in Accretion Discs
• Aspect ratio dependence in magnetorotational instability shearing box simulations
• Simulations of Magnetorotational Turbulence with a Higher-Order Godunov Scheme
• Empirical relation between angular momentum transport and thermal-to-magnetic pressure ratio in shearing box simulations
• Surface Structure in an Accretion Disk Annulus with Comparable Radiation and Gas Pressure
• Magnetorotational turbulence in stratified shearing boxes with perfect gas equation of state and finite thermal diffusivity

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• A New Approach to Constrain Black Hole Spins in Active Galaxies Using Optical Reverberation Mapping
• Testing the disk-corona interplay in radiatively-efficient broad-line AGN
• Wobbling and precessing jets from warped disks in binary systems
• A systematic study of the condensation of the corona and the application for ${\Gamma }_{2-10keV}-L_{\mathrm{bol}}/L_{\mathrm{Edd}}$ correlation in luminous active galactic nuclei
• Relativistic Turbulence with Strong Synchrotron and Synchrotron-Self-Compton Cooling
• Local models of two-temperature accretion disc coronae. I. Structure, outflows, and energetics
• Global simulations of magnetorotational turbulence II: turbulent energetics
• MRI turbulence in vertically stratified accretion discs at large magnetic Prandtl numbers
• Linear Growth of the Kelvin-Helmholtz Instability with an Adiabatic Cosmic-ray Gas
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