TY - JOUR

T1 - Heating of accretion-disk coronae and jets by general relativistic magnetohydrodynamic turbulence

AU - Chandran, Benjamin D.G.

AU - Foucart, Francois

AU - Tchekhovskoy, Alexander

N1 - Publisher Copyright:
© Cambridge University Press 2018

PY - 2018/6/1

Y1 - 2018/6/1

N2 - Turbulence in an accretion disk launches Alfvén waves (AWs) that propagate away from the disk along magnetic-field lines. Because the Alfvén speed varies with distance from the disk, the AWs undergo partial non-WKB reflection, and counter-propagating AWs subsequently interact, causing AW energy to cascade to small scales and dissipate. To investigate this process, we introduce an Elsasser-like formulation of general relativistic magnetohydrodynamics (GRMHD) and develop the theory of general relativistic reduced MHD in an inhomogeneous medium. We then derive a set of equations for the mean-square AW amplitude M+ and turbulent heating rate Q under the assumption that, in the plasma rest frame, AWs propagating away from the disk are much more energetic than AWs propagating toward the disk. For the case in which the background flow is axisymmetric and time independent, we solve these equations analytically to determine M+ and Q as functions of position. We find that, for an idealized thin disk threaded by a large-scale poloidal magnetic field, the AW energy flux is ∼(ρb/ρd)1/2βnet−1/d2 times the disk's radiative flux, where ρb and ρd are the mass densities at the coronal base and disk midplane, respectively, and βnet,d is the ratio (evaluated at the disk midplane) of plasma-plus-radiation pressure to the pressure of the average vertical magnetic field. This energy flux could have a significant impact on disk coronae and outflows. To lay the groundwork for future global simulations of turbulent disk coronae and jets, we derive a set of averaged GRMHD equations that account for reflection-driven AW turbulence using a sub-grid model.

AB - Turbulence in an accretion disk launches Alfvén waves (AWs) that propagate away from the disk along magnetic-field lines. Because the Alfvén speed varies with distance from the disk, the AWs undergo partial non-WKB reflection, and counter-propagating AWs subsequently interact, causing AW energy to cascade to small scales and dissipate. To investigate this process, we introduce an Elsasser-like formulation of general relativistic magnetohydrodynamics (GRMHD) and develop the theory of general relativistic reduced MHD in an inhomogeneous medium. We then derive a set of equations for the mean-square AW amplitude M+ and turbulent heating rate Q under the assumption that, in the plasma rest frame, AWs propagating away from the disk are much more energetic than AWs propagating toward the disk. For the case in which the background flow is axisymmetric and time independent, we solve these equations analytically to determine M+ and Q as functions of position. We find that, for an idealized thin disk threaded by a large-scale poloidal magnetic field, the AW energy flux is ∼(ρb/ρd)1/2βnet−1/d2 times the disk's radiative flux, where ρb and ρd are the mass densities at the coronal base and disk midplane, respectively, and βnet,d is the ratio (evaluated at the disk midplane) of plasma-plus-radiation pressure to the pressure of the average vertical magnetic field. This energy flux could have a significant impact on disk coronae and outflows. To lay the groundwork for future global simulations of turbulent disk coronae and jets, we derive a set of averaged GRMHD equations that account for reflection-driven AW turbulence using a sub-grid model.

KW - Astrophysical plasmas

KW - Plasma nonlinear phenomena

KW - Space plasma physics

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U2 - 10.1017/S0022377818000387

DO - 10.1017/S0022377818000387

M3 - Article

AN - SCOPUS:85065984430

SN - 0022-3778

VL - 84

JO - Journal of Plasma Physics

JF - Journal of Plasma Physics

IS - 3

M1 - 905840310

ER -