Heating of accretiondisk coronae and jets by general relativistic magnetohydrodynamic turbulence
Abstract
Turbulence in an accretion disk launches Alfvén waves (AWs) that propagate away from the disk along magneticfield lines. Because the Alfvén speed varies with distance from the disk, the AWs undergo partial nonWKB reflection, and counterpropagating AWs subsequently interact, causing AW energy to cascade to small scales and dissipate. To investigate this process, we introduce an Elsasserlike 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 meansquare AW amplitude +$ 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 largescale poloidal magnetic field, the AW energy flux is ∼(ρ_{b}/ρ_{d}) ^{1/2}β_{net,d}^{1/2} times the disk's radiative flux, where ρ_{b} and ρ_{d} are the mass densities at the coronal base and disk midplane, respectively, and β_{net,d} is the ratio (evaluated at the disk midplane) of plasmaplusradiation 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 reflectiondriven AW turbulence using a subgrid model.
 Publication:

Journal of Plasma Physics
 Pub Date:
 June 2018
 DOI:
 10.1017/S0022377818000387
 arXiv:
 arXiv:1707.06216
 Bibcode:
 2018JPlPh..84c9010C
 Keywords:

 astrophysical plasmas;
 plasma nonlinear phenomena;
 space plasma physics;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology;
 Physics  Plasma Physics
 EPrint:
 21 pages, no figures, accepted for publication in JPP. Version 2 contains more detail and a new section on a subgrid model for incorporating turbulence into GRMHD simulations