Helical and nonhelical large-scale dynamos in thin accretion discs

Abstract

The dynamics of accreting and outgoing flows around compact objects depends crucially on the strengths and configurations of the magnetic fields therein, especially of the large-scale fields that remain coherent beyond turbulence scales. Possible origins of these large-scale magnetic fields include flux advection and disc dynamo actions. However, most numerical simulations have to adopt an initially strong large-scale field rather than allow them to be self-consistently advected or amplified, due to limited computational resources. The situation can be partially cured by using sub-grid models where dynamo actions only reachable at high resolutions are mimicked by artificial terms in low-resolution simulations. In this work, we couple thin-disc models with local shearing-box simulation results to facilitate more realistic sub-grid dynamo implementations. For helical dynamos, detailed spatial profiles of dynamo drivers inferred from local simulations are used, and the nonlinear quenching and saturation is constrained by magnetic helicity evolution. In the inner disc region, saturated fields have dipole configurations and the plasma $\beta$ reaches $\simeq 0.1$ to $100$, with correlation lengths $\simeq h$ in the vertical direction and $\simeq 10h$ in the radial direction, where $h$ is the disc scale height. The dynamo cycle period is $\simeq 40$ orbital time scale, compatible with previous global simulations. Additionally, we explore two dynamo mechanisms which do not require a net kinetic helicity and have only been studied in shearing-box setups. We show that such dynamos are possible in thin accretion discs, but produce field configurations that are incompatible with previous results. We discuss implications for future general-relativistic magnetohydrodynamics simulations.

arXiv comment12 pages, 13 figures. Accepted by MNRAS

References in corpus (61)

• Astrophysical magnetic fields and nonlinear dynamo theory
• Hot Accretion Flows Around Black Holes
• Efficient Generation of Jets from Magnetically Arrested Accretion on a Rapidly Spinning Black Hole
• Magnetically Arrested Disk: An Energetically Efficient Accretion Flow
• Accretion disc viscosity: how big is alpha?
• GRMHD Simulations of Magnetized Advection Dominated Accretion on a Non-Spinning Black Hole: Role of Outflows
• The case for a distributed solar dynamo shaped by near-surface shear
• Sustained Magnetorotational Turbulence in Local Simulations of Stratified Disks with Zero Net Magnetic Flux
• The Influence of Magnetic Field Geometry on the Evolution of Black Hole Accretion Flows: Similar Disks, Drastically Different Jets
• Local Study of Accretion Disks with a Strong Vertical Magnetic Field: Magnetorotational Instability and Disk Outflow
• MHD simulations of jet acceleration from Keplerian accretion disks: the effects of disk resistivity
• A New Dynamical Mean-Field Dynamo Theory and Closure Approach
• Global simulations of axisymmetric radiative black hole accretion disks in general relativity with a sub-grid magnetic dynamo
• The Pencil Code, a modular MPI code for partial differential equations and particles: multipurpose and multiuser-maintained
• Magnetic diffusivity tensor and dynamo effects in rotating and shearing turbulence
• Large-Scale Poloidal Magnetic Field Dynamo Leads to Powerful Jets in GRMHD Simulations of Black Hole Accretion with Toroidal Field
• Transport of Large Scale Poloidal Flux in Black Hole Accretion
• Electromotive Force and Large-Scale Magnetic Dynamo in a Turbulent Flow with a Mean Shear
• An Incoherent $\alpha -\Omega$ Dynamo in Accretion Disks
• Nonlinear theory of a "shear-current" effect and mean-field magnetic dynamos
• A fully covariant mean-field dynamo closure for numerical 3+1 resistive GRMHD
• Generation of Magnetic Field by Combined Action of Turbulence and Shear
• The Turbulent Magnetic Prandtl Number of MHD Turbulence in Disks
• Plasmoid formation in global GRMHD simulations and AGN flares
• Local and nonlocal magnetic diffusion and alpha-effect tensors in shear flow turbulence
• Transport of magnetic flux and the vertical structure of accretion discs: I. Uniform diffusion coefficients
• Black-hole jets without large-scale net magnetic flux
• The large scale magnetic fields of advection dominated accretion flows
• The large scale magnetic fields of thin accretion disks
• Saturation of the magnetorotational instability in the unstratified shearing box with zero net flux: convergence in taller boxes
• Transport of magnetic flux and the vertical structure of accretion discs: II. Vertical profile of the diffusion coefficients
• The Surprisingly Small Impact of Magnetic Fields On The Inner Accretion Flow of Sagittarius A* Fueled By Stellar Winds
• Structured outflow from a dynamo active accretion disc
• Characterizing the mean-field dynamo in turbulent accretion disks
• Empirical relation between angular momentum transport and thermal-to-magnetic pressure ratio in shearing box simulations
• Testing the Propagating Fluctuations Model with a Long, Global Accretion Disk Simulation
• Test-field method for mean-field coefficients with MHD background
• Modelling MHD accretion-ejection - from the launching area to propagation scales
• A Local Model for Angular Momentum Transport in Accretion Disks Driven by the Magnetorotational Instability
• Accretion of a massive magnetized torus on a rotating black hole
• Modelling MHD accretion-ejection - episodic ejections of jets triggered by a mean-field disk dynamo
• Coronae as Consequence of Large Scale Magnetic Fields in Turbulent Accretion Disks
• Striped Blandford/Znajek jets from advection of small scale magnetic field
• A mean-field approach to the propagation of field patterns in stratified magneto rotational turbulence
• The influence of accretion disk thickness on the large-scale magnetic dynamo
• General relativistic magnetohydrodynamic dynamo in thick accretion disks: fully nonlinear simulations
• MRI-driven dynamo at very high magnetic Prandtl numbers
• Large-scale dynamo action due to $\alpha$ fluctuations in a linear shear flow
• Characterising the Dynamo in a Radiatively Inefficient Accretion Flow
• Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence
• Two-Temperature GRMHD Simulations of Black Hole Accretion Flows with Multiple Magnetic Loops
• Saturation of the magnetorotational instability and the origin of magnetically elevated accretion discs
• Magnetic Flux Transport in Radiatively Inefficient Accretion Flows and the Pathway towards a Magnetically Arrested Disk
• Accretion Disks and Dynamos: Toward a Unified Mean Field Theory
• Magnetic helicity fluxes from triple correlators
• Relativistic outflows from a GRMHD mean-field disk dynamo
• Compressible test-field method and its application to shear dynamos
• Variability Associated with Alpha Accretion Disc Theory for Standard and Advection Dominated Discs
• On the shear-current effect: toward understanding why theories and simulations have mutually and separately conflicted
• Generation of large-scale magnetic fields due to fluctuating $\alpha$ in shearing systems
• Magnetic field evolution in high and low $\beta$ disks with initially-toroidal fields

Cited by in corpus (1)

• Dynamics and Emission Properties of Flux Ropes from Two-Temperature GRMHD Simulations with Multiple Magnetic Loops