Shedding light on the MRI driven dynamo in a stratified shearing box

Abstract

We study the magneto-rotational instability (MRI) driven dynamo in a geometrically thin disc ($H/R\ll 1$) using stratified zero net flux (ZNF) shearing box simulations. We find that mean fields and EMFs oscillate with a primary frequency $f_{\mathrm{dyn}}=0.017$ ($\approx 9$ orbital period), but also have higher harmonics at $3f_{\mathrm{dyn}}$. Correspondingly, the current helicity, has two frequencies $2f_{\mathrm{dyn}}$ and $4f_{\mathrm{dyn}}$ respectively, which appear to be the beat frequencies of mean fields and EMFs as expected from the magnetic helicity density evolution equation. Further, we adopt a novel inversion algorithm called the `Iterative Removal Of Sources' (IROS), to extract the turbulent dynamo coefficients in the mean-field closure using the mean magnetic fields and EMFs obtained from the shearing box simulation. We show that an $\alpha -$effect (${\alpha }_{yy}$) is predominantly responsible for the creation of the poloidal field from the toroidal field, while shear generates back a toroidal field from the poloidal field; indicating that an $\alpha -\Omega$-type dynamo is operative in MRI-driven accretion discs. We also find that both strong outflow (${\bar{v}}_z$) and turbulent pumping (${\gamma }_z$ ) transport mean fields away from the mid-plane. Instead of turbulent diffusivity, they are the principal sink terms in the mean magnetic energy evolution equation. We find encouraging evidence that a generative helicity flux is responsible for the effective $\alpha$-effect. Finally, we point out potential limitations of horizontal ($x-y$) averaging in defining the `mean' on the extraction of dynamo coefficients and their physical interpretations.

16 pages, 13 figures, published in MNRAS

References in corpus (59)

• Astrophysical magnetic fields and nonlinear dynamo theory
• PLUTO: a Numerical Code for Computational Astrophysics
• An Unsplit Godunov Method for Ideal MHD via Constrained Transport
• Hydrodynamical Non-radiative Accretion Flows in Two-Dimensions
• Zonal Flows and Long-Lived Axisymmetric Pressure Bumps in Magnetorotational Turbulence
• Sustained Magnetorotational Turbulence in Local Simulations of Stratified Disks with Zero Net Magnetic Flux
• 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
• Mean-field concept and direct numerical simulations of rotating magnetoconvection and the geodynamo
• Constraints on the magnitude of alpha in dynamo theory
• Dynamic nonlinearity in large scale dynamos with shear
• Magnetic diffusivity tensor and dynamo effects in rotating and shearing turbulence
• Angular Momentum Transport in Accretion Disks: Scaling Laws in MRI-driven Turbulence
• Accretion disc dynamo activity in local simulations spanning weak-to-strong net vertical magnetic flux regimes
• Nonlinear theory of a "shear-current" effect and mean-field magnetic dynamos
• Testing Convergence for Global Accretion Disks
• The onset of a small-scale turbulent dynamo at low magnetic Prandtl numbers
• Emergent Mesoscale Phenomena in Magnetized Accretion Disc Turbulence
• Global MHD Simulations of Cylindrical Keplerian Disks
• Locality of MHD Turbulence in Isothermal Disks
• Magnetic helicity density and its flux in weakly inhomogeneous turbulence
• What Is The Numerically Converged Amplitude of MHD Turbulence in Stratified Shearing Boxes?
• On Self-Sustained Dynamo Cycles in Accretion Discs
• Convection Causes Enhanced Magnetic Turbulence in Accretion Disks in Outburst
• Simulations of Magnetorotational Turbulence with a Higher-Order Godunov Scheme
• Saturation of the magnetorotational instability in the unstratified shearing box with zero net flux: convergence in taller boxes
• Global bifurcations to subcritical magnetorotational dynamo action in Keplerian shear flow
• Structured outflow from a dynamo active accretion disc
• Turbulence in Global Simulations of Magnetized Thin Accretion Disks
• Characterizing the mean-field dynamo in turbulent accretion disks
• Radially Extended, Stratified, Local Models of Isothermal Disks
• Testing the Propagating Fluctuations Model with a Long, Global Accretion Disk Simulation
• A Local Model for Angular Momentum Transport in Accretion Disks Driven by the Magnetorotational Instability
• Large Scale Azimuthal Structures Of Turbulence In Accretion Disks - Dynamo triggered variability of accretion
• Resolution Dependence of Magnetorotational Turbulence in the Isothermal Stratified Shearing Box
• Modelling MHD accretion-ejection - episodic ejections of jets triggered by a mean-field disk dynamo
• Characterisation of the turbulent electromotive force and its magnetically-mediated quenching in a global EULAG-MHD simulation of solar convection
• Symmetries, scaling laws and convergence in shearing-box simulations of MRI driven turbulence
• A mean-field approach to the propagation of field patterns in stratified magneto rotational turbulence
• On the convergence of Magnetorotational turbulence in stratified isothermal shearing boxes
• The influence of accretion disk thickness on the large-scale magnetic dynamo
• Global simulations of magnetorotational turbulence I: convergence and the quasi-steady state
• Large scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability
• Characterising the Dynamo in a Radiatively Inefficient Accretion Flow
• Convective Quenching of Field Reversals in Accretion Disc Dynamos
• The dual role of shear in large-scale dynamos
• The magnetic shear-current effect: generation of large-scale magnetic fields by the small-scale dynamo
• Magnetic Flux Transport in Radiatively Inefficient Accretion Flows and the Pathway towards a Magnetically Arrested Disk
• Saturation of the magnetorotational instability and the origin of magnetically elevated accretion discs
• Coronae & Outflows from Helical Dynamos, Compatibility with the MRI, and Application to Protostellar Disks
• Magnetorotational instability with smoothed particle hydrodynamics
• Turbulent transport coefficients in galactic dynamo simulations using singular value decomposition
• Magnetic helicity fluxes from triple correlators
• Simulating the magnetorotational instability on a moving mesh with the shearing box approximation
• 3D global simulations of RIAFs: convergence, effects of azimuthal extent and dynamo
• Sensitivity of the Magnetorotational Instability to the shear parameter in stratified simulations
• Turbulent magnetic helicity fluxes in solar convective zone
• Finite-time response of dynamo mean-field effects in magnetorotational turbulence
• Steady states of the Parker instability: the effects of rotation

Cited by in corpus (3)

• Energy Extraction from a Black Hole by a Strongly Magnetized Thin Accretion Disk
• Iterative removal of sources to model the turbulent electromotive force
• MRI-driven $\alpha \Omega$ dynamo at high Pm numbers