Large scale dynamo action precedes turbulence in shearing box simulations of the magnetorotational instability

Abstract

We study the dynamo generation (exponential growth) of large scale (planar averaged) fields in unstratified shearing box simulations of the magnetorotational instability (MRI). In contrast to previous studies restricted to horizontal ($x$-$y$) averaging, we demonstrate the presence of large scale fields when either horizontal or vertical ($y$-$z$) averaging is employed. By computing planar averaged fields and power spectra, we find large scale dynamo action in the early MRI growth phase---a previously unidentified feature. Fast growing horizontal low modes and fiducial vertical modes over a narrow range of wave numbers amplify these planar averaged fields in the MRI growth phase, before turbulence sets in. The large scale field growth requires linear fluctuations but not nonlinear turbulence (as defined by mode-mode coupling) and grows as a direct global mode of the MRI. Only by vertical averaging, can it be shown that the growth of horizontal low wavenumber MRI modes directly feed-back to the initial vertical field providing a clue as to why the large scale vertical field sustains against turbulent diffusion in the saturation regime. We compute the terms in the planar averaged mean field equations to identify the individual contributions to large scale field growth for both vertical and horizontal averaging. The large scale fields obtained from such vertical averaging are found to compare well with global cylindrical simulations and quasilinear analytical analysis from a previous study by Ebrahimi \& Blackman. We discuss the potential implications of these new results for understanding large scale MRI dynamo saturation and turbulence.

12 pages, 9 figues, submitted to MNRAS, comments are welcome

References in corpus (9)

• Simulations of nonhelical hydromagnetic turbulence
• A self-sustaining nonlinear dynamo process in Keplerian shear flows
• Fluctuation dynamos and their Faraday rotation signatures
• The signature of the magnetorotational instability in the Reynolds and Maxwell stress tensors in accretion discs
• Magnetorotational instability driven dynamos at low magnetic Prandtl numbers
• Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow
• Helicity-Flux-Driven Alpha Effect in Laboratory and Astrophysical Plasmas
• Localized magnetorotational instability and its role in the accretion disc dynamo
• On characterizing nonlocality and anisotropy for the magnetorotational instability

Cited by in corpus (18)

• Dynamo theories
• Large-Scale Poloidal Magnetic Field Dynamo Leads to Powerful Jets in GRMHD Simulations of Black Hole Accretion with Toroidal Field
• Advances in mean-field dynamo theory and applications to astrophysical turbulence
• The effects of large-scale magnetic fields on the model for repeating changing-look AGNs
• Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?
• Characterising the Dynamo in a Radiatively Inefficient Accretion Flow
• Zero net flux MRI-turbulence in disks $-$ sustenance scheme and magnetic Prandtl number dependence
• Derivation and precision of mean field electrodynamics with mesoscale fluctuations
• Persistent mysteries of jet engines, formation, propagation, and particle acceleration: have they been addressed experimentally?
• The Magnetorotational Instability Prefers Three Dimensions
• Evolution of the magnetorotational instability on initially tangled magnetic fields
• Onset of Plasmoid Reconnection during Magnetorotational Instability
• Shedding light on the MRI driven dynamo in a stratified shearing box
• Unified treatment of mean-field dynamo and angular-momentum transport in magnetorotational instability-driven turbulence
• A generalized effective potential for differentially rotating plasmas
• Large Scale Dynamo in a Primordial Accretion Flow -- An Interpretation from Hydrodynamic Simulation
• Experimental Confirmation of the Standard Magnetorotational Instability Mechanism with a Spring-Mass Analogue
• Turbulence and order in magnetized flowing plasmas