Subcritical dynamos in shear flows

Abstract

Identifying generic physical mechanisms responsible for the generation of magnetic fields and turbulence in differentially rotating flows is fundamental to understand the dynamics of astrophysical objects such as accretion disks and stars. In this paper, we discuss the concept of subcritical dynamo action and its hydrodynamic analogue exemplified by the process of nonlinear transition to turbulence in non-rotating wall-bounded shear flows. To illustrate this idea, we describe some recent results on nonlinear hydrodynamic transition to turbulence and nonlinear dynamo action in rotating shear flows pertaining to the problem of turbulent angular momentum transport in accretion disks. We argue that this concept is very generic and should be applicable to many astrophysical problems involving a shear flow and non-axisymmetric instabilities of shear-induced axisymmetric toroidal velocity or magnetic fields, such as Kelvin-Helmholtz, magnetorotational, Tayler or global magnetoshear instabilities. In the light of several recent numerical results, we finally suggest that, similarly to a standard linear instability, subcritical MHD dynamo processes in high-Reynolds number shear flows could act as a large-scale driving mechanism of turbulent flows that would in turn generate an independent small-scale dynamo.

12 pages. Submitted to Astronomische Nachrichten (2007 Catania MHD Workshop special issue)

References in corpus (25)

• Dynamo action by differential rotation in a stably stratified stellar interior
• Simulations of small-scale turbulent dynamo
• Hydrodynamic turbulence cannot transport angular momentum effectively in astrophysical disks
• On magnetic instabilities and dynamo action in stellar radiation zones
• The ionization fraction in alpha-models of protoplanetary disks
• MHD simulations of the magnetorotational instability in a shearing box with zero net flux. II. The effect of transport coefficients
• Impact of dimensionless numbers on the efficiency of MRI-induced turbulent transport
• MHD simulations of the magnetorotational instability in a shearing box with zero net flux. I. The issue of convergence
• Turbulence transition and the edge of chaos in pipe flow
• Lower branch coherent states in shear flows: transition and control
• Local Hydrodynamic Stability of Accretion Disks
• Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers
• On the relevance of subcritical hydrodynamic turbulence to accretion disk transport
• Direct magnetic field detection in the innermost regions of an accretion disc
• A differential rotation driven dynamo in a stably stratified star
• The onset of a small-scale turbulent dynamo at low magnetic Prandtl numbers
• A self-sustaining nonlinear dynamo process in Keplerian shear flows
• Three Dimensional Compressible Hydrodynamic Simulations of Vortices in Disks
• Scale by scale budget and similarity laws for shear turbulence
• On the Phenomenology of Hydrodynamic Shear Turbulence
• On self-sustaining processes in Rayleigh-stable rotating plane Couette flows and subcritical transition to turbulence in accretion disks
• Subcritical dynamo bifurcation in the Taylor Green flow
• Helicity generation and alpha-effect by Vandakurov-Tayler instability with z-dependent differential rotation
• Anisotropy, inhomogeneity and inertial range scalings in turbulent convection
• Sustained magneto-shear instabilities in the solar tachocline

Cited by in corpus (21)

• Dynamo theories
• 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
• Nonlinear small-scale dynamos at low magnetic Prandtl numbers
• Periodic magnetorotational dynamo action as a prototype of nonlinear magnetic field generation in shear flows
• On the self-sustained nature of large-scale motions in turbulent Couette flow
• MHD simulations of the magnetorotational instability in a shearing box with zero net flux: the case Pm=4
• The shear dynamo problem for small magnetic Reynolds numbers
• Dissipative effects on the sustainment of a magnetorotational dynamo in Keplerian shear flow
• Nonmodal growth of the magnetorotational instability
• Supertransient magnetohydrodynamic turbulence in Keplerian shear flows
• Magnetorotational dynamo chimeras. The missing link to turbulent accretion disk dynamo models?
• Nonperturbative quasilinear approach to the shear dynamo problem
• MRI turbulence in accretion discs at large magnetic Prandtl numbers
• Zero net flux MRI-turbulence in disks $-$ sustenance scheme and magnetic Prandtl number dependence
• Transient growth of perturbations on scales beyond the accretion disc thickness
• High-speed shear driven dynamos. Part 1. Asymptotic analysis
• High-speed shear driven dynamos. Part 2. Numerical analysis
• Unified treatment of mean-field dynamo and angular-momentum transport in magnetorotational instability-driven turbulence
• Subcritical transition to turbulence in accretion disc boundary layer
• Nonlinear analysis of gravitational instability in a 3D gaseous disc