The magnetic shear-current effect: generation of large-scale magnetic fields by the small-scale dynamo

Abstract

A novel large-scale dynamo mechanism, the magnetic shear-current effect, is discussed and explored. The effect relies on the interaction of magnetic fluctuations with a mean shear flow, meaning the saturated state of the small-scale dynamo can drive a large-scale dynamo -- in some sense the inverse of dynamo quenching. The dynamo is nonhelical, with the mean-field $\alpha$ coefficient zero, and is caused by the interaction between an off-diagonal component of the turbulent resistivity and the stretching of the large-scale field by shear flow. Following up on previous numerical and analytic work, this paper presents further details of the numerical evidence for the effect, as well as an heuristic description of how magnetic fluctuations can interact with shear flow to produce the required electromotive force. The pressure response of the fluid is fundamental to this mechanism, which helps explain why the magnetic effect is stronger than its kinematic cousin, and the basic idea is related to the well-known lack of turbulent resistivity quenching by magnetic fluctuations. As well as being interesting for its applications to general high Reynolds number astrophysical turbulence, where strong small-scale magnetic fluctuations are expected to be prevalent, the magnetic shear-current effect is a likely candidate for large-scale dynamo in the unstratified regions of ionized accretion disks. Evidence for this is discussed, as well as future research directions and the challenges involved with understanding details of the effect in astrophysically relevant regimes.

References in corpus (22)

• 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
• Generation of Magnetic Field by Combined Action of Turbulence and Shear
• On Self-Sustained Dynamo Cycles in Accretion Discs
• Nonlinear current helicity fluxes in turbulent dynamos and alpha quenching
• Astrophysical Fluid Dynamics via Direct Statistical Simulation
• Nonlinear magnetic diffusivity and alpha tensors in helical turbulence
• Characterizing the mean-field dynamo in turbulent accretion disks
• Problems with kinematic mean field electrodynamics at high magnetic Reynolds numbers
• Magnetic-field generation in helical turbulence
• Numerical experiments on dynamo action in sheared and rotating turbulence
• Generation of large-scale magnetic fields by small-scale dynamo in shear flows
• Do mean-field dynamos in nonrotating turbulent shear-flows exist?
• Angular momentum transport and large eddy simulations in magnetorotational turbulence: the small Pm limit
• Statistical simulation of the magnetorotational dynamo
• The alpha effect with imposed and dynamo-generated magnetic fields
• On Large-Scale Dynamo Action at High Magnetic Reynolds Number
• Helicity-Flux-Driven Alpha Effect in Laboratory and Astrophysical Plasmas
• Localized magnetorotational instability and its role in the accretion disc dynamo
• Electromotive force due to magnetohydrodynamic fluctuations in sheared rotating turbulence
• Coherent nonhelical shear dynamos driven by magnetic fluctuations at low Reynolds numbers
• Nonlinear shear-current dynamo and magnetic helicity transport in sheared turbulence

Cited by in corpus (14)

• Dynamo theories
• The Turbulent Dynamo
• Advances in mean-field dynamo theory and applications to astrophysical turbulence
• Estimating activity cycles with probabilistic methods II. The Mount Wilson Ca H&K data
• Numerical evidence for a small-scale dynamo approaching solar magnetic Prandtl numbers
• Pressure-anisotropy-induced nonlinearities in the kinetic magnetorotational instability
• Hall current effects in mean-field dynamo theory
• Compressible test-field method and its application to shear dynamos
• Time-dependent relaxed magnetohydrodynamics -- inclusion of cross helicity constraint using phase-space action
• On large-scale dynamos with stable stratification and the application to stellar radiative zones
• On the shear-current effect: toward understanding why theories and simulations have mutually and separately conflicted
• On the existence of shear-current effects in magnetized burgulence
• Shedding light on the MRI driven dynamo in a stratified shearing box
• Gravitoturbulent dynamo in global simulations of gaseous disks