Mean-field dynamo action from delayed transport

Abstract

We analyze the nature of dynamo action that produces horizontally averaged magnetic fields in two particular flows that were studied by Roberts (1972, Phil. Trans. R. Soc. A 271, 411), namely his flows II and III. They have zero kinetic helicity either pointwise (flow II), or on average (flow III). Using direct numerical simulations, we determine the onset conditions for dynamo action at moderate values of the magnetic Reynolds number. Using the test-field method, we show that the turbulent magnetic diffusivity is then positive for both flows. However, we demonstrate that for both flows large-scale dynamo action occurs through delayed transport. Mathematically speaking, the magnetic field at earlier times contributes to the electromotive force through the off-diagonal components of the alpha tensor such that a zero mean magnetic field becomes unstable to dynamo action. This represents a qualitatively new mean-field dynamo mechanism not previously described.

arXiv comment11 pages, 12 figures, submitted to MNRAS

References in corpus (14)

• Mean-field concept and direct numerical simulations of rotating magnetoconvection and the geodynamo
• Electromotive Force and Large-Scale Magnetic Dynamo in a Turbulent Flow with a Mean Shear
• Nonlinear theory of a "shear-current" effect and mean-field magnetic dynamos
• Kinematic alpha effect in isotropic turbulence simulations
• Non-Fickian diffusion and tau approximation from numerical turbulence
• Alpha effect and turbulent diffusion from convection
• Memory effects in turbulent transport
• Contributions to the theory of a two-scale homogeneous dynamo experiment
• Modeling spatio-temporal nonlocality in mean-field dynamos
• Dynamo effect in parity-invariant flow with large and moderate separation of scales
• Mean-field effects in the Galloway-Proctor flow
• Mean-field diffusivities in passive scalar and magnetic transport in irrotational flows
• A mean field dynamo from negative eddy diffusivity
• Magnetic helicity fluxes in interface and flux transport dynamos

Cited by in corpus (16)

• Advances in mean-field dynamo theory and applications to astrophysical turbulence
• Laminar and turbulent dynamos in chiral magnetohydrodynamics. II. Simulations
• Galactic Dynamos
• Turbulent transport coefficients in spherical wedge dynamo simulations of solar-like stars
• Quenching and anisotropy of hydromagnetic turbulent transport
• Large-scale dynamo action due to $\alpha$ fluctuations in a linear shear flow
• Turbulent processes and mean-field dynamo
• Robustness of oscillatory ${\alpha }^2$ dynamos in spherical wedges
• Compressible test-field method and its application to shear dynamos
• Negative magnetic eddy diffusivities from test-field method and multiscale stability theory
• The nature of mean-field generation in three classes of optimal dynamos
• Moffatt drift driven large scale dynamo due to $\alpha$ fluctuations with nonzero correlation times
• Generation of large-scale magnetic fields due to fluctuating $\alpha$ in shearing systems
• Mean-field dynamos: the old concept and some recent developments
• Inverse cascade from helical and nonhelical decaying columnar magnetic fields
• Kinematic dynamos and resolution limits for Smoothed Particle Magnetohydrodynamics