Transition from large-scale to small-scale dynamo

Abstract

The dynamo equations are solved numerically with a helical forcing corresponding to the Roberts flow. In the fully turbulent regime the flow behaves as a Roberts flow on long time scales, plus turbulent fluctuations at short time scales. The dynamo onset is controlled by the long time scales of the flow, in agreement with the former Karlsruhe experimental results. The dynamo mechanism is governed by a generalized $\alpha$-effect which includes both usual $\alpha$-effect and turbulent diffusion, plus all higher order effects. Beyond the onset we find that this generalized $\alpha$-effect scales as $O(R{m}^{-1})$ suggesting the take-over of small-scale dynamo action. This is confirmed by simulations in which dynamo occurs even if the large-scale field is artificially suppressed.

4 pages, 3 figures

References in corpus (9)

• Kinematic alpha effect in isotropic turbulence simulations
• Nonlinear magnetic diffusivity and alpha tensors in helical turbulence
• A growing dynamo from a saturated Roberts flow dynamo
• Inverse cascades and alpha-effect at low magnetic Prandtl number
• Mean-field dynamos in random Arnold-Beltrami-Childress and Roberts flows
• Parametric instability of the helical dynamo
• Phenomenology of turbulent dynamo growth and saturation
• Oscillating Ponomarenko dynamo in the highly conducting limit
• An optimal scale separation for a dynamo experiment

Cited by in corpus (6)

• Shell Models of Magnetohydrodynamic Turbulence
• Energy transfers in dynamos with small magnetic Prandtl numbers
• Amplification of large-scale magnetic field in nonhelical magnetohydrodynamics
• The fate of alpha dynamos at large $Rm$
• Transition to turbulent dynamo saturation
• Dynamo saturation down to vanishing viscosity: strong-field and inertial scaling regimes