Convection-driven kinematic dynamos with a self-consistent shear flow

Abstract

It is widely accepted that astrophysical magnetic fields are generated by dynamo action. In many cases these fields exhibit organisation on a scale larger than that of the underlying turbulent flow (e.g., the eleven-year solar cycle). The mechanism for the generation of so-called large scale fields remains an open problem. In cases where the magnetic Reynolds number ($Rm$) is small, dynamo-generated fields are coherent but at (the astrophysically relevant) high $Rm$, the fields are overwhelmed by small scale fluctuating field. Recently Tobias and Cattaneo (2013) have shown that an imposed large scale shear flow can suppress the small scale fluctuations and allow the large scale temporal behaviour to emerge. Shear is also believed to modify the electromotive force by introducing correlations between the flow and the field. However in previous models at high $Rm$ the shear is often artificially imposed or driven by an arbitrary body force. Here we consider a simple kinematic model of a convective dynamo in which shear is self consistently driven by the presence of a horizontal temperature gradient (resulting in a thermal wind) and a rotation vector that is oblique to gravity. By considering a $2.5$-dimensional system, we are able to reach high $Rm$ so that the dynamo approaches the asymptotic regime where the growth rate becomes approximately independent of $Rm$. We find the flows studied here to be excellent small-scale dynamos, but with very little systematic behaviour evident at large $Rm$. We attribute this to being unable to self-consistently generate flows with both large (net) helicity and strong shear in this setup.

arXiv comment17 pages, 11 figures. Available open access from https://www.tandfonline.com/doi/full/10.1080/03091929.2018.1517210

References in corpus (10)

• Astrophysical magnetic fields and nonlinear dynamo theory
• Constraints on the magnitude of alpha in dynamo theory
• Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers
• Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo
• Generation of Magnetic Field by Combined Action of Turbulence and Shear
• Turbulent dynamos with shear and fractional helicity
• The shear dynamo problem for small magnetic Reynolds numbers
• Large-scale dynamos in rapidly rotating plane layer convection
• Mean flow generation in rotating anelastic two-dimensional convection
• Magnetic Helicities and Dynamo Action in Magneto-rotationally Driven Turbulence

Cited by in corpus (5)

• Dedalus: A Flexible Framework for Numerical Simulations with Spectral Methods
• Small-Scale Dynamo in Stably Stratified Turbulence
• The role of slow magnetostrophic waves in the formation of the axial dipole in planetary dynamos
• Generation of shear flows and vortices in rotating anelastic convection
• Effect of flow shear on the onset of dynamos