Abstract
ABSTRACTIt 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 11-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 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 -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.
Highlights
Astrophysical magnetic fields often exhibit a remarkable degree of order despite the high levels of turbulence
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 and a rotation vector that is oblique to gravity
Our model is similar in spirit, though different from, that considered by Ponty et al (2001), which described the interaction of convection and dynamos with an Ekman spiral flow
Summary
Astrophysical magnetic fields often exhibit a remarkable degree of order despite the high levels of turbulence. There are many papers (Yousef et al 2008, Kapyla and Brandenburg 2009, Sridhar and Singh 2010) that demonstrate this effect for imposed shear flows and turbulence at low Rm and attempt to characterise the nature of the new correlations by relating them back to the large-scale field via transport coefficients (this is possible in a purely kinematic/linear framework). In astrophysical situations the shear flows usually self-consistently emerge via the interaction of turbulence with rotation leading to correlations in the flow (see e.g. Brun and Browning 2017, and the references therein) This situation is much less widely studied at high Rm, though there are a large number of spherical convection dynamos that are investigated at lower Rm where differential rotation and magnetic fields emerge self-consistently (Passos and Charbonneau 2014, Augustson et al 2015). Our model is similar in spirit, though different from, that considered by Ponty et al (2001), which described the interaction of convection and dynamos with an Ekman spiral flow
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