On Large-Scale Dynamo Action at High Magnetic Reynolds Number

Abstract

We consider the generation of magnetic activity --- dynamo waves --- in the astrophysical limit of very large magnetic Reynolds number. We consider kinematic dynamo action for a system consisting of helical flow and large-scale shear. We demonstrate that large-scale dynamo waves persist at high $Rm$ if the helical flow is characterised by a narrow band of spatial scales and the shear is large enough. However for a wide band of scales the dynamo becomes small-scale with a further increase of $Rm$, with dynamo waves re-emerging only if the shear is then increased. We show that at high $Rm$ the key effect of the shear is to suppress small-scale dynamo action, allowing large-scale dynamo action to be observed. We conjecture that this supports a general "suppression principle" --- large-scale dynamo action can only be observed if there is a mechanism that suppresses the small-scale fluctuations.

arXiv commentAstrophysical Journal in Press

References in corpus (8)

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• Numerical experiments on dynamo action in sheared and rotating turbulence
• Shear-current effect in a turbulent convection with a large-scale shear
• The shear dynamo problem for small magnetic Reynolds numbers
• The alpha effect in rotating convection with sinusoidal shear
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