What is a large-scale dynamo?
2017
We consider kinematic dynamo action in a sheared helical flow at moderate to high values of the magnetic Reynolds number Rm. We find exponentially growing solutions which, for large enough shear, take the form of a coherent part embedded in incoherent fluctuations. We argue that at large Rm large-scale dynamo action should be identified by the presence of structures coherent in time, rather than those at large spatial scales. We further argue that although the growth-rate is determined by small-scale processes, the period of the coherent structures is set by mean-field considerations.
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