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 characterized 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. © 2014. The American Astronomical Society. All rights reserved.