Large scale dynamos with helicity loss through boundaries
Abstract
Dynamo action is investigated in simulations of locally isotropic and homogeneous turbulence in a slab between open boundaries. It is found that a vertical magnetic field (pseudo-vacuum) boundary condition leads to strong helicity fluxes which significantly reduce the amplitude of the resulting large scale field. On the other hand, if there is a conducting halo outside the dynamo-active region the large scale field amplitude can reach larger values, but the timescale after which this field is reached increases linearly with the magnetic Reynolds number. In both cases most of the helicity flux is found to occur on large scales. From the variety of models considered we conclude that open boundaries tend to lower the saturation field strength. No trends are found suggesting that open boundaries alleviate the problem of long timescales that result from magnetic helicity conservation.Keywords
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