Magnetic Helicities and Dynamo Action in Magneto-rotational Turbulence

Abstract

We examine the relationship between magnetic flux generation and magnetic helicity in a simple model of dynamo action. We consider dynamo action driven by Magneto-Rotational Turbulence (MRT) within the shearing-box approxima- tion. We consider magnetically open boundary conditions that allow a flux of helicity in or out of the computational domain. We circumvent the problem of the lack of gauge invariance in open domains by choosing a particular gauge – the winding gauge – that provides a natural interpretation in terms of average winding number of pairwise field lines. We use this gauge precisely to define and measure the helicity and helicity flux for several realizations of dynamo action. We find that the system as a whole does not break reflectional symmetry and the total helicity remains small even in cases when substantial magnetic flux is generated. We find no particular connection between the generation of magnetic flux – large-scale dynamo action – and the helicity or the helicity flux though the volume. We suggest that this result may be due to the essentially nonlinear nature of the dynamo processes in MRT.