Fabio Del Sordo, Gustavo Guerrero, Axel Brandenburg
Many astrophysical bodies harbor magnetic fields that are thought to be sustained by dynamo processes. However, it has been argued that the production of large-scale magnetic fields by a mean-field dynamo is strongly suppressed at large magnetic Reynolds numbers owing to the conservation of magnetic helicity. This phenomenon is known as catastrophic quenching. Advection of magnetic field toward the outer boundaries and away from the dynamo is expected to alleviate such quenching. Examples are stellar and galactic winds. Such advection might be able to overcome the constraint imposed by the conservation of magnetic helicity, transporting a fraction of it outside the domain in which the dynamo operates. We study how the dynamo process is affected by advection. In particular, we study the relative roles played by advective and diffusive fluxes of magnetic helicity. We do this by performing direct numerical simulations of a turbulent dynamo of alpha^2 type driven by forced turbulence in a Cartesian domain in the presence of a constant flow toward the upper and lower borders of the domain. We demonstrate that, within the range of magnetic Reynolds number examined (\Rm < 200, based on the wavenumber of the energy-carrying eddies), the resistive term still dominates over the advective one in the evolution equation of magnetic helicity. This means that for the Rm presently accessible, advection does not alleviate the quenching. Our results lead us to estimate that for Rm ~ 4.10^3. catastrophic quenching can be alleviated by the presence of advection. We also find that in the presence of advection the dynamo, otherwise stationary, becomes oscillatory.
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http://arxiv.org/abs/1205.3502
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