The dual role of shear in large-scale dynamos - AstrophysicsReport as inadecuate

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Abstract: The role of shear in alleviating catastrophic quenching by sheddingsmall-scale magnetic helicity through fluxes along contours of constant shearis discussed. The level of quenching of the dynamo effect depends on thequenched value of the turbulent magnetic diffusivity. Earlier estimates thatmight have suffered from the force-free degeneracy of Beltrami fields are nowconfirmed for shear flows where this degeneracy is lifted. For a dynamo that issaturated near equipartition field strength those estimates result in a 5-folddecrease of the magnetic diffusivity as the magnetic Reynolds number based onthe wavenumber of the energy-carrying eddies is increased from 2 to 600.Finally, the role of shear in driving turbulence and large-scale fields by themagneto-rotational instability is emphasized. New simulations are presented andthe 3pi-4 phase shift between poloidal and toroidal fields is confirmed. It issuggested that this phase shift might be a useful diagnostic tool inidentifying mean-field dynamo action in simulations and to distinguish thisfrom other scenarios invoking magnetic buoyancy as a means to explain migrationaway from the midplane.

Author: Axel Brandenburg


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