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arXiv · 1805.05548

Non-linear Galactic Dynamos and the Magnetic Rädler Effect

Luke Chamandy·Nishant K. Singh
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Abstract

We show that the magnetic analogue of the Rädler effect of mean-field dynamo theory leads to a non-linear backreaction that quenches a large-scale galactic dynamo, and can result in saturation of the large-scale magnetic field at near-equipartition with turbulent kinetic energy density. In a rotating fluid containing small-scale magnetic fluctuations, anisotropic terms in the mean electromotive force are induced via the Coriolis effect and these terms lead to a reduction of the growth rate in a predominantly $αΩ$-type galactic dynamo (Chamandy & Singh 2017). By including the generation of small-scale magnetic fluctuations by turbulent tangling of the large-scale magnetic field, one obtains a negative feedback effect that quenches the dynamo and leads to the saturation of the large-scale field. This saturation mechanism is found to be competitive with the dynamical $α$-quenching mechanism for realistic galactic parameter values. Furthermore, in the context of the dynamical $α$-quenching model, a separate non-linear term is obtained which has the same form as the helicity flux term of Vishniac & Cho (2001), but which depends on the strength of small-scale magnetic fluctuations. We briefly discuss the observational implications of the magnetic Rädler effect for galaxies.

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Luke Chamandy, Nishant K. Singh. 2018-08-24. Non-linear Galactic Dynamos and the Magnetic Rädler Effect. https://doi.org/10.1093/mnras%2Fsty2301

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