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Journal Article

Non-linear galactic dynamos and the magnetic Rädler effect


Singh,  Nishant K.
Max Planck Research Group in Solar and Stellar Magnetic Activity, Max Planck Institute for Solar System Research, Max Planck Society;

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Chamandy, L., & Singh, N. K. (2018). Non-linear galactic dynamos and the magnetic Rädler effect. Monthly Notices of the Royal Astronomical Society, 481(1), 1300-1319. doi:10.1093/mnras/sty2301.

Cite as: https://hdl.handle.net/21.11116/0000-0003-BAAC-A
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. 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, but which depends on the strength of small-scale magnetic fluctuations. We briefly discuss the observational implications of the magnetic Rädler effect for galaxies.