Magnetic helicity effects in astrophysical and laboratory dynamos

TL;DR

This paper explores magnetic helicity effects in astrophysical and laboratory dynamos, analyzing boundary conditions, scale separation, and the role of helicity fluxes in dynamo saturation and quenching.

Contribution

It presents new insights into how boundary conditions and helicity fluxes influence dynamo action and saturation in both laboratory and astrophysical contexts.

Findings

No large-scale field in Taylor-Green flows without scale separation.

Localized magnetic eddies with swirl can generate larger-scale fields.

Non-local alpha effects can be catastrophically quenched without helicity fluxes.

Abstract

Magnetic helicity effects are discussed in laboratory and astrophysical settings. First, dynamo action in Taylor-Green flows is discussed for different boundary conditions. However, because of the lack of scale separation with respect to the container, no large scale field is being produced and there is no resistively slow saturation phase as otherwise expected. Second, the build-up of a large scale field is demonstrated in a simulation where a localized magnetic eddy produces field on a larger scale if the eddy possesses a swirl. Such a set-up might be realizable experimentally through coils. Finally, new emerging issues regarding the connection between magnetic helicity and the solar dynamo are discussed. It is demonstrated that dynamos with a non-local (Babcock-Leighton type) alpha effect can also be catastrophically quenched, unless there are magnetic helicity fluxes.