Catastrophic alpha quenching alleviated by helicity flux and shear

TL;DR

This paper introduces a new simulation setup to study mean field dynamo action, showing that helicity flux and shear can mitigate catastrophic alpha quenching, with implications for solar magnetic field modeling.

Contribution

The study demonstrates that helicity flux and shear can alleviate alpha quenching in dynamo models, providing a more realistic simulation framework for solar-like magnetic activity.

Findings

Helicity flux reduces alpha quenching at moderate magnetic Reynolds numbers.

Closed boundary conditions lead to catastrophic alpha quenching.

Estimated solar helicity flux is about 0.001 G^2/s.

Abstract

A new simulation set-up is proposed for studying mean field dynamo action. The model combines the computational advantages of local cartesian geometry with the ability to include a shear profile that resembles the sun's differential rotation at low latitudes. It is shown that in a two-dimensional mean field model this geometry produces cyclic solutions with dynamo waves traveling away from the equator -- as expected for a positive alpha effect in the northern hemisphere. In three dimensions with turbulence driven by a helical forcing function, an alpha effect is self-consistently generated in the presence of a finite imposed toroidal magnetic field. The results suggest that, due to a finite flux of current helicity out of the domain, alpha quenching appears to be non-catastrophic -- at least for intermediate values of the magnetic Reynolds number. For larger values of the magnetic Reynolds number, however, there is evidence for a reversal of the trend and that $\alpha$ may decrease with increasing magnetic Reynolds number. Control experiments with closed boundaries confirm that in the absence of a current helicity flux, but with shear as before, alpha quenching is always catastrophic and alpha decreases inversely proportional to the magnetic Reynolds number. For solar parameters, our results suggest a current helicity flux of about 0.001 G^2/s. This corresponds to a magnetic helicity flux, integrated over the northern hemisphere and over the 11 year solar cycle, of about 10^{46}Mx^2.