The role of diffusivity quenching in flux-transport dynamo models

TL;DR

This paper investigates how suppressing turbulent magnetic diffusivity (eta-quenching) affects flux-transport solar dynamo models, revealing complex, non-linear, and spatially varied impacts on magnetic field amplification and cycle behavior.

Contribution

It is the first detailed study of eta-quenching effects in flux-transport dynamo models, highlighting their non-linear and spatially non-uniform influence on magnetic field evolution.

Findings

Magnetic field can be locally amplified up to 2.5 times at the tachocline.

Eta-quenching leads to unobservable effects and worsens magnetic field distribution.

Large eta-quenching can cause a transition to unsteady, segregation-dominated dynamo regimes.

Abstract

In the non-linear phase of a dynamo process, the back-reaction of the magnetic field upon the turbulent motion results in a decrease of the turbulence level and therefore in a suppression of both the magnetic field amplification (the alpha-quenching effect) and the turbulent magnetic diffusivity (the eta-quenching effect). While the former has been widely explored, the effects of eta-quenching in the magnetic field evolution have rarely been considered. In this work we investigate the role of the suppression of diffusivity in a flux-transport solar dynamo model that also includes a non-linear alpha-quenching term. Our results indicate that, although for alpha-quenching the dependence of the magnetic field amplification with the quenching factor is nearly linear, the magnetic field response to eta-quenching is non-linear and spatially non-uniform. We have found that the magnetic field can be locally amplified in this case, forming long-lived structures whose maximum amplitude can be up to ~2.5 times larger at the tachocline and up to ~2 times larger at the center of the convection zone than in models without quenching. However, this amplification leads to unobservable effects and to a worse distribution of the magnetic field in the butterfly diagram. Since the dynamo cycle period increases when the efficiency of the quenching increases, we have also explored whether the eta-quenching can cause a diffusion-dominated model to drift into an advection-dominated regime. We have found that models undergoing a large suppression of the diffusivity produce a strong segregation of magnetic fields that may lead to unsteady dynamo-oscillations. On the other hand, an initially diffusion-dominated model undergoing a small suppression in eta remains in the diffusion-dominated regime.