Shear-driven magnetic buoyancy in the solar tachocline: The mean electromotive force due to rotation

TL;DR

This paper investigates how magnetic buoyancy instability in a shear and rotation environment within the solar tachocline can generate a mean electromotive force, potentially contributing to the Sun's magnetic cycle without relying solely on traditional convective processes.

Contribution

It introduces a local, fully compressible model demonstrating magnetic buoyancy as an alternative mechanism for poloidal field regeneration in the solar dynamo.

Findings

Magnetic buoyancy can produce a mean EMF aligned with the magnetic field.

Rapid rotation enhances the alpha-effect in the EMF.

Magnetic buoyancy may directly contribute to large-scale poloidal field generation.

Abstract

The leading theoretical paradigm for the Sun's magnetic cycle is an $\alpha\omega$-dynamo process, in which a combination of differential rotation and turbulent, helical flows produces a large-scale magnetic field that reverses every 11 years. Most $\alpha\omega$ solar dynamo models rely on differential rotation in the solar tachocline to generate a strong toroidal field. The most problematic part of such models is then the production of the large-scale poloidal field, via a process known as the $\alpha$-effect. Whilst this is usually attributed to small-scale convective motions under the influence of rotation, the efficiency of this regenerative process has been called into question by some numerical simulations. Motivated by likely conditions within the tachocline, the aim of this paper is to investigate an alternative mechanism for the poloidal field regeneration, namely the magnetic buoyancy instability in a shear-generated, rotating magnetic layer. We use a local, fully compressible model in which an imposed vertical shear winds up an initially vertical magnetic field. The field ultimately becomes buoyantly unstable, and we measure the resulting mean electromotive force (EMF). For sufficiently rapid rotation, we find that a significant component of the mean EMF is aligned with the direction of the mean magnetic field, which is the characteristic feature of the classical $\alpha\omega$-dynamo model. Our results therefore suggest that magnetic buoyancy could contribute directly to the generation of large-scale poloidal field in the Sun.