Generation of twist on magnetic flux tubes at the base of the solar convection zone

TL;DR

This study uses magnetohydrodynamics simulations to explore how magnetic flux tubes at the solar convection zone's base can generate twist through vortex motions and magnetic instabilities, enabling them to rise without initial twist.

Contribution

It demonstrates a mechanism for twist generation in flux tubes via vortex motions and magnetic instabilities, allowing flux tubes to ascend without initial twist.

Findings

Flux tubes split into two parts with vortex motions.

Polarity field is amplified by magneto-rotational instability.

Flux tubes can rise without initial twist in weak poloidal fields.

Abstract

Using two-dimensional magnetohydrodynamics calculations, we investigate a twist gen- eration mechanism on a magnetic flux tube at the base of the solar convection zone based on the idea of Choudhuri, 2003, Sol. Phys., 215, 31 in which a toroidal mag- netic field is wrapped by a surrounding mean poloidal field. During generation of the twist, the flux tube follows four phases. (1) It quickly splits into two parts with vortex motions rolling up the poloidal magnetic field. (2) Owing to the physical mechanism similar to that of the magneto-rotational instability, the rolled-up poloidal field is bent and amplified. (3) The magnetic tension of the disturbed poloidal magnetic field re- duces the vorticity, and the lifting force caused by vortical motion decreases. (4) The flux tube gets twisted and begins to rise again without splitting. Investigation of these processes is significant because it shows that a flux tube without any initial twist can rise to the surface in relatively weak poloidal fields.