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

Department of Earth and Planetary Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, 113-0033 Tokyo, Japan
e-mail: hotta.h@eps.s.u-tokyo.ac.jp

Received: 26 July 2012
Accepted: 13 October 2012

Abstract

Using two-dimensional magnetohydrodynamics calculations, we investigate a twist generation 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 magnetic 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 reduces 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.