Magneto-Shear Instabilities in the Solar Tachocline
Abstract
Previously, linear theory has demonstrated that toroidal magnetic fields in the solar tachocline are destabilized by the presence of latitudinal differential rotation. For strong fields, such that the magnetic energy is comparable to the kinetic energy of the differential rotation, the most unstable modes are those with longitudinal wavenumber m=1 and these have been referred to as clam-shell or tipping instabilities for broad and banded toroidal field profiles respectively. Here we present nonlinear, three-dimensional simulations of these instabilities under both freely-evolving and forced conditions. Although the instabilities are allowed to have an arbitrary vertical structure, the dynamics remain quasi-2D, proceeding on horizontal surfaces which are nearly decoupled. The clam-shell instability saturates by opening up completely until horizontal loops of field become perpendicular to the equatorial plane. By contrast, the tipping instability saturates at relatively moderate tipping angles of 6-12 degrees by forming a jet of fluid along the axis of the tipped band. When the rotational shear and the magnetic field are maintained indefinitely by external forcing, clam-shell instabilities can operate continually, exhibiting a quasi-periodic behavior as mean toroidal fields alternately build up and destabilize.
- Publication:
-
American Astronomical Society Meeting Abstracts #210
- Pub Date:
- May 2007
- Bibcode:
- 2007AAS...210.4607M