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Title:
Shear-driven magnetic buoyancy oscillations
Authors:
Vermersch, V.; Brandenburg, A.
Affiliation:
AA(Centre de Physique Théorique, École Polytechnique, F-91128 Palaiseau cedex, France; NORDITA, AlbaNova University Center, Roslagstullsbacken 23, SE 10691 Stockholm, Sweden), AB(NORDITA, AlbaNova University Center, Roslagstullsbacken 23, SE 10691 Stockholm, Sweden; Department of Astronomy, AlbaNova University Center, Stockholm University, SE 10691 Stockholm, Sweden)
Publication:
Astronomische Nachrichten, Vol.330, Issue 8, p.797 (AN Homepage)
Publication Date:
08/2009
Origin:
AN
Keywords:
instabilities, magnetohydrodynamics (MHD), turbulence
DOI:
10.1002/asna.200911242
Bibliographic Code:
2009AN....330..797V

Abstract

The effects of uniform horizontal shear on a stably stratified layer of gas is studied. The system is initially destabilized by a magnetically buoyant flux tube pointing in the cross-stream direction. The shear amplifies the initial field to Lundquist numbers of about 200-400, but then its value drops to about 100-300, depending on the value of the sub-adiabatic gradient. The larger values correspond to cases where the stratification is strongly stable and nearly isothermal. At the end of the runs the magnetic field is nearly axisymmetric, i.e. uniform in the streamwise direction. In view of Cowling's theorem the sustainment of the field remains a puzzle and may be due to subtle numerical effects that have not yet been identified in detail. In the final state the strength of the magnetic field decreases with height in such a way that the field is expected to be unstable. Low amplitude oscillations are seen in the vertical velocity even at late times, suggesting that they might be persistent.
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