Astronomy and Astrophysics – Astrophysics
Scientific paper
2007-05-03
Astron.Astrophys.479:L33-L36,2008
Astronomy and Astrophysics
Astrophysics
accepted for publication in A&A
Scientific paper
10.1051/0004-6361:20077781
We investigate in isothermal MHD simulations the instability of toroidal magnetic fields resulting by the action of z-dependent differential rotation on a given axial field B^0 in a cylindrical enclosure where in particular the helicity of the resulting nonaxisymmetric flow is of interest. The idea is probed that helicity H is related to the external field and the differential rotation as H ~ B^0_i B^0_j Omega_i,j. The observed instability leads to a nonaxisymmetric solution with dominating mode m=1. With the onset of instability both kinematic and current helicity are produced which fulfill the suggested relation. Obviously, differential rotation dOmega/dz only needs an external axial field B^0_z to produce remarkable amounts of the helicities. Any regular time-dependency of the helicity could not be found. The resulting axial alpha-effect is mainly due to the current helicity, the characteristic time scale between both the values is of order of the rotation time. If the axial field is switched off then the helicity and the alpha-effect disappear.
Elstner Detlef
Gellert Marcus
Rudiger Günther
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