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Astron. Astrophys. 359, 1205-1210 (2000)

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The dynamo effect of magnetic flux tubes

M.A.J.H. Ossendrijver

Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79108 Freiburg, Germany (mathieu@kis.uni-freiburg.de)

Received 16 November 1999 / Accepted 30 May 2000


It is shown that toroidal magnetic flux tubes in a rotating star do not provide a net dynamo effect, even if they are subject to random forcing, unless the tubes are unstable to small displacements. The dynamo effect is produced by transverse helical waves that propagate along the flux tube, resulting in an electric current (anti-) parallel to the unperturbed magnetic field, equivalent to an alpha effect . For unstable flux tubes, the alpha effect is enabled by a systematic phase difference between the velocity and magnetic field perturbations. For stable flux tubes subject to random forcing, phase differences do occur, but they vanish in the mean. The requirement for instability is met if the magnetic field strength exceeds a threshold value. Therefore a stellar dynamo based on flux tubes is not self-excited, but needs triggering until the magnetic threshold is surpassed.

Key words: Magnetohydrodynamics (MHD) – stars: magnetic fields – stars: late-type – Sun: magnetic fields

© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000