Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...359.1205o&link_type=abstract
Astronomy and Astrophysics, v.359, p.1205-1210 (2000)
Astronomy and Astrophysics
Astrophysics
10
Magnetohydrodynamics (Mhd), Stars: Magnetic Fields, Stars: Late-Type, Sun: Magnetic Fields
Scientific paper
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.
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