École Nationale Supérieure de
Techniques Avancées

Abstract

The possibility of a dynamo in the radiative zone of the Sun is here investigated, according to the mecanism suggested by Spruit (2002) and completed by Zahn et al. (2007): the main difference with the classical dynamo theory is that the regeneration of the poloidal part of the field is caused by magnetic instabilities (Magneto-Rotational Instability or Tayler instability) instead of convection. A numerical set-up is created with the ASH code (Anelastic Spherical Harmonics, see Brun et al. (2004)). The initial differential rotation profile is cylindrical, either increasing to inside (MRI-unstable) or to outside (MRI-stable); it is maintained during the run. The initial magnetic field is a confined dipole to which is added random fluctuations. The stratified radiative zone studied belongs to a younger Sun than ours (to save calculation ressources). After the validation of the hydrodynamical model, magnetic field was added and parameters range to trigger the Tayler instability and the dynamo were found. Two parametrical studies were carried on the magnetic Reynolds number Rem and the drag time for differential rotation tau_Z. A threshold value was found for Rem to trigger the dynamo. tau_Z appears to be crucial for the regeneration of the field to happen: as the forcing goes stronger, the latter geos stronger along, and no forcing implies no regeneration. Moreover the regeneration seems to be unable to sustain itself on long times, though it is uncertain whether it is always true or only for our range of parameters. To conclude, it seems that a dynamo in a radiative zone is theoratically possible, but under a narrow range of parameters and by assuming there is a strong mecanism to enforce the differential rotation within the star, which is not very likely. Keywords: Sun - radiative zone - dynamo