Mickael Bourgoin

Laboratoire des Ecoulements Geophysiques et Industriels - Grenoble, CNRS & ENS-Lyon, France

Jueves 18/11/2010, 14 hs
Aula Federman, 1er piso, Pabellón I 

Dynamo action is the instability mechanism by which mechanical energy is partially converted into magnetic energy by the motion of an electrically conducting fluid. It is believed to be at the origin of the magnetic fields of planets and most astrophysical objects. Concerning the Earth, the most striking feature revealed by paleomagnetic studies, is the observation of irregular reversals of the polarity of its dipole field. This behaviour is allowed from the constitutive equations of magnetohydrodynamics and has been observed in numerical models. We report the observation of dynamo action in the von Karman sodium (VKS) experiment where the generation of a magnetic field is achieved by a strongly turbulent swirling flow of liquid sodium stirred with iron propellers. The VKS experiment has also given the first experimental observation of reversals of a dynamo field in a laboratory experiment. The self-sustained magnetic field randomly switches between two symmetric solutions B and (-B). We observe a hierarchy of time scales similar to that displayed by the Earth¿s magnetic field: the duration of the steady phases is widely distributed, but is always much longer than the time needed to switch polarity. In addition to reversals, we report events called excursions in the geophysical literature: the magnetic field starts to decrease as if about to change its polarity but returns to its initial value instead. Finally, we have found that the versatility of our experimental setup leads to a variety of dynamo regimes, some with geomagnetic similarities, as described above, but others which may be relevant for a wider range of astrophysical objects. From the point of view of physics, further analysis of the parameter space will help understand bifurcations and non-linear dynamics emerging from turbulent states.