[Todos] Coloquios del Departamento de Física

[pdmitruk en df.uba.ar]

---------------------------------------------------------------------------

                       COLOQUIOS DEL DEPARTAMENTO DE FÍSICA FCEyN - UBA

                                http://coloquios.df.uba.ar/

                                 Charla, café y galletitas
                       En el Aula Federman, 1er piso, Pabellón I,
                                     Ciudad Universitaria


                                   Jueves 18 de noviembre, 14hs,


                 The VKS experiment: geodynamo ported to laboratory scale


                                     Mickael Bourgoin


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




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.