Reversals of the magnetic field generated
by a turbulent flow
S. Fauve, E. Dormy, C. Gissinger and F. Pétrélis
Ecole Normale Supérieure, LPS - CNRS, 24 Rue Lhomond, 75005 Paris,
France.
Advances in Turbulence XII
Proceedings of the 12th EUROMECH European Turbulence Conference (ETC12).
Abstract.
When averaged on a few thousands years, the Earth's magnetic field can be
roughly described as the one of an axial dipole. As shown by paleomagnetic
records, it has frequently reversed its polarity on geological time
scales. Field reversals have also been reported in several numerical
simulations of the geodynamo and more recently, in a laboratory experiment
involving a von Karman swirling flow of liquid sodium (VKS). We first
recall some of the main experimental results and understand them using
phenomenological models and numerical simulations. In particular, we show
that all the regimes of the magnetic field observed in the experiments
reported so far, can be understood in the framework of low dimensional
dynamical system theory: two modes of the magnetic field, with dipolar
(respectively quadrupolar) symmetry, can be generated by the turbulent flow
of liquid sodium, and their interaction can lead to the observed dynamics
(oscillations, random reversals, symmetric or asymmetric bursts). Turbulent
fluctuations alone do not drive these dynamical regimes that only occur
when a symmetry of the flow is broken. Although the flow in the Earth's
core strongly differs from the one of the VKS experiment, a similar model
but based on a different broken symmetry, can be used. It explains several
features observed in paleomagnetic recordings of the Earth's magnetic field
reversals.