We report the observation of dynamo action in the VKS experiment, i.e., the generation of magnetic field by a strongly turbulent swirling flow of liquid sodium. Both mean and fluctuating parts of the field are studied. The dynamo threshold corresponds to a magnetic Reynolds number Rm ∼ 30. A mean magnetic field of order 40 G is observed 30 % above threshold at the flow lateral boundary. The rms fluctuations are larger than the corresponding mean value for two of the components. The scaling of the mean square magnetic field is compared to a prediction previously made for high Reynolds number flows.

PACS 91.25.Cw -Origins and models of the magnetic field; dynamo theories PACS 47.65.+a -Magnetohydrodynamics and electrohydrodynamicsAbstract. -We report the first experimental observation of reversals of a dynamo field generated in a laboratory experiment based on a turbulent flow of liquid sodium. The magnetic field randomly switches between two symmetric solutions B and −B. We observe a hierarchy of time scales similar to 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 excursions. Both coincide with minima of the mechanical power driving the flow. Small changes in the flow driving parameters also reveal a large variety of dynamo regimes.Dynamo action is the instability mechanism by which mechanical energy is partially converted into magnetic energy by the motion of an electrically conducting fluid [1]. It is believed to be at the origin of the magnetic fields of planets and most astrophysical objects. One of the most striking features of the Earth's dynamo, revealed by paleomagnetic studies [2], is the observation of irregular reversals of the polarity of its dipole field. This behaviour is allowed from the constitutive equations of magnetohydrodynamics [1] and has been observed in numerical models [3]. On the other hand, industrial dynamos routinely generate currents and magnetic fields from mechanical motions. In these devices, pioneered by Siemens [4], the path of the electrical currents and the geometry of the (solid) rotors are completely prescribed. As it cannot be the case for planets and stars, experiments aimed at studying dynamos in the laboratory have evolved towards relaxing these constraints. Solid rotor experiments [5] showed that a dynamo state could be reached with prescribed motions but currents free to self-organize. A landmark was reached in 2000, when the experiments in Riga [6] and Karlsruhe [7] showed that fluid dynamos could be generated by organizing favourable sodium flows, the electrical currents being again free to self-organize. For these experiments, the self-sustained dynamo fields had simple time dynamics (a steady field in Karlsruhe and an oscillatory field in Riga). No further dynamical evolution was observed. The search for more complex dynamics, such as exhibited by natural objects, has motivated most teams working on the dynamo problem to design experiments with less constrained flows and a higher level of turbulence [8]. The von Kármán sodium experiment (VKS) is one of them. It has recently shown regimes where a statistically stationary dynamo self-generates [9]. We report here the existence of other dynamical regimes and describe below the occurence of irregular reversals and excursions.