Magnetic field reversals in an experimental turbulent dynamo

Abstract

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.

Chaotic Dynamos Generated by a Turbulent Flow of Liquid Sodium

“…Recently, the VKS experiment [4] has shown a variety of dynamical behaviors. Depending on the amount of global rotation, both stationary and time dependent regimes, including field reversals [5], have been observed. We study here the secondary bifurcations between different dynamo regimes and we show that the dynamics can be understood as the result of a few competing modes in the vicinity of the dynamo threshold.…”

“…One of them consists in field reversals [5]. The three components of the magnetic field reverse at random intervals (see Fig.…”

Impact of time-dependent nonaxisymmetric velocity perturbations on dynamo action of von Kármán-like flows

“…In an idealizing model the mean axisymmetric flow between counter-rotating impellers comprises two toroidal and two poloidal eddies (so-called s2t2 topology), and it is well known that this flow is able to drive a dynamo [5,6]. Various attempts in different geometries have been made (numerically as well as experimentally) in order to examine dynamo action driven by such a flow [4,[7][8][9][10][11][12][13][14][15][16][17][18][19][20]. However, so far, experimental dynamo action driven by a von Kármán-like flow is obtained only at the VKS facility and only when at least one of the flow-driving impellers is made of soft iron with a large relative permeability.…”

A Low‐Order Solar Dynamo Model: Inferred Meridional Circulation Variations Since 1750

“…Priest (1984) showed that some of the dynamics of the solar dynamo can be explained in terms of dynamical systems analysis. Features, like magnetic field reversal, have also been recently observed in laboratory dynamo experiments ( Berhanu et al 2007;Dubrulle et al 2007) which indicate that, within certain approximations, some of these dynamics can be explained by low-order dynamo models (Mininni et al 2001;Pontieri et al 2003;WilmotSmith 2005).…”