Generation of a Magnetic Field by Dynamo Action in a Turbulent Flow of Liquid Sodium

Monchaux, Romain; Berhanu, Michaël; Bourgoin, Mickaël; Moulin, Marc; Odier, Philippe; Pinton, Jean‐François; Volk, Romain; Fauve, S.; Mordant, Nicolas; Pétrélis, François; Chiffaudel, Arnaud; Daviaud, François; Dubrulle, Bérengère; Gasquet, C.; Marié, Louis; Ravelet, Florent

doi:10.1103/physrevlett.98.044502

Cited by 392 publications

(355 citation statements)

References 37 publications

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“…So far, dynamo experiments based on a flow of a conducting fluid have been successfully conducted in Riga [2], Karlsruhe [3], and Cadarache [4]. The first two facilities made use of a more or less predetermined fluid flow essentially fixed by the forcing and the shape of the internal tubes.…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

Section: Introductionmentioning

confidence: 99%

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

2012

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We present numerical simulations of the kinematic induction equation in order to examine the dynamo efficiency of an axisymmetric von Kármán-like flow subject to time-dependent nonaxisymmetric velocity perturbations. The numerical model is based on the setup of the French von Kármán-sodium dynamo (VKS) and on the flow measurements from a water experiment conducted at the University of Navarra in Pamplona, Spain. The principal experimental observations that are modeled in our simulations are nonaxisymmetric vortexlike structures which perform an azimuthal drift motion in the equatorial plane. Our simulations show that the interactions of these periodic flow perturbations with the fundamental drift of the magnetic eigenmode (including the special case of nondrifting fields) essentially determine the temporal behavior of the dynamo state. We find two distinct regimes of dynamo action that depend on the (prescribed) drift frequency of an (m = 2) vortexlike flow perturbation. For comparatively slowly drifting vortices we observe a narrow window with enhanced growth rates and a drift of the magnetic eigenmode that is synchronized with the perturbation drift. The resonance-like enhancement of the growth rates takes place when the vortex drift frequency roughly equals the drift frequency of the magnetic eigenmode in the unperturbed system. Outside of this small window, the field generation is hampered compared to the unperturbed case, and the field amplitude of the magnetic eigenmode is modulated with approximately twice the vortex drift frequency. The abrupt transition between the resonant regime and the modulated regime is identified as a spectral exceptional point where eigenvalues (growth rates and frequencies) and eigenfunctions of two previously independent modes collapse. In the actual configuration the drift frequencies of the velocity perturbations that are observed in the water experiment are much larger than the fundamental drift frequency of the magnetic eigenmode that is obtained from our numerical simulations. Hence, we conclude that the fulfillment of the resonance condition might be unlikely in present day dynamo experiments. However, a possibility to increase the dynamo efficiency in the VKS experiment might be realized by an application of holes or fingers on the outer boundary in the equatorial plane. These mechanical distortions provoke an anchorage of the vortices at fixed positions thus allowing an adjustment of the temporal behavior of the nonaxisymmetric flow perturbations.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

2012

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“…The first attempts to isolate dynamo wave propagation from stellar activity data were undertaken in Berdyugina & Henry (2007) and Katsova et al (2010), and generation of oscillating magnetic field, probably indicating dynamo wave propagation, was obtained in the VKS dynamo experiment (Monchaux et al 2007). This opens new perspectives in the topic, in particular the opportunity to study dynamo waves as a physical, rather than a purely astronomical, phenomenon.…”

Section: Introductionmentioning

confidence: 99%

Resonances for activity waves in spherical mean field dynamos

Moss

Sokoloff

2013

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Context. It has been suggested that resonant phenomena may play a role in determining the properties of mean-field dynamos, such as operate in the Sun and other stars. Aims. We tried to identify resonances which are expected to occur when a mean-field dynamo excites waves of quasi-stationary magnetic field in two distinct spherical layers. Methods. We used a model with two physically separated dynamo active layers to study activity waves of the kind that determine cyclic magnetic activity of various stars, including the Sun, as a more general physical rather than a purely astronomical problem. Results. We isolated some features that can be associated with resonances in the profiles of energy or frequency plotted versus a dynamo governing parameter. Rather unexpectedly however the resonances in spherical dynamos take a much less spectacular form than resonances in many more familiar branches of physics. In particular, we found that the magnitudes of resonant phenomena are much smaller than seem detectable by astronomical observations, and that plausibly any related effects in laboratory dynamo experiments (which of course are not in gravitating spheres!) can also be inferred to be likely to be small. Conclusions. We discuss specific features relevant to resonant phenomena in spherical dynamos, and find parametric resonance to be the most pronounced type of resonance phenomena. Resonance conditions for these dynamo wave resonances are rather different from those found in more conventional branches of physics. We suggest that the relative insignificance of the phenomenon in this situation is because the phenomena of excitation and propagation of the activity waves are not well-separated from each other and this, together with the nonlinear nature of more-or-less realistic dynamos, suppresses the resonances and makes them much less pronounced than the resonant effects found, for example, in optics.

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“…The turbulent dynamo is important for the formation of the large-scale structure of the Universe (Ryu et al 2008;Miniati & Bell 2011;Iapichino & Brüggen 2012;Vazza et al 2014;Miniati & Beresnyak 2015;Beresnyak & Miniati 2016), in clusters of galaxies (Subramanian, Shukurov & Haugen 2006) and in the formation of the first cosmological objects in dark matter haloes . It determines the growth of magnetic energy in solar convection (Cattaneo & Hughes 2001;Pietarila Graham, Cameron & Schüssler 2010;Moll et al 2011), in the interior of planets (Roberts & Glatzmaier 2000) and is relevant for liquid metal experiments on Earth (Monchaux et al 2007). It may further explain the far-infrared-radio correlation in spiral galaxies (Schleicher & Beck 2013).…”

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confidence: 99%

Magnetic field amplification in turbulent astrophysical plasmas

2016

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Magnetic fields play an important role in astrophysical accretion discs and in the interstellar and intergalactic medium. They drive jets, suppress fragmentation in star-forming clouds and can have a significant impact on the accretion rate of stars. However, the exact amplification mechanisms of cosmic magnetic fields remain relatively poorly understood. Here, I start by reviewing recent advances in the numerical and theoretical modelling of the turbulent dynamo, which may explain the origin of galactic and intergalactic magnetic fields. While dynamo action was previously investigated in great detail for incompressible plasmas, I here place particular emphasis on highly compressible astrophysical plasmas, which are characterised by strong density fluctuations and shocks, such as the interstellar medium. I find that dynamo action works not only in subsonic plasmas, but also in highly supersonic, compressible plasmas, as well as for low and high magnetic Prandtl numbers. I further present new numerical simulations from which I determine the growth of the turbulent (un-ordered) magnetic field component (B turb ) in the presence of weak and strong guide fields (B 0 ). I vary B 0 over five orders of magnitude and find that the dependence of B turb on B 0 is relatively weak, and can be explained with a simple theoretical model in which the turbulence provides the energy to amplify B turb . Finally, I discuss some important implications of magnetic fields for the structure of accretion discs, the launching of jets and the star-formation rate of interstellar clouds.

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Generation of a Magnetic Field by Dynamo Action in a Turbulent Flow of Liquid Sodium

Cited by 392 publications

References 37 publications

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

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

Resonances for activity waves in spherical mean field dynamos

Magnetic field amplification in turbulent astrophysical plasmas

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