Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers

Schekochihin, A. A.; Iskakov, Alexey B.; Cowley, S. C.; McWilliams, James C.; Proctor, M. R. E.; Yousef, T. A.

doi:10.1088/1367-2630/9/8/300

Cited by 180 publications

(265 citation statements)

References 63 publications

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“…Resistive diffusion of the magnetic field is increasingly important on smaller scales, although our measured increase in the magnetic field shows that it is not dominant at the scales of interest. Indeed, the measured k-spectrum of the magnetic field is consistent with the Golitsyn (k −11/3 ) powerlaw dependence 24,25 , characteristic of magnetic fluctuations at low Pm (refs 25-27; Fig. 4).…”

supporting

confidence: 76%

“…As the mean flow velocity is larger than the (subsonic) velocity fluctuations excited by the grid, then, according to Taylor's hypothesis, we obtain ω ∼ v 0 k, where k is the wavenumber of the magnetic fluctuations. As a result of the proportionality relation between frequency and wavenumber, the magnetic energy spectrum thus exhibits the expected Golitsyn (k −11/3 ) power-law dependence [25][26][27] .…”

mentioning

confidence: 86%

“…The timescale for the resistivity-chaotic-tangling balance to be established at those scales is η/ 2 e , where e is of order 1 mm (which is the induction coil size, and therefore the measurement resolution limit). The fact that the Golitsyn spectrum extends at least to these scales is an indirect confirmation that turbulence is established 25 , with inertialrange motions tangling the magnetic field much faster than the motions at the energy-conserving scale (L e ). At scales of order e , the corresponding timescale is 50 ns in the laboratory, translating to 50 yr in the SNR over distances ∼0.25 pc.…”

mentioning

confidence: 92%

“…This suggests that turbulent motions are present and amplifying the field at subresistive scales. With a small magnetic Reynolds number, the amplification is due to the stochastic tangling of an imposed field, B 0 , by turbulent motions, and the saturated level is set by balancing this effect with Ohmic diffusion [25][26][27] . In our experiment, B 0 is the Biermann-batterygenerated baroclinic magnetic field in the homogeneous flow.…”

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confidence: 99%

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Turbulent amplification of magnetic fields in laboratory laser-produced shock waves

et al. 2014

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X-ray 1-3 and radio 4-6 observations of the supernova remnant Cassiopeia A reveal the presence of magnetic fields about 100 times stronger than those in the surrounding interstellar medium. Field coincident with the outer shock probably arises through a nonlinear feedback process involving cosmic rays 2,7,8 . The origin of the large magnetic field in the interior of the remnant is less clear but it is presumably stretched and amplified by turbulent motions. Turbulence may be generated by hydrodynamic instability at the contact discontinuity between the supernova ejecta and the circumstellar gas 9 . However, optical observations of Cassiopeia A indicate that the ejecta are interacting with a highly inhomogeneous, dense circumstellar cloud bank formed before the supernova explosion 10-12 . Here we investigate the possibility that turbulent amplification is induced when the outer shock overtakes dense clumps in the ambient medium 13-15 . We report laboratory experiments that indicate the magnetic field is amplified when the shock interacts with a plastic grid. We show that our experimental results can explain the observed synchrotron emission in the interior of the remnant. The experiment also provides a laboratory example of magnetic field amplification by turbulence in plasmas, a physical process thought to occur in many astrophysical phenomena.High-resolution X-ray images and radio polarization maps of Cassiopeia A show two distinct strong magnetic field regions [3][4][5][6]12 . Narrow X-ray filaments, a fraction of a parsec in width, are observed at the outer shock rim at a radius of about 2.5 pc. These structures are produced by synchrotron radiation from ultrarelativistic electrons (with teraelectronvolt energy) and can be explained by magnetic fields of the order of 100 µG or more 2,3 . The interior of the remnant contains a disordered shell (about 0.5 pc in width at a radius of 1.7 pc) of radio synchrotron emission by gigaelectronvolt electrons 4 . The inferred magnetic field in these radio knots is a few milligauss, about 100 times higher than expected from the standard shock compression of the interstellar medium 15 . Optical observations of Cassiopeia A show the presence of both rapidly moving (5,000-9,000 km s −1 ) and essentially stationary dense knots. Although the moving knots themselves have a high velocity, their overall pattern is nearly stationary 10 . This led to the suggestion 10 that a dense pre-existing inhomogeneous stationary cloud bank could be present. New rapidly moving knots predominantly appear at a position broadly coincident with the shell of bright radio emission 6 . Sizes of the observed small-scale features within the shell range from 0.01 to 0.1 pc arranged in larger patterns extending to 0.5-2 pc (ref. 16). Interaction between the ejecta and the cloud bank may excite the turbulence that amplifies the magnetic field and makes Cassiopeia A an exceptionally bright radio source 4 . The interaction is akin to the Rayleigh-Taylor instability otherwise proposed as a source of turbulenc...

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supporting

confidence: 76%

mentioning

confidence: 86%

mentioning

confidence: 92%

mentioning

confidence: 99%

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Turbulent amplification of magnetic fields in laboratory laser-produced shock waves

et al. 2014

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“…We emphasize that there are both observational (e.g., Trujillo Bueno et al 2004;Ishikawa & Tsuneta 2009;Danilovic et al 2010;Lites 2011;Shchukina & Trujillo Bueno 2011;Buehler et al 2013) and theoretical (Petrovay & Szakaly 1993;Cattaneo 1999;Schekochihin et al 2007;Vögler & Schüssler 2007;Pietarila Graham et al 2010;Brandenburg 2011;Rempel 2014, and references therein) hints that small-scale dynamo action plays a significant role in the small-scale magnetic activity of the quiet solar photosphere, especially concerning the tangled magnetic fields that Trujillo Bueno et al (2004) diagnosed via the Hanle effect (e.g., Stenflo 2012).…”

Section: Introductionmentioning

confidence: 92%

The impact of surface dynamo magnetic fields on the solar iron abundance

Shchukina

Bueno

2015

A&A

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Most chemical abundance determinations ignore that the solar photosphere is significantly magnetized by the ubiquitous presence of a small-scale magnetic field. A recent investigation has suggested that there should be a significant impact on the derived iron abundance, owing to the magnetically induced changes on the photospheric temperature and density structure (indirect effect). The three-dimensional (3D) photospheric models used in that investigation have non-zero net magnetic flux values and stem from magnetoconvection simulations without small-scale dynamo action. Here we address the same problem by instead using 3D models of the quiet solar photosphere that result from a state-of-the-art magneto-convection simulation with small-scale dynamo action, where the net magnetic flux is zero. One of these 3D models has negligible magnetization, while the other is characterized by a mean field strength of 160 Gauss in the low photosphere. With such 3D models we carried out spectral synthesis for a large set of Fe i lines to derive abundance corrections, taking the above-mentioned indirect effect and the Zeeman broadening of the intensity profiles (direct effect) into account. We conclude that if the magnetism of the quiet solar photosphere is mainly produced by a small-scale dynamo, then its impact on the determination of the solar iron abundance is negligible.

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Subcritical dynamos in shear flows

Rincon¹,

Ogilvie²,

Proctor³

et al. 2008

Astron Nachr

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Identifying generic physical mechanisms responsible for the generation of magnetic fields and turbulence in differentially rotating flows is fundamental to understand the dynamics of astrophysical objects such as accretion disks and stars. In this paper, we discuss the concept of subcritical dynamo action and its hydrodynamic analogue exemplified by the process of nonlinear transition to turbulence in non-rotating wall-bounded shear flows. To illustrate this idea, we describe some recent results on nonlinear hydrodynamic transition to turbulence and nonlinear dynamo action in rotating shear flows pertaining to the problem of turbulent angular momentum transport in accretion disks. We argue that this concept is very generic and should be applicable to many astrophysical problems involving a shear flow and non-axisymmetric instabilities of shear-induced axisymmetric toroidal velocity or magnetic fields, such as Kelvin-Helmholtz, magnetorotational, Tayler or global magnetoshear instabilities. In the light of several recent numerical results, we finally suggest that, similarly to a standard linear instability, subcritical MHD dynamo processes in high-Reynolds number shear flows could act as a large-scale driving mechanism of turbulent flows that would in turn generate an independent small-scale dynamo.Comment: 12 pages. Submitted to Astronomische Nachrichten (2007 Catania MHD Workshop special issue

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Fluctuation dynamo and turbulent induction at low magnetic Prandtl numbers

Cited by 180 publications

References 63 publications

Turbulent amplification of magnetic fields in laboratory laser-produced shock waves

Turbulent amplification of magnetic fields in laboratory laser-produced shock waves

The impact of surface dynamo magnetic fields on the solar iron abundance

Subcritical dynamos in shear flows

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