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Kinetic helicity needed to drive large-scale dynamos
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.ORCID iD: 0000-0002-7304-021X
2013 (English)In: Physical Review E. Statistical, Nonlinear, and Soft Matter Physics, ISSN 1539-3755, E-ISSN 1550-2376, Vol. 87, no 4, 043104- p.Article in journal (Refereed) Published
Abstract [en]

Magnetic field generation on scales that are large compared with the scale of the turbulent eddies is known to be possible via the so-called a effect when the turbulence is helical and if the domain is large enough for the a effect to dominate over turbulent diffusion. Using three-dimensional turbulence simulations, we show that the energy of the resulting mean magnetic field of the saturated state increases linearly with the product of normalized helicity and the ratio of domain scale to eddy scale, provided this product exceeds a critical value of around unity. This implies that large-scale dynamo action commences when the normalized helicity is larger than the inverse scale ratio. Our results show that the emergence of small-scale dynamo action does not have any noticeable effect on the large-scale dynamo. Recent findings by Pietarila Graham et al. [Phys. Rev. E 85, 066406 (2012)] of a smaller minimal helicity may be an artifact due to the onset of small-scale dynamo action at large magnetic Reynolds numbers. However, the onset of large-scale dynamo action is difficult to establish when the kinetic helicity is small. Instead of random forcing, they used an ABC flow with time-dependent phases. We show that such dynamos saturate prematurely in a way that is reminiscent of inhomogeneous dynamos with internal magnetic helicity fluxes. Furthermore, even for very low fractional helicities, such dynamos display large-scale fields that change direction, which is uncharacteristic of turbulent dynamos.

Place, publisher, year, edition, pages
2013. Vol. 87, no 4, 043104- p.
Keyword [en]
Mean-Field Electrodynamics, Magnetic Prandtl Numbers, Alpha-Omega-Dynamo, Turbulent Dynamos, Inverse Cascade, Abc Flows, Simulations, Diffusivity, Shear, Nonlinearity
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-122326DOI: 10.1103/PhysRevE.87.043104ISI: 000317591700009Scopus ID: 2-s2.0-84876929431OAI: oai:DiVA.org:kth-122326DiVA: diva2:622510
Funder
EU, European Research Council, 227952Swedish Research Council, 621-2007-4064
Note

QC 20130522

Available from: 2013-05-22 Created: 2013-05-20 Last updated: 2017-12-06Bibliographically approved

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Brandenburg, Axel

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