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Dynamo effect in decaying helical turbulence
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. University of Colorado, USA; Carnegie Mellon University, USA.
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Number of Authors: 62019 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 4, no 2, article id 024608Article in journal (Refereed) Published
Abstract [en]

We show that in decaying hydromagnetic turbulence with initial kinetic helicity, a weak magnetic field eventually becomes fully helical. The sign of magnetic helicity is opposite to that of the kinetic helicity-regardless of whether the initial magnetic field was helical. The magnetic field undergoes inverse cascading with the magnetic energy decaying approximately like t(-1/2). This is even slower than in the fully helical case, where it decays like t(-2/3). In this parameter range, the product of magnetic energy and correlation length raised to a certain power slightly larger than unity is approximately constant. This scaling of magnetic energy persists over long timescales. At very late times and for domain sizes large enough to accommodate the growing spatial scales, we expect a crossover to the t(-2/3) decay law that is commonly observed for fully helical magnetic fields. Regardless of the presence or absence of initial kinetic helicity, the magnetic field experiences exponential growth during the first few turnover times, which is suggestive of small-scale dynamo action. Our results have applications to a wide range of experimental dynamos and astrophysical time-dependent plasmas, including primordial turbulence in the early universe.

Place, publisher, year, edition, pages
2019. Vol. 4, no 2, article id 024608
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Physical Sciences
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URN: urn:nbn:se:su:diva-167672DOI: 10.1103/PhysRevFluids.4.024608ISI: 000459585000001OAI: oai:DiVA.org:su-167672DiVA, id: diva2:1302157
Available from: 2019-04-03 Created: 2019-04-03 Last updated: 2019-04-08Bibliographically approved

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Brandenburg, Axel
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