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Dynamo effect in decaying helical turbulence
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.ORCID iD: 0000-0002-7304-021X
Carnegie Mellon Univ, McWilliams Ctr Cosmol, 5000 Forbes Ave, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, 5000 Forbes Ave, Pittsburgh, PA 15213 USA.;Laurentian Univ, Dept Phys, Ramsey Lake Rd, Sudbury, ON P3E 2C, Canada.;Ilia State Univ, Abastumani Astrophys Observ, 3-5 Cholokashvili St, Tbilisi 0194, Rep of Georgia..
Carnegie Mellon Univ, McWilliams Ctr Cosmol, 5000 Forbes Ave, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, 5000 Forbes Ave, Pittsburgh, PA 15213 USA..
Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA.;Univ Colorado, Dept Aerosp Engn Sci, Boulder, CO 80303 USA..
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2019 (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
American Physical Society, 2019. Vol. 4, no 2, article id 024608
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Physical Sciences
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URN: urn:nbn:se:kth:diva-245908DOI: 10.1103/PhysRevFluids.4.024608ISI: 000459585000001Scopus ID: 2-s2.0-85062426577OAI: oai:DiVA.org:kth-245908DiVA, id: diva2:1296029
Note

QC 20190313

Available from: 2019-03-13 Created: 2019-03-13 Last updated: 2019-04-04Bibliographically approved

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