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Traces of large-scale dynamo action in the kinematic stage
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.ORCID iD: 0000-0002-7304-021X
2014 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 445, no 3, 2930-2940 p.Article in journal (Refereed) Published
Abstract [en]

Using direct numerical simulations (DNS), we verify that in the kinematic regime, a turbulent helical dynamo grows in such a way that the magnetic energy spectrum remains to high-precision shape-invariant, i.e. at each wavenumber k the spectrum grows with the same growth rate. Signatures of large-scale dynamo action can be identified through the excess of magnetic energy at small k, of one of the two oppositely polarized constituents. Also a suitably defined planar average of the magnetic field can be chosen such that its rms value isolates the strength of the mean field. However, these different means of analysis suggest that the strength of the large-scale field diminishes with increasing magnetic Reynolds number Re-M like Re(M)(-1/2)for intermediate values and like Re-M(-3/4) for larger ones. Both an analysis from the Kazantsev model including helicity and the DNS show that this arises due to the magnetic energy spectrum still peaking at resistive scales, even when helicity is present. As expected, the amplitude of the large-scale field increases with increasing fractional helicity, enabling us to determine the onset of large-scale dynamo action and distinguishing it from that of the small-scale dynamo. Our DNS show that, contrary to earlier results for smaller scale separation (only 1.5 instead of now 4), the small-scale dynamo can still be excited at magnetic Prandtl numbers of 0.1 and only moderate values of the magnetic Reynolds numbers (similar to 160).

Place, publisher, year, edition, pages
2014. Vol. 445, no 3, 2930-2940 p.
Keyword [en]
dynamo, magnetic fields, MHD-turbulence, Sun: magnetic fields, galaxies: magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-160074DOI: 10.1093/mnras/stu1954ISI: 000346962900063Scopus ID: 2-s2.0-84924599148OAI: oai:DiVA.org:kth-160074DiVA: diva2:791091
Funder
EU, European Research Council, 227952Swedish Research Council, 621-2011-5076Swedish Research Council, 2012-5797
Note

QC 20150225

Available from: 2015-02-26 Created: 2015-02-13 Last updated: 2017-12-04Bibliographically approved

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Brandenburg, Axel

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