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A unified large/small-scale dynamo in helical turbulence
Stockholms universitet, Naturvetenskapliga fakulteten, Institutionen för astronomi. Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita). University of Colorado, USA.ORCID-id: 0000-0002-7304-021X
Antal upphovsmän: 32016 (Engelska)Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 461, nr 1, s. 240-247Artikel i tidskrift (Refereegranskat) Published
Abstract [en]

We use high resolution direct numerical simulations (DNS) to show that helical turbulence can generate significant large-scale fields even in the presence of strong small-scale dynamo action. During the kinematic stage, the unified large/ small-scale dynamo grows fields with a shapeinvariant eigenfunction, with most power peaked at small scales or large k, as in Subramanian & Brandenburg. Nevertheless, the large-scale field can be clearly detected as an excess power at small k in the negatively polarized component of the energy spectrum for a forcing with positively polarized waves. Its strength (B) over bar, relative to the total rms field Brms, decreases with increasing magnetic Reynolds number, Re-M. However, as the Lorentz force becomes important, the field generated by the unified dynamo orders itself by saturating on successively larger scales. The magnetic integral scale for the positively polarized waves, characterizing the smallscale field, increases significantly from the kinematic stage to saturation. This implies that the small-scale field becomes as coherent as possible for a given forcing scale, which averts the Re-M-dependent quenching of (B) over bar /B-rms. These results are obtained for 1024(3) DNS with magnetic Prandtl numbers of PrM = 0.1 and 10. For PrM = 0.1, B/ Brms grows from about 0.04 to about 0.4 at saturation, aided in the final stages by helicity dissipation. For Pr-M = 10, (B) over bar /B-rms grows from much less than 0.01 to values of the order the 0.2. Our results confirm that there is a unified large/ small-scale dynamo in helical turbulence.

Ort, förlag, år, upplaga, sidor
2016. Vol. 461, nr 1, s. 240-247
Nyckelord [en]
dynamo, magnetic fields, MHD, turbulence, Sun: general, galaxies: magnetic fields
Nationell ämneskategori
Astronomi, astrofysik och kosmologi
Identifikatorer
URN: urn:nbn:se:su:diva-135047DOI: 10.1093/mnras/stw1257ISI: 000383272500021OAI: oai:DiVA.org:su-135047DiVA, id: diva2:1043995
Tillgänglig från: 2016-11-01 Skapad: 2016-10-31 Senast uppdaterad: 2022-02-28Bibliografiskt granskad

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Brandenburg, Axel

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Institutionen för astronomiNordiska institutet för teoretisk fysik (Nordita)
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Monthly notices of the Royal Astronomical Society
Astronomi, astrofysik och kosmologi

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