A mean field dynamo from negative eddy diffusivity
2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 432, no 2, 1651-1657 p.Article in journal (Refereed) Published
Using direct numerical simulations, we verify that Roberts-IV flow exhibits dynamo action dominated by horizontally averaged large-scale magnetic field. With the test-field method, we compute the turbulent magnetic diffusivity and find that it is negative and overcomes the molecular diffusivity, thus explaining quantitatively the large-scale dynamo for magnetic Reynolds numbers above approximate to 8. As expected for a dynamo of this type, but contrary to alpha-effect dynamos, the two horizontal field components grow independently of each other and have arbitrary amplitude ratios and phase differences. Small length-scales of the mean magnetic field are shown to be stabilized by the turbulent magnetic diffusivity becoming positive at larger wavenumbers. Oscillatory decaying or growing solutions have also been found in certain wavenumber intervals and sufficiently large values of the magnetic Reynolds number. For magnetic Reynolds numbers below approximate to 0.5, the turbulent magnetic diffusivity is confirmed to be positive, as expected for all incompressible flows. Earlier claims of a dynamo driven by a modified Taylor-Green flow through negative eddy diffusivity could not be confirmed.
Place, publisher, year, edition, pages
2013. Vol. 432, no 2, 1651-1657 p.
dynamo, magnetic fields, MHD, turbulence
Astronomy, Astrophysics and Cosmology
IdentifiersURN: urn:nbn:se:su:diva-92927DOI: 10.1093/mnras/stt590ISI: 000319832300062OAI: oai:DiVA.org:su-92927DiVA: diva2:644319
FunderEU, European Research Council, 227952