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Compressible Test-field Method and Its Application to Shear Dynamos
Aalto Univ, Dept Comp Sci, POB 15400, FI-00076 Aalto, Finland.;Max Planck Inst Solar Syst Res, Justus von Liebig Weg 3, D-37077 Gottingen, Germany.;Stockholm Univ, NORDITA, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden..
Aalto Univ, Dept Comp Sci, POB 15400, FI-00076 Aalto, Finland..
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden.;Stockholm Univ, Dept Astron, Oskar Klein Ctr, AlbaNova, SE-10691 Stockholm, Sweden.;Carnegie Mellon Univ, McWilliams Ctr Cosmol, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA..ORCID iD: 0000-0002-7304-021X
2022 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 932, no 1, p. 8-, article id 8Article in journal (Refereed) Published
Abstract [en]

In this study, we present a compressible test-field method (CTFM) for computing alpha-effect and turbulent magnetic diffusivity tensors, as well as those relevant for the mean ponderomotive force and mass source, applied to the full MHD equations. We describe the theoretical background of the method and compare it to the quasi-kinematic test-field method and to the previously studied variant working in simplified MHD (SMHD). We present several test cases using velocity and magnetic fields of the Roberts geometry and also compare with the imposed-field method. We show that, for moderate imposed-field strengths, the nonlinear CTFM (nCTFM) gives results in agreement with the imposed-field method. A comparison of different flavors of the nCTFM in the shear dynamo case also yields agreement up to equipartition field strengths. Some deviations between the CTFM and SMHD variants exist. As a relevant physical application, we study nonhelically forced shear flows, which exhibit large-scale dynamo action, and present a reanalysis of low-Reynolds-number, moderate shear systems, where we previously ignored the pressure gradient in the momentum equation and found no coherent shear-current effect. Another key difference is that in the earlier study we used magnetic forcing to mimic small-scale dynamo action, while here it is self-consistently driven by purely kinetic forcing. The kinematic CTFM with general validity forms the core of our analysis. We still find no coherent shear-current effect, but do recover strong large-scale dynamo action that, according to our analysis, is driven by incoherent effects.

Place, publisher, year, edition, pages
American Astronomical Society , 2022. Vol. 932, no 1, p. 8-, article id 8
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-314882DOI: 10.3847/1538-4357/ac5b78ISI: 000807756900001Scopus ID: 2-s2.0-85132736814OAI: oai:DiVA.org:kth-314882DiVA, id: diva2:1676805
Note

QC 20220627

Available from: 2022-06-27 Created: 2022-06-27 Last updated: 2023-03-22Bibliographically approved

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Brandenburg, Axel

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