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Reversed Dynamo at Small Scales and Large Magnetic Prandtl Number
KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Stockholm Univ, Dept Astron, AlbaNova Univ Ctr, SE-10691 Stockholm, Sweden.;Univ Colorado, JILA, Boulder, CO 80303 USA.;Univ Colorado, Lab Atmospher & Space Phys, Boulder, CO 80303 USA.;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
NCAR, High Altitude Observ, POB 3000, Boulder, CO 80307 USA..
2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 879, no 1, article id 57Article in journal (Refereed) Published
Abstract [en]

We show that at large magnetic Prandtl numbers, the Lorentz force does work on the flow at small scales and drives fluid motions, whose energy is dissipated viscously. This situation is the opposite of that in a normal dynamo, where the flow does work against the Lorentz force. We compute the spectral conversion rates between kinetic and magnetic energies for several magnetic Prandtl numbers and show that normal (forward) dynamo action occurs on large scales over a progressively narrower range of wavenumbers as the magnetic Prandtl number is increased. At higher wavenumbers, reversed dynamo action occurs, i.e., magnetic energy is converted back into kinetic energy at small scales. We demonstrate this in both direct numerical simulations forced by volume stirring and in large eddy simulations (LESs) of solar convectively driven small-scale dynamos. Low-density plasmas such as stellar coronae tend to have large magnetic Prandtl numbers, i.e., the viscosity is large compared with the magnetic diffusivity. The regime in which viscous dissipation dominates over resistive dissipation for large magnetic Prandtl numbers was also previously found in LESs of the solar corona, i.e., our findings are a more fundamental property of MHD that is not just restricted to dynamos. Viscous energy dissipation is a consequence of positive Lorentz force work, which may partly correspond to particle acceleration in close-to-collisionless plasmas. This is, however, not modeled in the MHD approximation employed. By contrast, resistive energy dissipation on current sheets is expected to be unimportant in stellar coronae.

Place, publisher, year, edition, pages
Institute of Physics Publishing (IOPP), 2019. Vol. 879, no 1, article id 57
Keywords [en]
dynamo, hydrodynamics, magnetohydrodynamics (MHD), Sun: corona, turbulence
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-255394DOI: 10.3847/1538-4357/ab24bdISI: 000474384700002Scopus ID: 2-s2.0-85069517127OAI: oai:DiVA.org:kth-255394DiVA, id: diva2:1339391
Note

QC 20190729

Available from: 2019-07-29 Created: 2019-07-29 Last updated: 2019-10-04Bibliographically approved

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Brandenburg, Axel

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