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Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Physics.
2014 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 791, no 1, 12- p.Article in journal (Refereed) Published
Abstract [en]

Using direct numerical simulations of three-dimensional hydromagnetic turbulence, either with helical or non-helical forcing, we show that the kinetic-to-magnetic energy dissipation ratio always increases with the magnetic Prandtl number, i.e., the ratio of kinematic viscosity to magnetic diffusivity. This dependence can always be approximated by a power law, but the exponent is not the same in all cases. For non-helical turbulence, the exponent is around 1/3, while for helical turbulence it is between 0.6 and 2/3. In the statistically steady state, the rate of energy conversion from kinetic into magnetic by the dynamo must be equal to the Joule dissipation rate. We emphasize that for both small-scale and large-scale dynamos, the efficiency of the energy conversion depends sensitively on the magnetic Prandtl number, and thus on the microphysical dissipation process. To understand this behavior, we also study shell models of turbulence and one-dimensional passive and active scalar models. We conclude that the magnetic Prandtl number dependence is qualitatively best reproduced in the one-dimensional model as a result of dissipation via localized Alfven kinks.

Place, publisher, year, edition, pages
2014. Vol. 791, no 1, 12- p.
Keyword [en]
accretion, accretion disks, hydrodynamics, magnetohydrodynamics (MHD), shock waves turbulence
National Category
Astronomy, Astrophysics and Cosmology
URN: urn:nbn:se:su:diva-107102DOI: 10.1088/0004-637X/791/1/12ISI: 000339657700012OAI: diva2:743911


Available from: 2014-09-05 Created: 2014-09-03 Last updated: 2014-09-05Bibliographically approved

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Brandenburg, Axel
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