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Nordita Astrophysics seminars

Magnetic Prandtl number dependence of kinetic to magnetic dissipation ratio

by Axel Brandenburg (Nordita)

122:026 ()


Using direct numerical simulations of three-dimensional hydromagnetic turbulence, either with helical or non-helical forcing, we show that the ratio of kinetic to magnetic energy dissipation always increases with 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 at large magnetic Prandtl numbers, the exponent is around 1/3, while for all other cases it is between 0.6 and 2/3. Characterizing the dynamo efficiency by the magnetic energy dissipation, we emphasize that our results imply a sensitivity of both small-scale and large-scale dynamos on the microphysical dissipation process. To understand this behavior, we also study shell models of turbulence and one-dimensional passive scalar 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.