http://arxiv.org/abs/1903.11869
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. 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 opposite to 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. We demonstrate this in both direct numerical simulations forced by volume stirring and in large eddy simulations of solar convectively driven small-scale dynamos. We speculate that the magnetic to kinetic energy conversion at small scales may partly correspond to particle acceleration, which is, however, not modelled in the magnetohydrodynamic approximation employed. By contrast, energy dissipation on current sheets is expected to be unimportant in stellar coronae.
A. Brandenburg and M. Rempel
Fri, 29 Mar 19
47/78
Comments: 8 pages, 6 figures, 2 tables
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