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We show that at large magnetic Prandtl numbers, the Lorentz force does workon the flow at small scales and drives fluid motions, whose energy isdissipated viscously. This situation is opposite to that in a normal dynamo,where the flow does work against the Lorentz force. We compute the spectralconversion rates between kinetic and magnetic energies for several magneticPrandtl numbers and show that normal (forward) dynamo action occurs on largescales over a progressively narrower range of wavenumbers as the magneticPrandtl number is increased. At higher wavenumbers, reversed dynamo actionoccurs, i.e., magnetic energy is converted back into kinetic energy at smallscales. We demonstrate this in both direct numerical simulations forced byvolume stirring and in large eddy simulations of solar convectively drivensmall-scale dynamos. Low density plasmas such as stellar coronae tend to havelarge magnetic Prandtl numbers, i.e., the viscosity is large compared with themagnetic diffusivity. The regime in which viscous dissipation dominates overresistive dissipation for large magnetic Prandtl numbers was also previouslyfound in large eddy simulations of the solar corona, i.e., our findings are amore 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-collisionlessplasmas. This is, however, not modeled in the MHD approximation employed. Bycontrast, resistive energy dissipation on current sheets is expected to beunimportant in stellar coronae.

Reversed dynamo at small scales and large magnetic Prandtl number