  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, 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 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 large eddy simulations 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.
