  Using direct numerical simulations of the equations of magnetohydrodynamics,
we study reversals of the magnetic field generated by the flow of an
electrically conducting fluid in a sphere. We show that at low magnetic Prandtl
numbers, Pm=0.5, the decrease of magnetic energy, ohmic dissipation and power
of the Lorentz force during a reversal is followed by an increase of the power
injected by the force driving the flow and an increase of viscous dissipation.
Cross correlations show that the Lorentz energy flux is in advance with respect
to the other energy fluxes. We also observe that during a reversal, the maximum
of the magnetic energy density migrates from one hemisphere to the other and
comes back to its initial position, in agreement with recent experimental
observations. For larger magnetic Prandtl numbers (Pm= 1, 2), the magnetic
field reversals do not display these trends and strongly differ one from
another.
