  The magnetohydrodynamic dynamo effect explains the generation of
self-sustained magnetic fields in electrically conducting flows, especially in
geo- and astrophysical environments. Yet the details of this mechanism are
still unknown, e.g., how and to which extent the geometry, the fluid topology,
the forcing mechanism and the turbulence can have a negative effect on this
process. We report on numerical simulations carried out in spherical geometry,
analyzing the predicted velocity flow with the so-called Singular Value
Decomposition, a powerful technique that allows us to precisely identify
vortexes in the flow which would be difficult to characterize with conventional
spectral methods. We then quantify the contribution of these vortexes to the
growth rate of the magnetic energy in the system. We identify an axisymmetric
vortex, whose rotational direction changes periodically in time, and whose
dynamics are decoupled from those of the large scale background flow, is
detrimental for the dynamo effect. A comparison with experiments is carried
out, showing that similar dynamics were observed in cylindrical geometry. These
previously unexpected eddies, which impede the dynamo effect, offer an
explanation for the experimental difficulties in attaining a dynamo in
spherical geometry.
