  We study numerically the dynamo transition of an incompressible electrically
conducting fluid filling the gap between two concentric spheres. In a first
series of simulations, the fluid is driven by the rotation of a smooth inner
sphere through no-slip boundary conditions, whereas the outer sphere is
stationary. In a second series a volume force intended to simulate a rough
surface drives the fluid next to the inner sphere within a layer of thickness
one tenth of the gap width. We investigate the effect of the boundary layer
thickness on the dynamo threshold in the turbulent regime. The simulations show
that the boundary forcing simulating the rough surface lowers the necessary
rotation rate, which may help to improve spherical dynamo experiments.
