  The geo and solar magnetic fields have long been thought to be very different
objects both in terms of spatial structure and temporal behavior. The recently
discovered field structure of a fully convective star is more reminiscent of
planetary magnetic fields than the Sun's magnetic field (Donati J.-F. et al.,
Science, 311 (2006) 633), despite the fact that the physical and chemical
properties of these objects clearly differ. This observation suggests that a
simple controlling parameter could be responsible for these different
behaviors. We report here the results of three-dimensional simulations which
show that varying the aspect ratio of the active dynamo region can yield sharp
transition from Earth-like steady dynamos to Sun-like dynamo waves.
