  We show that hemispherical dynamos can result from weak equatorial symmetry
breaking of the flow in the interior of planets and stars. Using a model of
spherical dynamo, we observe that the interaction between a dipolar and a
quadrupolar mode can localize the magnetic field in only one hemisphere when
the equatorial symmetry is broken. This process is shown to be related to the
one that is responsible for reversals of the magnetic field. These seemingly
very different behaviors are thus understood in a unified framework.
