  We present a new mechanism for oscillatory or random reversals of the
magnetic field that occur from a stationary dynamo state. The basic ingredient
is the existence of two nearly critical dynamo eigenmodes, a stable and an
unstable one, that collide during a saddle-node bifurcation. Above this
bifurcation, a finite amplitude limit cycle is generated. In the neighbourhood
of the bifurcation, even a small amount of turbulent fluctuations is enough to
generate random reversals of the magnetic field. When the two competing
eigenmodes have different symmetries, this scenario requires a broken symmetry
of the flow that generates the dynamo. This is in good agreement with the
observations of the VKS experiment. It can also explain the dynamics of
planetary or stellar magnetic fields and can be used to understand reversals of
large scale flows that often develop on a turbulent background.
