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

Excitability in the vicinity of a saddle-node bifurcation: a mechanism for reversals