  We measure the decay rates of magnetic field modes in a turbulent flow of
liquid sodium below the dynamo threshold.We observe that turbulent fluctuations
induce energy transfers between modes with different symmetries (dipolar and
quadrupolar). Using symmetry properties, we show how to measure the decay rate
of each mode without being restricted to the one with the smallest damping
rate.We observe that the respective values of the decay rates of these modes
depend on the shape of the propellers driving the flow. Dynamical regimes,
including field reversals, are observed onlywhen the modes are both nearly
marginal. This is in linewith a recently proposed model.
