  We consider the generation of magnetic field by a turbulent flow. For the
linear induction equation (i.e. the kinematic dynamo problem), we show that the
statistical moments of the magnetic field display multiscaling and in
particular moments of different order turn unstable for different values of the
control parameter. On a canonical example, we map the problem onto the
calculation of the injected power by a time correlated fluctuating force acting
on a Brownian particle. We are then able to calculate analytically the growth
rate of the moments of the magnetic field and explain the origin of this
intermittency. We finally show that the onset for the nonlinear problem is
predicted by the linear onset of the moment of order 0 + (i.e. the logarithm of
the magnetic field)
