Impact of impellers on the axisymmetric magnetic mode in the VKS2 dynamo experiment

In the VKS2 (von K\'arm\'an Sodium 2) successful dynamo experiment of September 2006, the magnetic field that was observed showed a strong axisymmetric component, implying that non axisymmetric components of the flow field were acting. By modeling the induction effect of the spiraling flow between the blades of the impellers in a kinematic dynamo code, we find that the axisymmetric magnetic mode is excited and becomes dominant in the vicinity of the dynamo threshold. % The control parameters are the magnetic Reynolds number of the mean flow, the coefficient measuring the induction effect, $\alpha$, and the type of boundary conditions (vacuum for steel impellers and normal field for soft iron impellers). We show that using realistic values of $\alpha$, the observed critical magnetic Reynolds number, $Rm^c \approx 32$, can be reached easily with ferromagnetic boundary conditions. We conjecture that the dynamo action achieved in this experiment may not be related to the turbulence in the bulk of the flow, but rather to the alpha effect induced by the impellers.