Dynamo action with inhomogeneous magnetic diffusivity

Dynamo instability in an unconstrained flow was first observed in the VKS (von Kármán Sodium) experiment in 2006 te{VKS}. In this experiment, dynamos are excited only when soft iron impellers are used. The dynamo mode is not the one predicted by the numerical simulation based on the mean flow. Moreover when one impeller rotates faster than the other dynamical regimes are observed (including field reversals). These dynamical regimes can be modeled by two magnetic modes interacting coupled by noise te{Petrelis}. When only one soft iron impeller is used, only statistically stationary dynamo regimes are observed. In order to understand the role of soft iron impellers in the VKS experiment, we have study experimentally the influence of soft iron on induction mechanisms in a vK flow. We have found that gradient of magnetic diffusivity have to be taken into account te{Verhille}. Based on these observations, we propose a mechanism for dynamo generation in the VKS experiment, compatible with all yet observed features.