Jérémy Dargent, University of Pisa, Pisa, Italy, Nicolas Aunai, Laboratoire de Physique des Plasmas (UMR7648) CNRS/Ecole Polytechnique/UPMC/Univ. Paris Sud/Obs. de Paris Paris (France, Paris, France, Benoit Lavraud, IRAP/CNRS, Toulouse, France and Francesco Califano, Universita' di Pisa, Pisa, Italy