1. Department of Aerospace Science and Technology, Politecnico di Milano, via La Masa 34, I-20156 Milano, Italy
2. Laboratoire de Physique des Plasmas, École Polytechnique, route de Saclay, F-91128 Palaiseau, France
3. Aeronautics and Aerospace Department, von Karman Institute for Fluid Dynamics, Waterloosesteenweg 72, B-1640 Sint-Genesius-Rode, Belgium