1. Laboratoire de Physique des Plasmas, CNRS, Sorbonne Universités, UPMC Univ Paris 06, Univ Paris-Sud, Ecole Polytechnique, 91128 Palaiseau, France
2. Centre National d'Etudes Spatiales (CNES), F-31401 Toulouse, France
3. Department of Physics and Astronomy, University of Iowa, Iowa City, Iowa 52242, USA