1. Code 672, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
2. GPHI, Astronomy Department, University of Maryland, College Park, MD 20742, USA
3. Dipartimento di Fisica, Università della Calabria, Rende 87036, Italy
4. Laboratoire de Physique des Plasmas, CNRS-UPMC, Ecole Polytechnique, Route de Saclay, Palaiseau 91128, France