1. Dipartimento di Fisica, Università degli Studi di Cagliari , SP Monserrato-Sestu km 0.7, 09042 Monserrato, Italy
2. Department of Astronomy, University of Maryland , College Park, MD 20742, USA
3. Astrophysics Science Division, NASA Goddard Space Flight Center , Greenbelt, MD 20771, USA
4. MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology , Cambridge, MA 02139, USA
5. Space Science Division, Naval Research Laboratory , Washington, DC 20375-5352, USA
6. National Space Institute, Technical University of Denmark , Elektrovej 327-328, 2800 Lyngby, Denmark
7. Università degli Studi di Palermo, Dipartimento di Fisica e Chimica , via Archirafi 36, 90123 Palermo, Italy
8. INAF/IASF Palermo , via Ugo La Malfa 153, I-90146 - Palermo, Italy
9. School of Physics and Astronomy, University of Southampton , Southampton, Hampshire SO17 1BJ, UK
10. Department of Physics, Faculty of Science and Engineering, Chuo University , 1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan