[1] INAF IFSI, Ist Fis Spazio Interplanetario, I-00133 Rome, Italy
[2] Univ Michigan, Dept Atmospher Ocean & Space Sci, Ann Arbor, MI 48109 USA
[3] Observ Paris, Lab Etud Spatiales & Instrumentat Astrophys, Observ Paris, F-92195 Meudon, France
[4] Russian Acad Sci IKI, Space Res Inst, Moscow 117810, Russia