[1] Univ Coll London, Mullard Space Sci Lab, Dorking RH5 6NT, Surrey, England
[2] UCL Birkbeck, Ctr Planetary Sci, London WC1E 6BT, England
[3] CNRS, Inst Rech Astrophys & Planetol, F-31028 Toulouse 4, France
[4] Univ Toulouse, UPS OMP, Inst Rech Astrophys & Planetol, F-31028 Toulouse, France
[5] Inst Astron Fis Espacio, Buenos Aires, DF, Argentina
[6] Swedish Inst Space Phys, S-75121 Uppsala, Sweden
[7] Univ London Imperial Coll Sci Technol & Med, Space & Atmospher Phys Grp, Blackett Lab, London SW7 2AZ, England
[8] KFKI Res Inst Particle & Nucl Phys, H-1121 Budapest, Hungary
[9] Johns Hopkins Univ, Appl Phys Lab, Laurel, MD 20723 USA
[10] Acad Athens, Off Space Res, Athens 11527, Greece
[11] SW Res Inst, San Antonio, TX 78228 USA