[1] Univ Politecn Cataluna, Dept Fis & Engn Nuc, E-08800 Barcelona, Spain
[2] UPC, IEEC, E-08034 Barcelona, Spain
[3] CSIC, IEEC, E-08034 Barcelona, Spain
[4] CSIC, Inst Ciencies Espai, E-08034 Barcelona, Spain
[5] Washington Univ, Dept Phys, St Louis, MO 63130 USA
[6] Washington Univ, Space Sci Lab, St Louis, MO 63130 USA
[7] Washington Univ, Dept Earth & Planetary Sci, Planetary Chem Lab, St Louis, MO 63130 USA
[8] Washington Univ, McDonnell Ctr Space Sci, St Louis, MO 63130 USA