[1] CNR, Ist Tecnol & Studio Radiaz Extraterr, I-40129 Bologna, Italy
[2] CNR, Ist Fis Cosm & Applicaz Informat, I-90146 Palermo, Italy
[3] European Space Agcy, ESTEC, Dept Space Sci, Div Astrophys, NL-2200 AG Noordwijk, Netherlands
[4] Univ Ferrara, Dept Phys, I-44100 Ferrara, Italy