[1] Univ Autonoma Barcelona, E-08193 Barcelona, Spain
[2] Univ Padua, Dipartimento Fis, I-35100 Padua, Italy
[3] Inst Fis Altes Energies, Barcelona, Spain
[4] Max Planck Inst Phys & Astrophys, D-80805 Munich, Germany
[5] Univ Wurzburg, D-97070 Wurzburg, Germany
[6] Yerevan Phys Inst, Cosm Ray Div, Yerevan 375036, Armenia
[7] Univ Complutense Madrid, E-28040 Madrid, Spain
[8] Univ Calif Davis, Davis, CA 95616 USA
[9] Univ Siena, Dipartimento Fis, I-53100 Siena, Italy
[10] Univ Lodz, Div Expt Phys, PL-90131 Lodz, Poland
[11] Univ Gesamthsch Siegen, Fachbereich Phys, D-5900 Siegen, Germany
[12] Potchefstroom Univ Christian Higher Educ, Space Res Unit, ZA-2520 Potchefstroom, South Africa
[13] Tuorla Observ, Piikkio, Finland
[14] Crimean Astrophys Observ, UA-334413 Nauchnyi, Ukraine
[15] Ist Nazl Fis Nucl, Sez Padova, I-35121 Padua, Italy
[16] Ist Nazl Fis Nucl, Sez Pisa, Pisa, Italy