1. Department of Meteorology1, Stockholm University, S-106 91 Stockholm, Sweden
2. Institut für Experimentalphysik, Universität Wien, Strudlhofgasse 4, A-1090 Wien, Austria
3. FISBAT-CNR, Via de Castagnoli 1, 40126 Bologna, Italy
4. Department of Nuclear Physics, University of Lund, Sölvegatan 14, S-223 62 Lund, Sweden