[1] Rutherford Appleton Lab, Didcot, Oxon, England
[2] Univ Strathclyde, Glasgow, Lanark, Scotland
[3] Inst Super Tecn, Lisbon, Portugal
[4] Ruhr Univ Bochum, Theoret Phys 4, Bochum, Germany
[5] Swedish Inst Space Phys, Uppsala, Sweden
[6] Univ Calif Los Angeles, Dept Phys, Los Angeles, CA 90024 USA
[7] UCSD, Jacobs Sch Engn, La Jolla, CA USA