[1] INAF OAR Natl Inst Astrophys, Via Frascati 33, I-00078 Monte Porzio Catone, RM, Italy
[2] INAF IAPS Natl Inst Astrophys, Via Fosso Cavaliere 100, I-00133 Rome, Italy
[3] Univ Sheffield, Sch Math & Stat, Solar Phys & Space Plasma Res Ctr SP2RC, Sheffield S3 7RH, S Yorkshire, England
[4] Univ Oslo, Inst Theoret Astrophys, POB 1029, NO-0315 Oslo, Norway
[5] Natl Solar Observ, Boulder, CO 80303 USA
[6] Univ Catania, Dept Phys & Astron, Via S Sofia 78, I-95125 Catania, Italy