[1] Katholieke Univ Leuven, Dept Math, Ctr Math Plasma Astrophys, Celestijnenlaan 200B, B-3001 Leuven, Belgium
[2] Consiglio Nazl Ric ISTP CNR, Ist Sci & Tecnol Plasmi, Via Amendola 122-D, I-70126 Bari, Italy
[3] Univ Calabria, Dipartimento Fis, I-87036 Arcavacata Di Rende, CS, Italy