[1] Inst. F. Theor. Physik Astrophysik, Universität Potsdam, PF 601553, D-14415 Potsdam, Germany
[2] Institute for Plasma Research, University of Maryland, College Park, MD 20742, United States
[3] Depts. Phys. Astron. and of Math., University of Kansas, Lawrence, KS 66045, United States