1. Department of Mathematics, University of Surrey, 388 Stag Hill, Guildford, Surrey GU2 7XH, United Kingdom
2. Department of Nuclear, Plasma, and Radiological Engineering, 116 Talbot Laboratory, 104 S. Wright, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
3. Centre for Mathematical Plasma Astrophysics, Department of Mathematics, KULeuven, Celestijnenlaan 200b, 3001 Leuven, Belgium