1. School of Mathematics and Statistics, University Place, University of Glasgow, Glasgow G12 8QQ, UK
2. Department of Mathematics, University of Trento, I-38123 Povo, Italy
3. School of Physics and Astronomy, Kelvin Building, University of Glasgow, Glasgow G12 8QQ, UK
4. Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern, NO-0315 Oslo, Norway