1. Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
2. Department of Physics and Astronomy, University of Victoria, Victoria, BC V8W 2Y2, Canada
3. Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, The Netherlands
4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
5. Department of Physics & Astronomy, Michigan State University, East Lansing, MI 48824-2320, USA