1. Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
2. Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281, B–9000 Gent, Belgium
3. School of Chemistry and Physics, University of KwaZulu-Natal, Durban 4000, South Africa
4. Department of Applied Mathematics and Statistics, Colorado School of Mines, Golden, Colorado 80401-1887, USA