1. Department of Geography, Geoinformatics and Meteorology, University of Pretoria, Lynwood Road, Pretoria 0002, South Africa
2. National Laser Centre, Council for Scientific and Industrial Research, P.O. Box 395, Pretoria 0001, South Africa
3. Discipline of Physics, School of Chemistry and Physics, University of KwaZulu Natal, Westville, Durban 4000, South Africa
4. Department of Physics, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia