1. Chemical Resource Beneficiation, North-West University, Potchefstroom, South Africa
2. Department of Applied Physics, University of Eastern Finland, Kuopio, Finland
3. School of Mathematical and Statistical Sciences, North-West University, Potchefstroom, South Africa
4. Finnish Meteorological Institute, Helsinki, Finland
5. Earth Sciences Division, NASA/Goddard Space Flight Center, Greenbelt, MD, USA
6. Department of Physics, University of Helsinki, Helsinki, Finland