1. RUPF–Department of Physics, Faculty of Sciences of Tunis, University of Tunis El Manar , 2092 EL Manar Tunis, Tunisia
2. Institute of Computer Graphics and Knowledge Visualization, Graz University of Technology , Inffeldgasse 16c, A-8010 Graz, Austria
3. Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory , SE-439 92 Onsala, Sweden