1. LEMAP, Department of Physics, Faculty of Science University of Yaounde 1 Yaounde Cameroon
2. L2MPS, Department of Physics, Faculty of Science University of Dschang Dschang Cameroon
3. Earth System Physics Section, The Abdus salam ICTP Trieste Italy
4. Department of Physics, Faculty of Science University of Douala Douala Cameroon