1. a Institute of Electronics, Laser Radars Laboratory, Bulgarian Academy of Sciences , Sofia , Bulgaria
2. b Environmental Science and Technology Section, Linnaeus University , Kalmar , Sweden
3. c Department of Physics , University of Mining and Geology ‘St. Ivan Rilski’ , Sofia , Bulgaria
4. d Department of Meteorology and Geophysics, Faculty of Physics , Sofia University ‘St. Kliment Ohridsky’ , Sofia , Bulgaria
5. e Central Laboratory of Solar-Terrestrial Influences, Bulgarian Academy of Sciences , Sofia , Bulgaria