1. a Institute of Electronics, Bulgarian Academy of Sciences , 72, Tsarigradsko shosse Blvd, Sofia, 1784, Bulgaria
2. b Central Laboratory of Solar–Terrestrial Influences , Bulgarian Academy of Sciences , Sofia, Bulgaria
3. c University of Mining and Geology ‘St Ivan Rilski’ , Sofia, Bulgaria
4. d Indian Institute of Tropical Meteorology , Pune, 411 008, India