1. a Eötvös University , Department of Environmental Physics, and MTA–ELTE Research Group for Geoinformatics and Space Sciences , Pázmány P. sétány 1/A, H-1117 Budapest, Hungary E-mail:
2. b Space Research Group, Geophysical Department , Eötvös University , Pázmány P. sétány 1/A, H-1117 Budapest, Hungary
3. c MTA-ELTE Research Group for Geoinformatics and Space Sciences , Pázmány P. sétány 1/A, H-1117 Budapest, Hungary