1. Department of Physical and Chemical Sciences University of L'Aquila L'Aquila Italy
2. Mining and Geological Survey of Hungary Budapest Hungary
3. Institute of Geophysics Polish Academy of Sciences Warsaw Poland
4. Space Research Group, Department of Geophysics and Space Sciences Eötvös Loránd University Budapest Hungary
5. Geodetic and Geophysical Institute RCAES, Hungarian Academy of Sciences Sopron Hungary