1. Institute of Geophysics, The Czech Academy of Sciences, Boční II, CZ-14100, Praha 4, Czech Republic
2. Helmholtz-Institut für Strahlen- und Kernphysik, University of Bonn, Nussallee 14-16, D-53115 Bonn, Germany
3. Faculty of Mathematics and Physics, Astronomical Institute, Charles University in Prague, V Holešovičkách 2, CZ-180 00 Praha 8, Czech Republic