1. Faculty of Mathematics and Physics; Charles University; Prague Czech Republic
2. Department of Space Physics, Institute of Atmospheric Physics; The Czech Academy of Sciences; Prague Czech Republic
3. Royal Belgian Institute for Space Aeronomy; Brussels Belgium
4. Laboratoire de Physique des Plasmas; Ecole Polytechnique, CNRS; Palaiseau France
5. LESIA; Observatoire de Meudon; Meudon France