1. Department of Radiophysics, University of Nizhny Novgorod, 23 Gagarin Ave., Nizhny Novgorod 603950, Russia
2. MTA Research Centre for Astronomy and Earth Sciences, Csatkai E. u. 6–8, Sopron H-9400, Hungary
3. Department of Nuclear Physics, R. E. Alekseev State Technical University of Nizhny Novgorod, 24 Minin St., Nizhny Novgorod 603950, Russia