1. V V Akimov, N L Grigorov, ID Rapoport, V Ye Nesterov, IA Savenko, G A Skuridin and A F Titenkov,Geomagn. Aeron. 11, 327 (1971)
2. V K Balasubrahmanyan and J F Ormes,Astrophys. J. 186, 109 (1973)
3. E Miusson,Astrophys. J. 191, 331 (1974)
4. J A Lezniak and W R Webber,Astrophys. J. 223, 676 (1978)
5. A V Apanasenkoet al, RUNJOB collaboration,Proc. 26th Int. Cosmic Ray Conf. Salt Lake City 3, 163 (1999);3, 167 (1999)