1. Cantu, A.M., Godoli, G. and Poletto, G.: 1970,Solar Phys. 15, 356
2. Leftus, V. and Ruzickova-Topolova: 1980,Bull. Astron. Inst. Czechosl. 31, 232
3. Leroy, J.L. and Trellis, M.: 1974,Astron. Astrophys. 35, 283
4. No�ns, J.C. and Leroy, J.L.: 1981,Solar Phys. 73, 81
5. No�ns, J.C.et al.: 1991,Physics of Solar Corona (Leningrad).