1. Arnaud, M. and Rothenflug, R.: 1985, Astron. Astrophys. Suppl. 60, 425.
2. Dwivedi, B.N. and Raju, P.K.: 1980, Solar Phys. 68, 111.
3. Dwivedi, B.N. and Gupta, A.K.: 1990, Adv. Space Res., in press.
4. Ewlert, G. and Raju, P.K.: 1975, Astrophys. Space Sci. 38, 369.
5. Elzner, L.R.: 1976, Astron. Astrophys. 47, 9.