1. Abrami, A.: 1970, Solar Phys. 11, 104.
2. Achong, A.: 1972, UWI Physics Department, Report No. 50.
3. Barrow, C. H. and Morrow, D. P.: 1968, Astrophys. J. 152, 593.
4. Barrow, C. H. and Saunders, H.: 1972, Astrophys. Letters 12, 211.
5. Böhme, A. and Krüger, A.: 1973, Report UAG 28, Part I, World Data Center A, NOAA, p. 260.