1. H. Zirin, The Solar Atmosphere [Russian translation], Izd. Mir, Moscow (1969).
2. I. S. Shklovskii, Astron. Zh.,41, 676 (1964).
3. S. L. Mandelstam, Space Sci. Rev.,4, 587 (1965).
4. M. Landini, D. Russo, and G. L. Tagliaferri, Space Res.,712, 1281 (1967).
5. P. R. Sengupta, Sol. Phys.,17, 160 (1971).