1. Ananthakrishnan, S. and Ramanathan, K. R.: 1969, Nature (in press).
2. Burnight, T. R.: 1949, Phys. Rev.
76, 165.
3. Labeyrie, J.: 1968, in Electromagnetic Radiation in Space (ed. by J. G. Emming), D. Reidel Publ. Co., Dordrecht, Holland, p. 125.
4. Whitten, R. C. and Poppoff, I. G.: 1965, Physics of the Lower Ionosphere, Prentice-Hall, Inc., p. 30.