1. A. V. Gurevich, D. I. Fishchuk, and E. E. Tsedilina, Geomagn. Aeron.,13, No. 1, 31 (1973).
2. P. A. Bradley and J. R. Dudeney, J. Atm. Terr. Res.,35, No. 12, 2131 (1973).
3. W. Becker, Radio Sci.,2, No. 10, 1205 (1967).
4. S. S. Chavdarov, Yu. K. Chasovitin, S. P. Chernysheva, et al., Medium Latitude Sporadic E-layer of the Ionosphere [in Russian], Nauka, Moscow (1975).
5. O. Ovezgel'dyev and G. Kelov, Izv. Vyssh. Uchebn. Zaved., Radiofiz.,18, No. 12, 1794 (1975).