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3. J. La?tovi?ka and J. Bo?ka, J. Atm. Terr. Phys.,44, No. 9, 793 (1982).
4. Yu. G. Mizun, The Lower Ionosphere at High Latitudes [in Russian], Nauka, Leningrad (1983).
5. A. V. Frank-Kamenetskii, in: Trudy Arkt. i Antarkt. Nauch. Issled. Inst., Vol. 366, Leningrad (1980), p. 58.