1. Bannister, P. R. et al.: 1979, Extremely Low Frequency (ELF) Propagation, Naval Underwater Systems Center, New London, Connecticut.
2. Barr, R.: 1971, J. Atmos. Terr. Phys. 33 1577.
3. Barr, R.: 1975a, J. Atmos. Terr. Phys. 37 1405.
4. Barr, R.: 1975b, Int. J. Electronics
34, 241.
5. Behroozi-Toosi, A. B. and Booker, H. G.: 1983, Space
Sci. Rev. 35, 91 (this issue).