Far-Field Extremely Low Frequency (ELF) Propagation Measurements, 1970-1972

For the past 3 years, we have participated in various extremely low frequency (ELF) propagation tests. During these tests, receiving sites were located in Connecticut, North Carolina, Maine, Utah, Nova Scotia, California, Greenland, the Virgin Islands, Alaska, Norway, Hawaii, Greece, and Saipan. Measurements have also been taken sporadically in Connecticut since June 1970. At each location, the horizontal magnetic field strengths were measured at a band of frequencies centered at 45 Hz and 75 Hz in order to determine the average attenuation rates and relative excitation factors for daytime and nighttime propagation conditions. The U.S. Navy ELF Wisconsin Test Facility was the transmission source. The principal results obtained from these measurements were 1) the daytime attenuation rate is usually higher than the nighttime attenuation rate at both 45 and 75 Hz; 2) the relative excitation factors are quite different for daytime and nighttime propagation conditions; 3) nighttime propagation is more variable than daytime propagation; 4) there is a seasonal variation in the ELF propagation parameters; and 5) auroral-zone effects appear to be present at both frequencies.