Abstract
ELF radio wave propagation measurements at northeastern U.S. sites within 1600 km of the U.S. Navy test transmitter in Wisconsin occasionally display an unexpected early-morning null behavior in which a decrease of more than 6 dB in signal strength occurs over a period of several hours. These severe ELF disturbances occur only occasionally during the several days following magnetic storms when similar, but less pronounced behavior is found to coincide with phase disturbances on VLF paths across the northern U.S. These disturbances appear to be caused by energetic electrons precipitated into middle latitudes from the radiation belt. The magnitude of the severe ELF disturbances requires an explanation involving a twofold discontinuity in the Earth-Ionosphere waveguide. It is suggested that the effective ionosphere reflecting height is lowered immediately to the east of the receiving locations by particulate bombardment but then is raised further east at the geomagnetic conjugate of the South Atlantic anomaly, beyond which quasi-trapped particles are no longer available at lower D-region heights.
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