Focusing and shadowing of ELF signals

Phase velocity transitions in the earth‐ionosphere waveguide can focus or shadow ELF signals and give rise to anomalies less than a megameter in extent. This paper develops a hybrid method to calculate those effects: it uses full‐wave theory to determine local TEM mode parameters, but traces rays to describe the signal's lateral structure. The method is applied to models of the terminator and disturbed polar cap boundary, including one based on the November 22, 1982 solar proton event (SPE). During that event a receiver in the Gulf of Alaska measured a severe signal fade. That fade probably was caused by refraction of the signal away from the polar cap boundary and into the disturbed cap, where the TEM‐mode phase velocity is slowest. Focusing and shadowing, lasting about an hour, are predicted for signals whose great‐circle path grazes the terminator.