Localized ELF propagation anomalies

Two methods are used to analyze extremely low frequency (ELF) propagation near lateral nonhomogeneities in the earth-ionosphere waveguide. The first, a full-wave integral equation, is accurate although expensive to solve and the second, a two-dimensional ray trace, is only semiquantitative, but provides physical insight. Both methods are applied to several model solar proton events (SPEs), including one based on the 23 November 1982 event, which is of special interest because simultaneous measurements of ELF signal anomalies and incident proton fluxes are available. Calculations predict a region of weak field near the Gulf of Alaska, where a submarine-borne receiver measured an unusually severe signal loss. That behavior is caused by lateral refraction, which bends the signal away from the gulf and into the disturbed polar cap, where the phase velocity of the TEM mode is lowest. The theory also predicts a region of strong field just inside the polar cap boundary, but no data are available to test that result. Localized regions of weak or strong fields will occur on links where the great-circle path is nearly tangent to a boundary between large zones with different waveguide phase velocities.