Effects of the ionosphere on ELF signals during polar cap absorption events: Comparison of theory and experiments

In order to calculate extremely low frequency (ELF) propagation when the earth-ionosphere waveguide is not stratified, a hybrid method is developed that uses full-wave theory to determine local parameters of the TEM mode, but uses ray theory to describe the field's horizontal variations. The method is applied to several model solar proton events (SPEs), including one based on the weak 23 November 1982 event. Calculations predict diminished fields near the Gulf of Alaska, where a submarine-borne receiver measured an unusually severe signal reduction. That behavior is caused by lateral refraction, which bends the signal away from the polar cap boundary and into the central cap where, during an SPE, the phase velocity of the TEM mode is slowest. The theory also predicts an enhanced field just inside the polar cap boundary, but no data are available to test that result.