Extremely Low Frequency (ELF) Electromagnetic Wave Propagation

Abstract Extremely low frequency (ELF) electromagnetic waves are currently the lowest frequency waves used routinely for wireless communication. Their ability to cover the globe with low transmitted power and to penetrate deeply into electrically conducting material makes them indispensable in a variety of applications, including communication with submerged submarines. ELF waves are also used in a variety of scientific applications, from geological remote sensing to the study of lightning, which is a strong natural ELF wave source. Transmitting controlled electromagnetic power at such long wavelengths is tricky and requires antennas rather different from those used at higher frequencies. ELF waves transmitted near the surface of the Earth are strongly reflected by the ground and the ionosphere, and thus propagate as guided waves in this spherical waveguide. The article discusses ELF transmission and reception, ELF applications, and describes in detail techniques for mathematically modeling, with varying degrees of simplicity and approximation, ELF electromagnetic wave propagation in this Earth‐ionosphere waveguide.