ELF propagation in disturbed environments

The effects on long-range ELF propagation from ionospheric disturbances such as solar particle events or nuclear environments, which substantially constrict the Earth-ionosphere waveguide are described. A few results pertain to more common ionospheric phenomena, such as sporadic-E patches. Stratified disturbances usually cause both the excitation factor and the attenuation rate of the TEM waveguide mode to increase. Thus, the signal increases somewhat at short distances, but decreases by several decibels at long distances. The main loss mechanism is Ohmic heating of heavy ions in the lower ionosphere. Numerical solutions show that a localized disturbance behaves like a cylindrical lens filling a narrow aperture. Lateral diffraction, focusing, and reflection can cause the TEM mode to exhibit a transverse pattern of maxima and minima beyond the disturbance, and a standing wave pattern in front of it. Such phenomena can contribute to spatial fluctuations occasionally observed on ELF transmissions. The focusing and diffraction diminish when the transverse dimension of the disturbance approaches the width of the first Fresnel zone--typically, several megameters.