Propagation of ELF waves under normal and naturally disturbed conditions

Abstract

This study presents a theoretical analysis of the propagation of electromagnetic waves in the frequency range 30–1000 Hz in the earth-ionosphere waveguide. Full wave methods incorporating the vertical inhomogeneity of the ionosphere are used in conjunction with model ionospheres corresponding to ambient and disturbed (PCA) conditions. For ambient conditions, the character of the assumed ion-density profile below 60 km substantially affects the computed attenuation rates, phase velocities, Joule heating rates, and field strengths for waves at frequencies lower than a few hundred Hz. For example, at a wave frequency of 50 Hz, 30% to 50% of the attenuation is due to losses associated with ion heating. At frequencies of a few hundred Hz or higher the losses, under ambient conditions, are related almost entirely to electron heating. Under moderate PCA conditions the losses are due mainly to ion-heating for the entire ELF frequency range, and the attenuation rates are substantially higher than for undisturbed situations. The ionospheric height ranges which influence the ELF mode structure are quite broad being typically many tens of kilometers in thickness. The influence of the geomagnetic field upon the mode characteristics is substantial in the undisturbed nighttime, slight in the undisturbed daytime, and essentially nil under PCA conditions.