EM Ground-Wave Propagation over the Curved Lunar Surface

Abstract

VLF and LF electromagnetic (EM) ground-wave propagation between points on the surface of a smooth lunar sphere which is surrounded by free space or a cold isotropic plasma is investigated. Following Ward's analysis of recent lunar geophysical data, two mathematical models which are most likely to actually exist were chosen as either a cold homogeneous moon or a hot wet moon. The hot wet model is conceived as a radial stratification of material consisting of a dry outer shell, a wet shell, and a hot interior. Further subdivision results when the uppermost part of the wet shell is assumed to be either frozen (permafrost model) or unfrozen (wet rock model). The conductivities and dielectric constants in the hot wet model were assumed to vary continuously at several different rates within each of the layers, finally attaining the values suggested by Ward. Using the permafrost and wet rock models, the range of variation of the surface impedance and the subsequent variations in the ground-wave attenuation function were calculated versus frequency in the VLF and LF bands on the moon's dark side, and for frequencies above the solar-wind plasma frequency on its sunlit side. It is concluded that conditions for long-distance point-to-point ground-wave propagation are considerably better for the permafrost model. Above 100 kHz there is little difference between any of the assumed models, and the effect of the deeper subsurface stratification becomes second order.