Optical and equivalent paths in a stratified medium, treated from a wave standpoint

Abstract

In a series of recent papers, Appleton has examined various methods of investigating tire structure of the Heaviside layer, lazing particular stress on the exact nature of the information which can be obtained from the experimental observations made by different methods. One particular point of importance in the interpretation of the observations emerges from this examination, namely the distinction between the "optical path" or "phase path" ∫μ ds = c ∫ ds / V and the "equivalent path" or "group path" c ∫ ds / U, where V, U are the wave and group velocities and the integrations are taken along the geometrical ray between transmitter and receiver. The analysis also shows clearly that the latter is the quantity which is observed by most methods, and in particular by the frequency-change method used by Appleton himself in his observational work.