The mechanism ofF-layer propagated back-scatter echoes

An analysis of the mechanism of F-layer propagated (2F) back-scatter is given, together with the results of multifrequeney experimental observations. It is shown that when scattering is assumed to occur at the surface of the earth (rather than in the E-region, as some have thought) theory agrees very well with the experimental evidence. The behavior of back-scatter for transmission near the critical frequency confirms the ground-origin hypothesis and is particularly difficult to explain in terms of E-region origin. It is shown that the leading edge of the echo corresponds to energy returned from scattering centers beyond the edge of the skip-zone. Energy from the skip-zone edge is returned with greater delay, the time-differential becoming larger as the F-layer vertical-incidence critical frequency is approached. Great care is clearly needed if meaningful skip-distance information is to be obtained from back-scatter observations. The rapid build-up in echo amplitude which follows a sharply-defined minimum time-delay is explained in terms of a time-delay focusing phenomenon.