Excerpt from Characteristics of the Earth-Ionosphere Waveguide for Vlf Radio WavesPropagation of vlf radio waves to very great distances is made possible by the high reflectivity of the lower ionosphe re at oblique incidence. The latter is due to the relatively sharp gradient of the electron density in the D-region of the ionosphe re. In fact, for many purposes, the assumption of an abrupt lower edge of the ionized region has permitted an analytical approach to the problem, which has produced useful results. In the main, these are confirmed experimentally. However, a number of systematic discrepancies have been observed which suggest that the sharply bounded model is not entirely adequate.In recent years, evidence from a number of independent experi mental approaches has indicated that the profile of the electron density in the undisturbed lower iono Sphere can be approximately described by an exponential function of height. Some of this evidence is indicated briefly below.About the PublisherForgotten Books publishes hundreds of thousands of rare and classic books. Find more at www.forgottenbooks.comThis book is a reproduction of an important historical work. Forgotten Books uses state-of-the-art technology to digitally reconstruct the work, preserving the original format whilst repairing imperfections present in the aged copy. In rare cases, an imperfection in the original, such as a blemish or missing page, may be replicated in our edition. We do, however, repair the vast majority of imperfections successfully, any imperfections that remain are intentionally left to preserve the state of such historical works.