ABSTRACT
The ionosphere is composed of a number of ionised regions above the Earth’s surface which play a most important part in the propagation of radio waves. These regions are believed to influence radio waves mainly because of the presence of free electrons, which are arranged in approximately horizontally stratified layers. For reasons related to the historical development of ionospheric research, the ionosphere is divided into three regions or layers designated D, E and F, respectively, in order of increasing altitude (see Figure 4.1). Subdivisions of these regions may exist under certain conditions, for example F1 and F2 layers. From the viewpoint of HF propagation, the E-and F-regions act mainly as radio wave reflectors, and permit long range propagation between terrestrial terminals. The D-region acts principally as an absorber, causing signal attenuation in the HF range, although VLF and ELF waves are reflected at D-region altitudes.
