achieved by ionospheric reflection of radio waves back towards the earth. This mode of propagation is called sky wave propagation and is used by short wave broadcast services. The ionosphere is so called because of the presence of a large number of ions or charged particles. It extends from a height of ~ Km to about Km above the earth’s surface.
Ionisation occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules. The ionosphere is further subdivided into several layers, the details of which are given in Table . . The degree of ionisation varies with the height.
The density of atmosphere decreases with height. At great heights the solar radiation is intense but there are few molecules to be ionised. Close to the earth, even though the molecular concentration is very high, the radiation intensity is low so that the ionisation is again low. However, at some intermediate heights, there occurs a peak of ionisation density.
The ionospheric layer acts as a reflector for a certain range of frequencies ( to MHz). Electromagnetic waves of frequencies higher than MHz penetrate the ionosphere and escape. These phenomena are shown in the Fig. .
. The phenomenon of bending of em waves so that they are diverted towards the earth is similar to total internal reflection in optics * . Troposphere D (part of stratosphere) E (part of Stratosphere) F (Part of Mesosphere) F (Thermosphere) Name of the stratum (layer) Approximate height over earth’s surface Exists during km - km km - km km at night, - km during daytime Day and night Day only Day only Daytime, merges with F at night Day and night T ABLE . D IFFERENT LAYERS OF ATMOSPHERE AND THEIR INTERACTION WITH THE PROPAGATING ELECTROMAGNETIC WAVES Frequencies most affected VHF (up to several GHz) Reflects LF, absorbs MF and HF to some degree Helps surface waves, reflects HF Partially absorbs HF waves yet allowing them to reach F Efficiently reflects HF waves, particularly at night P A