What is sky wave propagation ?

Explanation

Sky Waves : Long distance communication between two points on the earth is achieved through reflection of electromagnetic waves by Ionosphere , Such waves are called sky waves .
This mode of propagation 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 `~65` Km to about 400 km above the earth's surface .
The degree of ionisation varies with the height. The density of atmospgere decreases with height. At greater heights the solar radiation is intense but there are few molecules to be ionized .Closed to the earth, the radiation intensity is low so that the ionization is again low. However at some intermediate heights, there occurs a peak of ionization density. The ionosoheric layer acts as a reflector for a certain range of frequencies (3 to30 MHz). Electromagnetic waves od frequencies higher than 30 MHz penetarate ionosphere and escape.This phenomenon is shown in the Fig.The phenomenon of bending of em waves is so that they are diverted towards the earth which is similar to Total Internal Reflection in options.