In sky wave propagation, the ionosphere plays a very important role'- explain.

Explanation

The ionosphere extending from a height of 80 km to about 400 km above earth surface plays a significant role for sky wave propagation. The reason is as follows-
The ionisation of air in the ionosphere is mainly due to solar radiation in day time and cosmic rays of much less intensity during night. the rate of ionisation gradually decreases from the upper to the lower layers. Higher the ionisation rate in a layer, higher is the velocity of electromagnetic waves through it i.e., the refractive index (`mu`) of this layer decreases according to the relation,
`mu = (1 - (81"n")/(f^(2)))^(1//2)`
where n = electron number density, a measure of rate of ionisation and f = frequency of the wave.
Whenever a sky wave, with a higher slope propagates through ionosphere from a higher refractive index to lower refractive index layer, it suffers total internal reflection. thus the wave is reflected back to the earth surface. Hence lies the importance of ionosphere for sky wave propagation.