Distinguish between 'sky wave' and ' space wave' modes of propagation. Why is the sky wave mode of propagation restricted to frequencies upto 40 MHz ?

Explanation

Sky wave mode of propagation is ionospheric propagation. In the frequency range from a few MHz upto 30 MHz to 40 MHz, long distance communication can be achieved by ionospheric reflection of radio waves back towards the earth.Hence, sky wave mode of propagation is restricted to frequencies upto 40 MHz.