Why cannot the (i) ground waves (ii) space waves and (iii) sky waves

Why cannot the (i) ground waves (ii) space waves and (iii) sky waves be sustained for long-distance communication beyond 10 to 20 MHz?

What is meant by critical frequency for skywave propagation? Write an expression for the critical frequency in terms of the maximum electron density of the ionosphere.

The absorption of waves (loss of energy) by atmosphere increases with increasing frequency that is why ground waves, space waves and sky waves cannot be sustained for long-distance communication beyond 10 to 20 MHz.

Critical Frequency: Critical frequency is the highest frequency of the radio waves which when sent normally towards a given layer of ionosphere gets reflected from ionosphere and returns to the earth.
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where nmax is the maximum number density of electrons in the given layer of the ionosphere.