Which mode of propagation is used by short wave broadcast services?

- Hint: The movement of sky waves is favored because the wave needs to travel a longer path. The wave is transmitted into the atmosphere here, where it is mirrored back to the planet again. The waves are seen here to be emitted from one location and from which other receivers absorb it.

Complete step-by-step solution -

Sky wave propagation is used for short wave broadcast systems ranging from a few MHz to 30 MHz. Sky waves can cover a very long distance and even can cover the earth.
There is a layer in the atmosphere known as the ionosphere. This is a region of electrically charged atoms or ions. It is a simplistic explanation; in fact there are a variety of distinct ionosphere levels leading to various altitudes.
SW, MW, LW radio waves interact actively with the charged ions in the ionosphere and are successfully transmitted back to Earth. This is generally referred to as 'skip' or 'jump.'

Additional information- When you have a much higher frequency signal, the interference is negligible and the radio waves do not transmit, they travel straight through (to space). The atmosphere of the planet will also mirror these radio waves up into the sky-so it is probable that a single wave will undergo several skips.

Note- Naturally, if there is a receiving radio in the middle of the skip, there will be no signal received. Going back to the ionosphere. This is heavily influenced by the Sun. And the multiple layers of the ionosphere all interact with different wavelengths. So at some times of the day, it may not be possible to use certain frequencies to reflect off the ionosphere and this mechanism becomes unusable.