Sky-wave propagation is when a signal is returned to Earth by the ionosphere. The D region of the ionospheric is closest to the Earth. The D region of the ionosphere is mainly responsible for absorbing MF/HF radio signals during the daytime.
The F2 region of the ionosphere is mainly responsible for long-distance sky-wave radio communications.
In relation to sky-wave propagation, the term "maximum usable frequency" (MUF) is the highest frequency signal that will reach its intended destination.
If you are receiving a weak and distorted signal from a distant station on a frequency close to the maximum usable frequency, scatter propagation is probably occurring.
Ground-wave propagation is when signal travels along the surface of the Earth.
A skip zone is an area that is too far away for ground-wave propagation, but too close for sky-wave propagation.
The more sunspots there are, the greater the ionization in the ionosphere. An average sunspot cycle is 11 years.