The ionosphere occupies the part of the atmosphere between 50 and 600km above the surface of the Earth. It assists in the propagation of radio waves around the curved surface of the Earth in the following way.
The refractive index of the ionosphere increases with the density of free electrons, (there are many free electrons in the ionosphere, which is why it is called the ionosphere) which can reach
allowing some radio waves to be refracted by enough to bring them down to Earth.
Gradual refraction in the ionosphere allows radio signals to be propagated long distances. It is often convenient to imagine the process as a reflection with an imaginary reflection point at some virtual height above the actual refracting region.
DX communication, popular among amateur radio enthusiasts, is a term given to communication over great distances. The ionosphere can be utilized to "reflect" a transmitted signal down to ground. Transcontinental HF-connections rely on up to 5 reflections.