Radio propagation that rely on reflection (actually refraction) of radio waves from the ionosphere. The ionosphere is a region of the atmosphere from about that contains layers of charged particles (ions) which can refract a radio wave back toward the Earth. A radio wave directed at an angle into the sky can be reflected back to Earth beyond the horizon by these layers, allowing long distance radio transmission. The F2 layer is the most important ionospheric layer for long-distance, multiple-hop HF propagation, though F1, E, and D-layers also play significant roles. The D-layer, when present during sunlight periods, causes significant amount of
Rarefaction local medium. This expansion behaviour is in contrast to the behaviour of pressure increases, which gets narrower with time, until they steepen into shock waves. A natural example of rarefaction occurs in the layers of Earth's atmosphere. Because the atmosphere has mass, most atmospheric matter is nearer to the Earth due to the Earth's gravitation. Therefore, air at higher layers of the atmosphere is less dense, or "rarefied", relative to air at lower layers. Thus rarefaction can refer either to a reduction in density over space at a single point of time, or a reduction of density over time for