Ionosphere

Ionosphere or Thermosphere, name given to a layer or layers of ionized air in the atmosphere extending from almost 90 km (55mi) above the surface of the Earth to altitudes of 640 km (400 mi) and more. At these altitudes the air is extremely thin, having about the density of the gas in a vacuum tube. When the atmospheric particles undergo ionization by ultraviolet radiation from the Sun or by other radiation, they tend to remain ionized, because few collisions occur between ions.

The ionosphere exerts a great influence on the propagation of radio signals. Energy that is radiated from a transmitter upwards towards the ionosphere is in part absorbed by the ionized air and in part refracted, or bent downwards again, towards the surface of the Earth. The bending effect makes possible the reception of radio signals at distances much greater than would be possible for waves that travelled along the surface of the Earth.

Such refracted waves, however, reach the Earth only at certain definite distances from the transmitter; the distance depends on the angle of refraction and the altitude. Hence, a radio signal may be inaudible at 100 km (60 mi) from the transmitter but audible at 500 km (300 mi). This phenomenon is known as skip. In certain other areas the ground-wave signals and the refracted signals from the ionosphere may reach the receiver and interfere with each other, producing the phenomenon known as fading.

The amount of refraction in the ionosphere decreases with an increase in frequency and for very high frequencies is almost non-existent. Therefore long-distance transmission of high-frequency radio waves is limited to the line of sight. Both television and frequency-modulation (FM) radio use high-frequency waves. Long-distance transmission can be achieved only in a direct line, such as between the Earth and a communications satellite; the signal then may be relayed from the satellite to a distant point on the Earth.

The ionosphere is usually divided into two main layers: a lower layer, designated the E layer (sometimes called the Heaviside layer or Kennelly-Heaviside layer), which is between about 80 and 113 km (50 and 70 mi) above the Earth's surface and which reflects radio waves of low frequency; and a higher layer, the F, or Appleton, layer, which reflects higher-frequency radio waves. The latter is further divided into an F₁ layer, which begins at about 180 km (112 mi) above the Earth; and an F₂ layer, which begins at about 300 km (186 mi) from the surface. The F layer rises during the night and therefore changes its reflecting characteristics.