IONOSPHERE
Within the earth's atmosphere are layers in which the concentration of free electrons is above the average atmospheric value. These layers constitute the ionosphere. The electrons are due to the ionization of atmospheric molecules and atoms by solar ultraviolet and X-ray photons.
Radio waves of certain wavelengths, for example the AM band of conventional broadcasting, transmitted by ground stations are reflected between the ionosphere and the earth's surface. This makes possible long-distance communication between stations that are not along a direct line of sight because of the earth's curvature. Radio wavelengths greater than about 10 meters, or frequencies less than about 30 megahertz, are turned back by the ionospheric layers. Shorter wavelengths, or higher frequencies, such as radar signals, pass through the ionosphere into space with little or no bending.
