IONOSPHERE

region of the Earth's atmosphere in which the number of ions, or electrically charged particlesresulting from the action of extraterrestrial (primarily solar) radiation on the neutral atoms and molecules of the airis large enough to affect the propagation of radio waves. The ionosphere begins at a height of about 50 km (30 miles) above the surface but is most distinct at altitudes above about 80 km (50 miles). Discovery of the ionosphere extended over nearly a century. As early as 1839, the German mathematician Carl Friedrich Gauss speculated that an electrically conducting region of the atmosphere could account for observed variations of the Earth's magnetic field. The notion of a conducting region was reinvoked by others, notably in 1902 by the American engineer Arthur E. Kennelly and the British physicist Oliver Heaviside to explain the transmission of radio signals around the curve of the Earth's surface, before definitive evidence was obtained in 1925. For some years the ion-rich region was referred to as the Kennelly-Heaviside layer. The ionosphere consists of three layers: the D region, the lowest, is strongly ionized during the day and is responsible for the attenuation of shortwave and broadcast-band radio signals, an effect that largely disappears at night; the E layer, extending from 90 to 140 km (56 to 90 miles), is a region of ionized molecules and strong electric currents; and the F region, from 140 km up, is characterized by ionized atoms and contains the stratum (designated as F2) in which ion concentration reaches a maximum. The E and F regions are responsible for the long-distance propagation, by reflection, of radio signals in the shortwave and broadcast bands. Extending upward from the F layer is a region called the magnetosphere, wherein the Earth's magnetic field largely controls the movement of ions. The magnetosphere contains the highly energetic charged particles of the Van Allen radiation belts, though its ion content is little affected by them. At distances of 8 to 14 Earth radii (roughly 51,000 to 89,000 km, or 31,700 to 55,500 miles) on the sunward side and farther out elsewhere, the magnetosphere and the ionosphere end at a boundary known as the magnetopause, beyond which is the domain of the solar wind (continual flow of electrons and atomic nuclei from the Sun). Ionization in the ionospheric region is chiefly effected by solar radiation at X-ray and ultraviolet wavelengths. The particles ionized are mainly molecular nitrogen and molecular and atomic oxygen, but at a height of about 1,000 km (620 miles) there is a region, sometimes called the protonosphere, in which protons (hydrogen nuclei) constitute the dominant ionic species. Additional reading