ionosphere (Īŏnˈəsfēr) [key], series of concentric ionized layers forming part of the upper atmosphere of the earth from around 30 to 50 mi (50 to 80 km) to 250 to 370 mi (400 to 600 km) where it merges with the magnetosphere, the region of the Van Allen radiation belts. The degree of ionization and the heights of the ionized layers fluctuate on a daily and a seasonal basis and show latitudinal variations as well. Causes for other variations in characteristics may include changes in the amount of ultraviolet radiation received from the sun and effects of the earth's magnetic field. Ionization of nitrogen and oxygen molecules from X-rays and ultraviolet radiation from the sun produces a layer of charged particles which allows radio waves to be reflected around the world. Such activity makes possible long-distance wireless communication. The layers comprising the ionosphere are the D layer, E layer, and F layer (divided into F-1 and F-2). The lower layers have the lowest concentration of charged particles and reflect low frequency waves. The middle layers are called the Kennelly-Heaviside layers (named after Oliver Heaviside in England and A. E. Kennelly in the United States who independently discovered the existence and effects of the ionosphere); while the Appleton, or highest layer, has the highest concentration of charged particles due to the low density of gases.