Ionosphere, << eye ON uh sfeer, >> is a part of Earth’s atmosphere that has many ions (electrically charged atoms and groups of atoms) and free electrons. Cosmic rays and radiation from the sun produce these ions. The ionosphere extends through the layers of the atmosphere known as the mesosphere and the thermosphere.
Several ionized regions make up the ionosphere. The lowest one, called the D region, begins at an altitude of 34 to 55 miles (55 to 89 kilometers). The E region has an altitude of 55 to 90 miles (89 to 145 kilometers). The F region begins at 90 miles (145 kilometers) and extends up to about 190 miles (306 kilometers). The height and ionization of these regions vary from day to night and with changes in solar radiation. For example, the D region almost disappears at night. The other regions rise and become less strongly ionized at night because no solar radiation reaches the atmosphere.
The ionosphere makes long-distance radio communication possible because it reflects certain radio waves thousands of miles back to Earth. At night, when the regions of the ionosphere rise, radio waves can be received even farther from the transmitter than during the day.
See also Air ; Radio (Transmitting radio waves) ; Appleton, Sir Edward Victor .
