ionosphere

ionosphere The upper levels of the atmosphere, above about 80 km, act as an absorber for harmful ultraviolet radiation and high-energy particles emitted by the Sun. Although some ultraviolet radiation passes through his level to be absorbed by ozone in the stratosphere, the most harmful radiation is absorbed at altitudes above 80 km. As this stream of particles and radiation from the Sun collides with atmospheric molecules, it causes some electrons to be split from their atoms and molecules. The resulting particles are ions. Positive ions are atoms and molecules that have had electrons removed, leaving them with a net positive electrical charge. Negative ions are those that have absorbed the free electrons, producing a net negative charge.

The ionosphere is the layer extending from about 80 km to the outer reaches of the atmosphere that includes charged particles. Since the air at these levels is electrically conductive it reflects some radio waves. Long wavelengths, for example those used in AM radio, are reflected at lower levels than shorter wavelengths, for example, the short-wave transmissions used by ‘ham’ radio operators. Very short wavelengths, such as those used for television and FM radio, are not reflected at all. The reflection of radio waves means that AM radio reception is possible at great distances from the transmitter.

The Aurora Borealis, or Northern Lights (Aurora Australis in the southern hemisphere) is also a manifestation of the ionosphere. It is produced when atoms and molecules which are excited (that is, put into a state of higher energy) by collision with particles from the Sun decay to their original state by emitting light.

Charles N. Duncan

Bibliography

Slanger, T. G. (1994) Energetic molecular oxygen in the atmosphere. Science, 265 (5180), 1817–18.