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Man continues to extend his activities into more distant layers of the atmosphere. Of particular interest in environmental science are advances in use of communications satellite and remote sensing instruments, which scan the earth's surfaces. Certain physical phenomena of the outer atmospheric region are thus of importance.

Of particular interest in the development of radio communication on a global scale is a layer known as the ionosphere, located in the altitude range of 50 to 250 miles (80 to 400 km). The ionosphere is essentially identical in position with the lower thermosphere. The ionosphere consists of a number of layers in which the process of ionization takes place.

Here highly energetic gamma rays and X-rays from the solar radiation spectrum are absorbed by molecules and atoms of nitrogen and oxygen. In the absorption process, each molecule or atom gives up an electron, becoming a positively charged ion. The electrons thus released form an electric current that flows freely on a global scale within the ionosphere. Of particular interest in the geography of radio communication is the ability of the layers of ions to reflect radio waves and thus to turn them back toward the earth. Most of the important reflection of long-wave radio waves takes place in the lower part of the ionosphere, which bears the name of Kennelly-Heaviside layer. Without such reflection, long-distance radio communication would not be possible. Because the process of ionization requires direct solar radiation, the ionospheric layers, of which there are five, are developed on the sunlight side of the earth. On the dark side, under nighttime conditions, the layers tend to weaken and disappear.

Yet another phenomenon, one of vital concern to Man and all other life forms on earth, is the presence of an ozone layer largely occurring in the region from 12 to 21 mi (20 to 35km) elevation, but also extending upward to an elevation of 30 to 35 mi (50 to 55km). The ozone layer thus extends from the upper stratosphere into the mesosphere. The ozone layer is a region of concentration of the form of oxygen molecule known as ozone (0³), in which three oxygen atoms are combined instead of the usual two atoms (0²). Ozone is produced by the action of ultraviolet rays upon ordinary oxygen atoms. The ozone layer thus serves as a shield, protecting the troposphere and earth's surface from most of the ultraviolet radiation found in the sun's radiation spectrum. If these ultraviolet rays were to reach the earth's surface in full intensity, all exposed bacteria would be destroyed and animal tissues severely burned. Thus the presence of the ozone layer is an essential element in man's environment. It is also interesting to note that the high temperatures of the mesosphere are produced by the absorption of the ultraviolet rays in the upper part of the ozone layer.

Environmental importance of the magnetosphere to man and all other life forms at the earth's surface lies in the shielding action of the lines of force of that magnetic field. Should the earth's magnetic field weaken or disappear, as it may have done for short periods in the geologic past, the intensity of ionizing radiation would be increased, with possible important effects upon the genetic materials of organisms. It has been speculated that periods of rapid evolutionary change, including extinction of species, have been bought about by weakening of the magnetic field at time when the polarity of the entire field has been reversed.

Thus we see that while physical geography is aimed at understanding man's physical environment close to the earth's surface, it has been necessary to extend our inquiry far into outer space in search of forces that determine quality of surface environment.

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