DEFINITION OF IONOSPHERE

An electrified region in the upper atmosphere

where a large number of ions and free electrons exist. The

ionosphere is above approximately 37–50 miles (60–80 km)

and extends to the outer reaches of the atmosphere. It

absorbs much of the harmful ultraviolet radiation and highenergy

particles from the Sun, protecting life on Earth from

their ill effects. Some of the ultraviolet radiation that passes

through this region of the atmosphere is absorbed by the

ozone layer. The ionosphere forms where high-energy particles

from the Sun begin colliding with molecules in the

Earth’s atmosphere, splitting some electrons off their

molecules, forming ions. The ionosphere has both positively

charged ions that have lost electrons and negatively charged

ions that have absorbed electrons.

The auroras are produced in the ionosphere when atmospheric

ions charged by interaction with solar particles decay

to their former state, emitting light in the process. The ionosphere

also plays a major role in radio communications. The

lower part of the ionosphere reflects AM radio waves, while

absorption in the daytime weakens the signal. At night, the

radio waves penetrate more deeply and are not strongly

absorbed, so at night AM radio signals are able to travel much

farther than in the day. FM radio waves have a shorter wavelength

than AM waves and are able to penetrate the ionosphere

without reflection. See also ATMOSPHERE; AURORA.

island arc See CONVERGENT PLATE MARGIN PROCESSES;

JAPAN’S PAIRED METAMORPHIC BELT.

isograd See METAMORPHISM.

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