the region of the earth's atmosphere between the stratosphere and the exosphere, consisting of several ionized layers and extending from about 50 to 250 miles (80 to 400 km) above the surface of the earth.

1925–30; iono- + -sphere

ionospheric [ahy-on-uh-sfer-ik] , adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
ionosphere (aɪˈɒnəˌsfɪə)
D region E region See also F region a region of the earth's atmosphere, extending from about 60 kilometres to 1000 km above the earth's surface, in which there is a high concentration of free electrons formed as a result of ionizing radiation entering the atmosphere from space

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American Heritage
Science Dictionary
ionosphere  [%PREMIUM_LINK%]     (ī-ŏn'ə-sfîr')  Pronunciation Key 
A region of the Earth's upper atmosphere, extending from a height of 70 km (43 mi) to 400 km (248 mi) and containing atoms that have been ionized by radiation from the Sun. The ionosphere lies mostly in the lower thermosphere and is subdivided into three regions, the D region (70 km to 90 km; 43 to 56 mi), the E region (90 km to 150 km; 56 to 93 mi), and the F region (150 km to 400 km; 93 to 248 mi). The concentration of ionized atoms is lowest in the D region, intermediate in the E region, and highest in the F region. The ionosphere is useful for radio transmission because radio waves, which normally propagate in straight lines, are reflected off the ionized gas particles, thereby being transmitted long distances across the Earth's curved surface. See more at D region, E region, F region.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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American Heritage
Cultural Dictionary
ionosphere [(eye-on-uh-sfeer)]

A region of the atmosphere that begins at an altitude of about thirty miles.

Note: In this region, free particles carrying an electrical charge, atoms ionized (see ionization) by radiation from the sun, reflect radio waves. “Bouncing” radio waves off the ionosphere makes communication possible over long distances of the surface of the Earth.
The American Heritage® New Dictionary of Cultural Literacy, Third Edition
Copyright © 2005 by Houghton Mifflin Company.
Published by Houghton Mifflin Company. All rights reserved.
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Example sentences
Those charged particles can excite atoms in the ionosphere, which emit light as
  they return to their unexcited state.
Electric currents in the ionosphere induce electric currents in the ground and
  in pipelines.
Mantle is responsible for sustaining our air, water and our atmosphere and
But the ionosphere can cause slight and random delays, introducing error into
  the receiver's calculations.
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