Definition of ionosphere
: the part of the earth's atmosphere in which ionization of atmospheric gases affects the propagation of radio waves, which extends from about 30 miles (50 kilometers) to the exosphere, and which is contiguous with the upper portion of the mesosphere and the thermosphere
also
: a comparable region of charged particles surrounding another celestial body (such as Venus)
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Merriam-Webster unabridged
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Time Traveler for ionosphere
The first known use of ionosphere was in 1926
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