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American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(General Physics) the lowest region of the ionosphere, extending from a height of about 60 kilometres to about 90 kilometres: contains a low concentration of free electrons and reflects low-frequency radio waves. See also ionosphere
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014
the lowest region of the ionosphere, at an altitude of ab. 50 mi. (80 km).
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|Noun||1.||D region - the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio waves|
ionosphere - the outer region of the Earth's atmosphere; contains a high concentration of free electrons
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