d-region

D region

noun
the lowest region of the ionosphere, in which the D layer develops.

Origin:
1925–30

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Collins
World English Dictionary
D region or D layer
 
n
See also ionosphere 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
 
D layer or D layer
 
n

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
D region  
The region of the atmosphere that lies in the upper mesosphere, extending from about 70 to 90 km (43 to 56 mi) above the Earth. The D region is responsible for the daytime absorption of high-frequency radio signals; at night, oxygen ions recombine with free electrons and form oxygen molecules that are transparent to such signals, which bounce off of the E region and back to Earth. Also called D layer. Compare E region, F region.

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