From Glossary of Meteorology
The lowest region of the ionosphere.
The term is used somewhat loosely to describe the ionization, beginning about 70 km and merging with the E-region, that does not usually produce an echo on normal ionosonde recordings. The main effect of the D-region on radio waves is one of absorption, thus inhibiting long- distance propagation of HF and VHF radio waves in daytime, when D-region ionization is most intense. At low and middle latitudes, the D-region is produced mainly by the action of solar radiation on nitric oxide (NO). At high magnetic latitudes energetic particles of solar or auroral origin may be the principal source, in which case radio waves can be strongly absorbed at all times of day. The term D-layer is used occasionally by analogy with the higher E- and [[F<INF>1</INF>-layer|F-layers]], which produce sharply defined echoes on ionosonde recordings.
Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact firstname.lastname@example.org. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.