From AMS Glossary
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.