Definitions for "D Region"
In solar-terrestrial terms, a daytime layer of the earth's ionosphere approximately 50 to 90 km in altitude.
A daytime layer of the Earth's ionosphere approximately 50 to 90 km in altitude. The D layer is effective as a reflector only for frequencies below VLF. It is the primary cause of absorption for signals in the HF band.
the lowest region of the ionosphere (35 to 50 miles up) that reflects low-frequency radio waves