D-region meaning

The lowest atmospheric zone within the ionosphere, at an altitude of c. 55 to 90 km (c. 34 to 56 mi), containing the D layer.

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.