The D region is the portion of the ionosphere that exists approximately 50 to 95 km above the surface of the Earth.
Note: Attenuation of radio waves, caused by ionospheric free-electron density generated by solar radiation, is pronounced during daylight hours. Because solar radiation is not present at night, ionization ceases, and hence attenuation of radio waves ceases.
Source: from Federal Standard 1037C and from MIL-STD-188