D Region

In solar-terrestrial terms, a daytime layer of the earth's ionosphere approximately 50 to 90 km in altitude.

That portion of the ionosphere existing 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.