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

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Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "D region". Image Credit.