A weak background emission of light over the whole sky, resulting mainly from the excitation of atmospheric oxygen at an altitude of around 100 km by solar radiation; also known as nightglow. Airglow emissions vary with the time of day. Green oxygen emission at a wavelength of 557.7 nm predominates at night. Sodium and red oxygen emissions are prominent in the twilight airglow. Airglow is also produced during daylight, and is 1000 times as intense as at night.