AIRGLOW

faint luminescence of the Earth's upper atmosphere that is caused by air molecules' and atoms' selective absorption of solar ultraviolet and X-radiation. Most of the airglow emanates from the region about 70 to 300 km (42 to 180 miles) above the surface of the Earth. Unlike the aurora, airglow does not exhibit structures such as arcs and is emitted from the entire sky at all latitudes at all times. The nocturnal phenomena is called nightglow. Dayglow and twilight glow are analogous terms. Nightglow is very feeble in the visible region of the spectrum; the illumination it gives to a horizontal surface at the ground is only about the same as that from a candle at a height of 91 m (300 feet). It is possibly about 1,000 times stronger in the infrared region. Observations from the Earth's surface and data from rockets and satellites indicate that much of the energy emitted during nightglow comes from recombination processes. In one such process, radiant energy is released when oxygen atoms recombine to form molecular oxygen, O2, which had originally become dissociated upon absorbing sunlight. In another process, free electrons and ions (notably ionized atomic oxygen) recombine and emit light. In the daytime and during twilight, the process of resonance scattering of sunlight by sodium, atomic oxygen, nitrogen, and nitric oxide seems to contribute to airglow. Moreover, interactions between cosmic rays from deep space and neutral atoms and molecules of the upper atmosphere may play a role in both the nocturnal and daytime phenomena in the high latitudes.