It never gets truly dark at night because of an effect called airglow. Airglow is similar in color to the aurora, but you don’t have to visit a polar region to see it. While the aurora is light released by the interaction between magnetosphere and the solar wind, airglow is a form of chemiluminescence. Even without light pollution, the best terrestrial telescope peers at space through a thin veil of light because the atmosphere glows in the dark!
•Airglow or nightglow is a type of chemiluminescence that makes the atmosphere glow.
•Light is produced when atoms and molecules in air absorb radiation and release photons.
•The most common color of airglow is green, but red and blue also occur.
Swedish physicist Anders Ångström first described airglow in 1868. Laboratory experiments verified chemical reactions between gases in air release light. The energy for the reactions comes from cosmic rays and photoionization of gases during daytime. While airglow is sometimes called nightglow, it’s present on both the day and night sides of the planet. In fact, the glow is about a thousand times brighter when the Sun is shining on the atmosphere. Because the glow at night is dim, it’s best viewed in an area free of light pollution.
Airglow Color and Causes
The most common airglow colors are green, red, and blue. However, other colors also occur. The colors are characteristic of photochemical reactions occurring at different levels of the atmosphere. Most of the effect comes from oxygen in air.
Green light (wavelength 558 nm) is the brightest airglow. It comes from excited oxygen atoms located between 90 to 100 km (56 to 62 miles) high. This band of green is readily observable from spacecraft looking back toward earth.
Blue airglow comes from excited molecular oxygen (O2) around 95 km high. The blue band is weaker than the green one, but it also observable from space.
Red airglow comes from atomic oxygen (O) excitation at 150 to 300 km.
Sodium atoms release yellow light in an atmospheric layer found at 92 km.
OH radicals found in a layer around 86 to 87 km high emit red and infrared light.
While the intensity of airglow mainly depends on whether it’s day or night, it also varies according to the 11-year solar cycle. Airglow is brighter near the solar maximum.
How to See Airglow
If you live in an area with a dark night sky, you may be able to see airglow after you give your eyes time to adjust to the darkness. Blue often appears as a faint blue wash to the night sky. Red appears as a faint glow that resembles the light dome over a city. Green appears like a faint glow from an aurora, but if you see it at the lower latitudes, it’s probably airglow.
Across most of the world (except within a bright city) you can photograph nighttime airglow. To see it, use a camera set to a long exposure (20 to 30 seconds) and the widest possible aperture. While success is practically guaranteed using a good digital camera with a fast lens, airglow may also be observed using a smart phone or GoPro set to night mode.
Airglow viewed from the Earth’s surface often has a rippled appearance or forms rays. This is due to gravity waves in the atmosphere. The effect is particularly easy to spot in a long exposure photograph.
Airglow From the International Space Station
The International Space Station (ISS) has an ongoing view of aurora and airglow. The airglow appears as the green limb along the arc of the Earth’s curve and sometimes as a blue glow closer to the surface.
Airglow on Other Planets
Earth isn’t the only world with airglow. The Venus Express spacecraft detected near-IR light from the upper atmosphere of Venus. The light comes from the interaction between radiation and molecular oxygen and nitric oxide (NO). Ultraviolet emissions were also detected.
NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) probe photographed nightglow on Mars. The glow is in the ultraviolet region of the spectrum and caused by nitric oxide emission.
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