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Have you noticed a characteristic green glow at Earth’s limp seen from space? That’s light emitted from exited atoms in the upper atmosphere. Green colour comes from oxygen atoms and there are other colours as well. But first take a look at the detail in the featured image – the astronaut looking through International Space Station’s window.

The green line in the photograph marks the boundary where space begins. Below it, lies the layer of air surrounding our planet, our Atmosphere.

How thick is this protective layer? About 100 km. That boundary is very close over our heads, in other words: “Space isn’t remote at all. It’s only an hour’s drive away if your car could go straight upwards” (Sir Fred Hoyle, British astronomer).

Image Credit: NASA, Expedition 28

What causes atoms in our atmosphere to emit light? Airglow occurs when atoms and molecules in the upper atmosphere, excited by extreme ultraviolet radiation from the sun, emit light to shed their excess energy. Another cause is chemiluminescence, when exited atoms and molecules collide and react chemically emitting light in the process.

Airglow comes in a variety of colours in various altitudes. Watch the video below to see it in green, red, yellow and blue.

Time-Lapse View of Earth’s Limb from the ISS, watch the different colors of airglow, green, red, yellow and blue.

Let’s try to explain the creation of all those colours.

The major effect of Sun’s ultraviolet radiation in the upper atmosphere is to break oxygen molecules (O2: two oxygen atoms bound together). The energy splits the molecules apart into individual atoms and these atoms have a little bit of extra energy — we say they are in an excited state. So they want to give off this extra energy and they can do this in two ways: they can either emit light or lose their energy by collisions, bumping into other atoms and molecules.

An oxygen atom if undisturbed can shed its extra energy by emitting green light in less than a second after becoming excited. Or by emitting red light but on much longer timescales, like minutes.

Up to an altitude of 95 km the atmosphere is thick enough that collisions between atoms happens all the time (in microseconds). So oxygen exited atoms don’t have time to emit light, because when they get exited it’s not long before another atom or molecule bumps to them.

But at heights of 95 – 100 km or so, collisions happen less frequently, giving the oxygen atom time to emit a green photon. So that’s the height we see the green glow.

Further up the oxygen atoms are much farther apart because the density is lower. So time between collisions can be pretty long, long enough to give the oxygen atoms time to emit red photons. That’s why we see that red glow higher up, at 150 – 300 km where collisions are so infrequent that the excited atoms have time to radiate away their energy in the red part of the spectrum.

The region of Earth’s atmosphere where atoms get ionized from solar radiation is called Ionosphere. Ionosphere pass the 100 km mark (the green line in the picture below) and extends into space.

Airglow different colours. You can find a detailed explanation of the airglow spectrum at the ιστοχώρο Atmospheric Optics.

Image Credit: Alex Rivest

Below the green line is a somewhat fuzzier yellow glow in a layer at 92 km. It turns out that’s from sodium, which emits yellow light when excited. The sodium source is meteors, tiny rocks from space burning up in our upper atmosphere and leaving sodium behind.

A fainter blue line lies at an altitude of 95 km. That’s light emitted from molecular oxygen. When two oxygen atoms recombine to form an O2 molecule, it has a bit of residual energy left over, and it can get rid of that by emitting a blue photon.

There is also a narrow faint red layer centered at 86-87 km, from vibrationally and rotationally excited OH radicals emitting red and infra-red light. That is probably the red colour in airglow photographs taken from the surface of Earth (and not the exited oxygen red at 150-300 km). OH radicals are produced when oxygen atoms react chemically with hydrogen and nitrogen in the atmosphere (chemiluminescence).

From a remote dark location airglow is visible to the eye, but not in color. Sensitive digital cameras reveal its color and eerie Atmospheric Gravity Waves. See the photo from Yuri Beletsky (Carnegie Las Campanas Observatory, TWAN)

Because of airglow night sky is never fully dark. If you removed light pollution, the moon, stars and galaxies, there would still be a very faint colorful glow. From a remote dark location airglow is visible to the eye, but not in color. Sensitive digital cameras reveal its color and eerie Atmospheric Gravity Waves, oscillations in air analogous to those created when a rock is thrown in calm water. Gravity waves are triggered from disturbances far below in the troposphere, for example wind flow over mountain ranges or violent thunderstorms.

Another photo of colourful Airglow over the Azores from Miguel Claro (TWAN)

The airglow phenomenon is similar to auroras, but where auroras are driven by high-energy particles originating from the solar wind and confined to the poles, airglow is energized by ordinary, day-to-day solar radiation and seen allover the globe.

Konstantinos Sakkas