Airglow: all you need to know about this firework lighting up our night sky

Have you ever wondered why the background night night sky isn't pitch black at night, much like in space? Here are all the answers to unravel a strange and mind-boggling phenomenon!

Also called nightglow, airglow is quite common all over the world and above our heads. During the day, the atoms of the upper atmosphere (mainly mesosphere and thermosphere) receive a high dose of short-waved electromagnetic radiations that bombard the atoms with high energy. Most of these radiations, starting with the most energetic ones are absorbed by the atoms at their fundamental state (little dense and very cold matter). Only the near-visible UV's (of wavelength between 300 and 400nm) can pass the barrier and strike the surface of the Earth with all living things on it.

© Adrien Mauduit

The electrons of the low-energy atoms only absorb an electromagnetic radiation if it corresponds to one of its quantified energy transition, meaning the energy required for an electron to go from an energy level to another. Each atom has its own unique and quantified levels of energy and absorbing properties. In the picture above I took the example of an oxygen atom that absorbs a short wavelength UV radiation during the day. Because it has received energy, the electron is partly released from the electromagnetic interaction that kept it bound to the nucleus (protons), and is 'flanked' out to an outer shell corresponding to a higher energy level. We say that the electron, and thus the atom, has gotten to an unstable 'excited' state.

© Adrien Mauduit

Since mother nature always aims at coming back to a stable state, the atom will try and dissipate out the accumulated energy in any way it can. There are two major ways:

- The first one is to simply fall back to an inner shell with at a lower energy state. This transition causes the electron to lose some energy in the form of an electromagnetic wave and a photon (quantum of light) with a wavelength dependent of the level of energy lost. Some of them will be visible in the form of colored light if the wavelength is situated in the visible spectrum (400-800nm), and some of them are invisible (UV or IR).

- The second is to bump into other molecules, atoms and radicals, transferring the energy either in the form of a transfer of electrons (ionization), or electron sharing (formation of molecules such as Nitrogen oxide NO), exciting the electrons of the entities. In turn these entities will also eventually get rid of this energy surplus by emitting photons or continue the chain reaction and collide with other entities. The mesosphere, ionosphere and thermosphere are places where highly energetic, fundamental and intricate chemical reactions take place under the influence of the radiations from the sun. It is thus normal that we can observe phenomena such as airglow, noctilucent clouds, aurorae...

© Adrien Mauduit

Most of the energy absorbed by the atoms and molecules of the upper atmosphere is released shortly afterwards, still during the day. This light is then too faint for us to see, because the light from the sun washes it away. However, because of its chain reaction nature, and because the transfers of energy are weirdly enough very long for the atomic scale standards (>100s sometimes), the emission of light is also delayed. Some of the atoms and molecules, because of their configuration, retain some of the energy for several hours and can release it after nightfall. At night though, the energy left to release is up to a thousand times less than that during the day, producing only a very faint glow. Our eyes cannot pick up the colors (only rarely when the glow is very strong), but can definitely tell that our night sky is far from being pitch black. Part of the light comes from light pollution, but even when you travel to the darkest areas, the grayish glow lingers... On the other hand, most mirrorless and DSLR can not only catch the light, but also the faint colors of airglow because they are more light-sensitive than our retinas.

Depending on the nature of the chemical species concerned, its electrons, the amount of energy received and released, the atmosphere will produce different colors that can be identified using an atomic ID: the emission spectrum:

Emission spectrum © Adrien Mauduit

Below the mesosphere (>80km), the atmosphere is denser, but the most of the concerned UV's have been absorbed with the entities above, so no significant airglow can be produced there. From 80 to 100 km of height, sodium atoms and compounds produce a yellow-orange glow. There, OH radicals can also emit in the faint reds and invisible IR. The brightest and predominant emission is green (at 558nm) produced by the single oxygen atoms around 90-100 km, visible from both ground and orbit. With sodium airglow, green airglow is often very well spotted from the International Space Station: