What is "Airglow"?

update: 17 Nov 2015

Among the many recent Google hits [the number changes too much to be repeated here] on "airglow definition", too many are unfortunately misleading by trying to give too much wrong (and too little correct) information. Even some photos by NASA purportedly showing airglow (and the full moon) do in fact show nothing like that (except the moon), but only Rayleigh scattering from the atmosphere: the "blue sky". The proportion of "rare words" Google hits from dictionary sites has probably increased, where some are honest enough to admit they don't really know, like one site from Sweden saying "Ledsen! I ordlistan "airglow" inte funnit förklaringen!".

1. Short definition

Luminous layer(s) in the upper earth atmosphere, in the visible and infrared.
See photo taken by the Clementine spacecraft (with a monochrome star sensor; coloring is artificial!; those low light levels below the threshold of colour vision should be painted "gray"...). Recently, a true colour photograph taken by ISS crew #28 may have been the first really good airglow image from earth orbit, which even allows to distinguish different colours, according to height in the mesopause region. Since exposure conditions had to accomodate bright foreground features, this may be why the airglow appeared as a fine line, without the luminous veil down to the horizon caused by parts of the airglow layer at different distance than the tangent point.

2. A very precise definition

had been given by the American Meterological Society (although the link has changed; but beware: the explanation for the origin of OH is not valid for the mesopause region, where the airglow comes from; the AMS is quite clear about that. For the upper atmosphere OH and O2 reaction, see below, item 6!)

3. An airglow student's definition (Jürgen Scheer)

Airglow is the emission of light by atoms and molecules excited through chemical processes in the upper atmosphere.

This does not include phenomena like
   aurora, where the excitation stems (directly or indirectly) from the collision by charged particles precipitating along geomagnetic field lines at high latitudes,
   sunlight scattered       by atmospheric neutral gas (the Rayleigh scattering which makes the sky look blue), or
                                   .....by interplanetary dust (zodiacal light),
    Cerenkov light and other emissions due to relativistic particles from cosmic radiation hitting the atmosphere, and the
   diffusion of city lights in the troposphere (occasionally called "sky glow"),
   stellar background, and whatever else.

That is, airglow is essentially due to chemoluminescence, the conversion of chemical energy into light (ultraviolet, visible or infrared). It is not a "glow" like the thermal emission by an incandescent lamp, but "cold" light like from a Light Emitting Diode!

The source of the chemical energy is ultraviolet sunlight, which photodissociates molecular oxygen into individual atoms, during daytime.

Atomic oxygen cannot efficiently recombine, so that it has a very long lifetime in the upper atmosphere. Therefore, atomic oxygen represents a store of the chemical energy that powers the airglow during the night.

Different chemical reaction chains are responsible for the production of the excited states of atmospheric species that are the sources of airglow. These species include atoms (atomic oxygen, sodium) and neutral molecules (OH, O2, NO). (Ions like N2+ have also been considered as airglow emitters, but since precipitating electrons are involved in their excitation, those emissions should rather be classified as aurora).

While mentioning ions: airglow OH must not be confounded with hydroxyl anions OH- that exist in aqueous solutions.

There are considerable photochemical differences between sunlit and dark conditions in the atmosphere, so that the distinction between dayglow and nightglow goes beyond technical differences in observing conditions (observation requires special measures for sunlight rejection, during daytime). Nightglow has much more practical importance than dayglow, so that we here focus on nightglow, when talking of airglow. On the other hand, nightglow can not be used synonymously to airglow, because dayglow and twilightglow must not be ignored.

The distinction between airglow emissions that are permanently present all over the globe, on the one hand, and polar lights (aurora) on the other hand, which are only occasionally observed outside their high latitude "home", was only made in the 1920s by Robert John Strutt, the fourth Baron Rayleigh. Lord Rayleigh talked of "permanent aurora", and the term "airglow" was introduced later by Otto Struve and Sidney Chapman (so the rumour goes; however, Chapman himself attributed the expression to C.T. Elvey [The Rayleigh Archives Dedication, J.N. Howard, ed., AFCRL Special Report 63, 1967]).

Molecules of OH and O2 were identified by Meinel (in 1950) as the sources of the most important airglow emissions with bands that together cover much of the visible and near infrared spectrum.

It was later established that the OH emissions arise from a narrow layer centered around an altitude of about 87 km, while the O2 emissions come from 95 km.

The relative narrowness of the emission layers (6-10 km full width at half maximum) is mainly a consequence of the variation of atmospheric density with height:
below the height of maximum emission, collisional de-excitation (quenching) increases with rising density, and
above the emission maximum, the emitting species concentration drops, as density goes down (in fact, even faster than that, if there's a decrease in the mixing ratio of the emitting species).

Density follows an exponential law with a scale height (corresponding to a 1/e change) of about 6 km, at the height of these airglow emissions (because temperature is of the order of 200 K, far below the 300 K at ground level where scale height is 8 km).

Nowadays, airglow studies are regularly performed at many observing sites of the world, as well as from orbiting space platforms, in order to elucidate the causes of variation of the state (including temperature) of the upper atmosphere.

Both, OH and O2 airglow brightness varies by more than an order of magnitude, in such an irregular way, and without an obvious relation between both, so that predictions would be risky.

The known causes of variation (and especially, their mutual interactions) are still not sufficiently well understood to allow more than probabilistic forecasts (based on long series of observations, like those we have from El Leoncito), e.g.,
"high O2 intensities occur mainly in April",
"high OH intensity is most often seen in mid-winter".

The presence of quasi-periodic oscillations can only be stated after the fact ("we had good tidal signatures, or monochromatic gravity waves, last night!"), but we can't be sure how things will turn out, the next night.

While airglow is a valuable source of information about upper atmospheric dynamics, and therefore useful for aeronomy, it is mainly a nuisance for observational astronomy. Except that airglow features may help in wavelength calibration of astronomical spectroscopy (they are not nearly as much Doppler shifted as astronomical spectra often are).

Airglow can overwhelm astronomical signals, especially in the infrared, and sophisticated filtering schemes are being developed for airglow supression.

Therefore, airglow affects the suitability of sky conditions for astronomy, and it is unfortunately not true that its effect can be reasonably quantified in terms of effective stellar magnitude per unit area. This is because of the amount and irregularity of airglow variations, and the different behaviour of different airglow emissions.
Some emissions vary considerably with solar activity (those originating at greater thermospheric heights, like the 630 nm line, or the thermospheric contribution to the 558 nm line, both due to atomic oxygen) and therefore have an 11-year modulation, but some do not at all, like OH (at least at low-latitude sites like El Leoncito).

4. Notation issues

OH is a molecule, but it would not be stable in the laboratory. This is why chemists call it a "radical": it would very much prefer to combine into hydrogen peroxide HOOH, if there was a chance. But since the peroxide reaction is exothermic, the excess energy must be removed unless the newly formed molecule flies apart. Because of the low ambient density, the three-body collison necessary for such an energy removal is unlikely, in the mesopause region.
The tropospheric OH produced by photodissociation of water vapour (and other processes) can only survive in the electronical and vibrational ground state, so that it does not emit, but can be observed by absorption spectroscopy.

Sometimes, the OH responsible for upper atmospheric airglow is therefore written as OH*, where the star superscript signals an excited state. This is still the electronic ground state (a doublett-Pi), but it is vibrationally excited (at a vibrational level between 1 and 9).

Different vibrational transitions lead to different emission bands. Each band corresponds to a series of rotational transitions, occupying a certain range of wavelengths in the visible or infrared.

Strictly speaking, O2 is also unable to emit (if it's in the electronic ground state). A symmetrical diatomic molecule (in ground state) lacks a dipole moment and therefore does not couple (strongly) to the electromagnetic field, that is, it is "spectroscopically invisible" (unless we excite it electronically into a state with non-zero dipole moment).

The O2 airglow we are talking of originates from such electronically excited states (for which the star superscript notation is not in use; the spectroscopic notation for the specific excited state is used, instead). However, such a molecule would not produce airglow visible from the ground, if it remained in the vibrational ground state, because of massive absorption by near-ground-level oxygen (which, as everybody knows, forms 21% of dry tropospheric air; it's a major constituent). Therefore, O2 transitions that lead to airglow observable from the ground must involve a change of vibrational level. The O2b(0-1) band is such a case, because it can only be re-absorbed by oxygen molecules in the first vibrational level, but in the lower atmosphere there is none.

Atomic oxygen is only capable of electronic excitation. It cannot vibrate, because with respect to what? Rotations are its private affairs without physical consequences, just like a point that rotates might get dizzy, but for an external observer that doesn't make a difference ;-).

Some people seem to believe that iodine is involved in OI and NaI (forming a diatomic molecule like OH), but they have fallen into another notation trap: the O I 558 nm line (for example) is due to a plain solitary oxygen atom. The Roman number I is the ionization level +one, in spectroscopic notation. For example, a spectral line in the solar corona labelled Fe XIII is due to a 12-times ionized iron ion (don't read this aloud!). That means, O I is oxygen with ionization level zero, or neutral oxygen (not an ion). This is all a late collateral damage of the missing zero in the Roman number system... ;-)

5. Frequently Questionable Answers

Many, many web sites try to explain airglow, often with too little success (see, e.g., wikipedia in English or, worse, in Spanish ; both variants had until recently been "adorned" by artist's views of dayglow and nightglow which pretended to be photographs. Latest ISS photos are now so popular that the illustration problem has gone away, but not the explanation problem). They frequently succeed in stating what airglow is not:

Airglow cannot be excited during daytime and emit during the night, because a night lasts in the order of 40 thousand seconds, while optical lifetimes of excited atoms and molecules range from sub-nanoseconds to hundreds of seconds. The excitation would have decayed completely before night falls!

Atomic oxygen is not the most important emitter of airglow. It is directly responsible for only a few airglow emission lines (at 558, 630, 636 nm, also present in the aurora).
However, it plays a very important part in the airglow game, as a source and store of chemical energy:
Solar ultraviolet radiation is converted into chemical energy during daytime by dissociating oxygen molecules, in the thermosphere. This process is also the reason for the positive temperature gradient in the thermosphere, and the high thermospheric temperatures.
The altitude in the thermosphere with maximum solar heat input from this mechanism is exactly where the second derivative of the vertical temperature profile vanishes, but not, where the maximum thermospheric temperature is reached (which is only very much higher up, in the exosphere, where absorption is negligible).

But, isn't airglow faint?
What is faint? Unless you clearly state what you are comparing with, there is no use saying airglow is faint! Several megarayleighs is a lot of light, the same order of magnitude as integrated star light! It can easily be measured. But, no doubt, sunlight is much brighter. Would you like to say that "star light is the faint light emitted by far-away celestial bodies"?

Isn't airglow "more ore less stable"?
Same argument: more or less than what? It is certainly not stable enough that you could take its typical intensity for granted. Its brightness is occasionally quite low (compared to average), but its maximum possible brightness is unknown (values about 15 times minimum occur several times per year). There have been reports of airglow events visible to the unaided eye!

The OH responsible for Meinel band airglow is not produced by photodissociation of water vapor, but by a reaction between ozone and atomic hydrogen (the Bates-Nicolet process).
This reaction supplies the energy to boost OH into as high as the ninth vibration level, from where the excitation "cascades down" (by photon emission and excitation exchange through collisions) to result in OH molecules with lower vibrational quantum numbers.

6. The chemical reactions

leading to the OH Meinel and O2(0-1) Atmospheric airglow bands from the mesopause region

7. Illuminating digression

Another phenomenon, familiar to everyone, has nevertheless a history of incomplete understanding and public appreciation similar to that of airglow: fire (example: the common camp fire). Fire only develops naturally under the influence of gravity. Gravity controls the transport of combustible gases into the flame, and of hot combustion products out of it, so the process can be sustained. This is not unlike the effect that gravity has on confining airglow to certain atmospheric heights...

    1. current explanation over-simplified (see some of the 2.6 million Google hits on "what is fire"). Many more people seem to have strong opinions about fire than there are opinions about airglow, but if they all agreed, there would be no need for so much internet presence. For example, flame colours are not simply a temperature effect, as some opinioners claim (otherwise a blue flame would need to have an unreasonably high temperature: not even the hottest stars are as blue as some flames! More precisely: a purely thermal spectrum would not be as blue as flames occasionally are. You can see in that picture that even infinite temperature would only lead to some whitish blue, not the deep blue to the far lower left).
    2. flames contain OH emission bands (see G.H. Dieke and H.M. Crosswhite, J. Quant. Spectrosc. Radiat. Transfer 2, 97-199, 1962: "...there can be no doubt that the neutral OH radical is the emitter"). That's natural, since the so-called "carbohydrates" are no hydrates at all. These molecules (like cellulose) do not contain H2O but HCOH groups.
    3. depends on gravity. Without the orderly gas transport imposed by gravity, maintaining a fire under zero gravity is tricky, because oxygen cannot diffuse fast enough into the area of hot gas (the flame is electrically conductive; it's a plasma!) to keep the fire burning. Buoyancy of hot gas is necessary to remove combustion products from the reaction zone.
    4. and, believe it or not, there are people who claim that the plasma in a flame is "an unfamiliar state of matter". Homo sapiens and our predecessor ape men have been in the habit of gathering around campfires and looking into the flames, for millions of years. Shall we really believe that we still have not familiarized ourselves with the stuff? Familiarity is not the same thing as understanding but, alas, often counterproductive to understanding!