Among the approximately 61 thousand recent Google hits 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". That the number of Google hits has now hit a new record (more than 200 thousand) is not necessarily a sign of more interest in the topic but possibly only a growing interest in "rare words". Citation needed? I cite the Bee Gees, "it's only words...", and J.W. Goethe, "mit Worten laesst sich's trefflich streiten, mit Worten ein Systhem bereiten...". At any rate, we should try to clarify which "Begriff muss bei dem Worte sein"...
This does not include phenomena like
aurora, where the excitation stems (directly or
indirectly) from the collision by 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), molecules (OH, O2, NO), and even some ions (like N2+).
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 may 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; I keep my fingers crossed, because statements about "firsts" often prove wrong when the original sources -unaccessible to me- are consulted).
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 are impossible.
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
"high O2 intensites 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 ("Ha, 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).
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 there is
none, in the lower atmosphere.
Atomic oxygen is only capable of electronic excitation. It cannot vibrate, because with respect to what? Rotations are its private affairs without physical comsequences, just like a point that rotates might get dizzy, but an external observer won't ever notice ;-).
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... ;-)
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 are now "adorned" by artist's views of dayglow and nightglow which pretend to be photographs...). 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 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! It can easily be
measured. But, no doubt, sunlight is much brighter.
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.
Similarities:
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: even at infinite temperature, a purely thermal
spectrum would
not be so blue).
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 H-C-OH 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 ("flame") to keep the fire
burning.
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!
