optics of the upper atmosphere

Optics of the Upper Atmosphere

D. M. Hunten

Present knowledge of the upper atmosphere is reviewed, and the various absorption and emission phe-nomena are discussed. Topics included are: chemical composition, ionization, temperatures, aurora,night airglow, and twilight and day airglow. A short bibliography lists the standard reference works onthis material.

1. Introduction

The optical effects associated with the upper at-mosphere are conveniently classified as emission andabsorption. The most spectacular emission is aurora,but there is also a relatively steady component, the air-glow. The latter can be further divided into night, twi-light, and day airglows, or nightglow, twilightglow, anddayglow. Observations of these emissions can bemade from the ground, or in some cases only from out-side the lower atmosphere. The wavelength region ab-sorbed in the upper atmosphere is about 1-2000 A;these energetic photons produce ionization and chemicalchanges, and also heating. The last is important -eventhough the energy flux is small, because the density isalso small. The ionization is readily observed from theground, and is the agent of long-distance radio com-munication, but most of the other effects, and thesolar radiation itself, must be observed from rocketsand satellites.

The division between the lower and upper atmosphereis nebulous, and perhaps properly so. Phenomenacharacteristic of the upper atmosphere are ionization,changes in chemical composition, and light emission.These are also present near ground level, but canusually be ignored. The ozone layer, with its maxi-mum near 30 km, is located in the lower atmosphere butis conveniently treated along with other chemical phe-nomena characteristic of the upper atmosphere. Withthis exception, the upper atmosphere can be consideredto begin at a height of 50-70 km. Atmospheric no-menclature, and the run of temperature with height,are shown in Fig. 1.

The author is with Kitt Peak National Observatory, Tucson,Arizona, operated by the Association of Universities for Researchin Astronomy, Inc., under contract with the National ScienceFoundation.

Received 10 September 1963.Contribution No. 39 from the Kitt Peak National Observatory.

Much of this article is devoted to the physics andchemistry of the upper atmosphere, in order to form abackground for the later discussion of the various emis-sion phenomena. A number of excellent books, orchapters in books, are available and are listed in thebibliography at the end. The most relevant are Rat-cliffe' and Chamberlain 2 ; between them, they coverthe physics of the upper atmosphere and of the auroraand airglow rather thoroughly. The chemistry and theionization are discussed by Bates.3 The older work inthe subject is well reviewed by Mitra.4 Good discus-sions at a less technical level are given by Bates5 andMassey and Boyd6 ; the latter also consider the electriccurrents flowing in the ionosphere and their magneticand dynamo effects.

References to the original papers are omitted here,because copious bibliographies are already available insome of the booksl- 4 mentioned. iore recent ref-erences are hardly needed for most of the topics dis-cussed, since no very startling advances have been madein the last three years. However, the proceedings of a1962 symposium on theoretical interpretation of upperatmosphere emissions7 may be of interest, and somereferences on dayglow work are given in Sec. VII.

II. Chemical Composition

It is well known that the variation of atmosphericpressure with height z can be approximated by an ex-ponential law: p = p exp( -z/H), where p is thepressure at z = 0 and H is called the scale height. Foran ideal gas of mean molecular mass m at temperatureT, H is equal to kT/mg, where k is Boltzmann's con-stant and g the acceleration of gravity. The exponen-tial law holds strictly only if H is constant; in the realatmosphere this is not so, because T, m, and g all varywith height. But they vary slowly, and an exponentialexpression with varying H gives a convenient repre-sentation of the actual atmosphere. Occasionally, itis necessary to remember that under these conditions

February 1964 / Vol. 3, No. 2 / APPLIED OPTICS 167

500

400

300

200

IU

LU

100

00 500 1000

TEMPERATURE (K)

1500

Fig. 1. Atmospheric temperature and nomenclature. Thedetails of the temperature distribution above 150 km are some-what uncertain, and there are certainly large variations in the

temperature of the exosphere.

the density and the pressure vary with different scaleheights.

Up to about 100 km, the scale height varies withinthe range 6-8 km. At greater heights, a rapid increaseof T and decrease of m give an increasing H, which mayreach 100 km at a height of 800 km.

Apart from variations in water vapor, carbon dioxide,and other minor constituents, the lower atmosphere iscompletely mixed: that is, its composition does notchange with height. But this cannot continue indefi-nitely, because the rate of mixing is more or less inde-pendent of height, whereas the rate of diffusion increaseswith height in proportion to the mean free path. In astatic atmosphere, each gas would be distributed ac-cording to its own scale height, depending on its ownmolecular mass; thus, the heavier gases would "settleout". This is called the case of diffusive equilibrium,and contrasted with mixing equilibrium. For conve-nience of description, it is common to postulate a dif-fusion level, above which there is pure diffusive equilib-rium, and below which there is perfect mixing. Whenthe stratosphere was discovered by the first balloonflights, it was assumed that its base was the diffusionlevel, hence the name. For a long time it was thoughtthat above 100 km the atmosphere must be essentiallypure hydrogen. One of the first pieces of evidenceagainst this was the discovery of oxygen and nitrogenemissions from the aurora.

All efforts to observe a diffusion level failed untilrocket mass-spectrometer measurements finally placedit near 110 km. As we shall soon see, at this heightoxygen is mostly in the atomic form; its scale height is

larger than that of N2 by 28/16 = 1.75. This isenough to make the concentration of 0 atoms equalthat of N2 molecules by about 220 km (Fig. 2), and toexceed it tenfold at about 600 km. Somewhere be-tween 1000 km and 2000 km, H becomes the dominantconstituent (by numbers of atoms), and there is someevidence for a region of helium dominance in this range.The H is presumably produced from methane or watervapor near 80 km by solar ultraviolet.

Let us now consider the dissociation of 02 by sunlightand some of the resulting chemical reactions. In anoxygen-nitrogen atmosphere, the main effects will beproduction of atomic oxygen and ozone. Minor con-stituents, particularly hydrogen, may change theequilibrium appreciably, but we shall not consider themin detail. Dissociation of N'2 is never large, because thethreshold wavelength is so short that the intensity ofsunlight is insufficient.

Atmospheric 02 is dissociated by absorption in thecontinua of two band systems-the Schumann-Rungeand the Herzberg. The former has a threshold at1760 A and is responsible for the opacity of laboratoryair in this region. In full sunlight, the mean life of amolecule against dissociation by the Schumann-Rungecontinuum is 6 X 104 sec or 16 hr. The absorption isso strong that little of this wavelength region penetratesbelow 90-100 km, even though much of the oxygen is inatomic form (Fig. 3). Herzberg bands arise from a for-bidden transition (they are observed in the airglow:see Sec. VI) and have a dissociation threshold at 2420 A.

iiI-

1)

I

3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 IS 19 20

loge of (Particle concentration per cm3

)

Fig. 2. Atmospheric composition and ionization. The curvesare somewhat schematic and omit fine details. The uncertaintiesgrow rapidly above 100 km, and there is evidence for largevariations in this region, some of them depending on solaractivity. This is particularly true of the electron concentration,

but also holds for the neutral particles.

168 APPLIED OPTICS Vol. 3, No. 2 / February 1964

[ |__EXOSPHERE

THERMOSPHERE

/I10NO PHEREt / ~~~~~~F REGION

AURORA ! 227'MESOSPHERE D REGION

STRATOSPHERE eOZCNE LAYER

' , ,S, , 8 , FROcp6OAPHETq ,

WAVELENGTH-ANGSTROMS

Fig. 3. Penetration of solar radiation as a function of wave-length, after Friedman (in ref. 1). At this height the intensityis reduced to 1/e for overhead sun. (Figure reproduced courtesy

of Academic Press.)

The mean life against dissociation by the Herzbergcontinuum in sunlight is 2.3 X 109 see or 740 years.This radiation penetrates to about 30 km, where it is ab-sorbed by ozone. It is responsible for the presence ofthe ozone layer near 30 km, and smaller amounts ofozone and atomic oxygen between 30 km and 90 km.

Once atomic oxygen is produced, it may be convertedinto ozone by

+ 02 + M > ()3 + M, (1)

where M is a third body, needed to help carry away theenergy released. Below about 40 km this reaction is sorapid that most of the oxygen atoms released becomebound into ozone molecules, and the "ozone layer" isformed. The main reactions reforming 02 are

+ + M >0°2 + A/ (2)and

0 + 03 02 + 02. (3)The first is more important above about 80 km, and thesecond below. Ozone is dissociated by sunlight into0 + 02; the oxygen atom thus produced will prob-ably enter into (1) again, reforming the 03, but may in-stead combine with 03 (3). This is the chief means bywhich ozone is removed in the 30-km region, but it isvery inefficient, and the lifetime of an ozone moleculemay be several months. Considerable amounts of non-equilibrium ozone can accumulate near 20 km undercertain conditions. This gives ozone its interest as ameteorological tracer.

Returning to the upper atmosphere, we note that therate of destruction of 0 by reactions (1), (2), and (3)falls off rapidly with increasing height, while the rate ofphotodissociation increases. At about 90 km the oxy-gen is about half dissociated, and above this height thedominant form is atomic. The transition is ratherrapid, but is probably smoothed out by mixing, andamounts of 02 Up to 10% of the 0 appear to be presentup to at least 120 km (Fig. 2).

Large diurnal variations are present at certainheights. Below about 90 km the 0 is rapidly convertedinto 03 at night, and between 70 km and 90 km theozone concentration may rise by a factor of 100 aftersunset. The increase in total ozone, as observed fromthe ground, is still negligible.

The optical absorption of ozone occurs in the strongHartley continuum below about 3000 A (Fig. 3), andthe weak Chappuis continuum in the visible. The for-mer is responsible for the shortwave cutoff of the solarspectrum (as well as stellar, auroral, and airglowspectra). The latter gives the blue color of twilight;the absorption band is very prominent in low-disper-sion twilight spectra.

III. Ionization

At wavelengths shorter than about 1000 A, solarradiation becomes capable of ionizing the atmosphericconstituents. The most important thresholds are':

N2 796.Ao 910 02 1027 ANO 1345A

The inclusion of nitric oxide in this listing may be sur-prising, but it is important because it can be ionized byLyman-a (1216 A). By a remarkable coincidence,Lyman-a can penetrate to about 75 km through a"window" in the absorption spectrum of 02. It ap-pears that enough NO is present between 75 km and100 km to account for the D region of the ionosphere bythis process, with daytime electron concentrations of 103to 104 electrons/cm'. The only other ionizing wave-lengths that can penetrate low enough are soft x rayswith wavelengths below 10 A; these do seem to bepresent during solar flares, producing radio fadeoutsbecause of the absorption of radio waves.

When the thresholds of the major constituents arereached, the absorption is so strong that much of theenergy is absorbed at 170 km and above, forming theF region. The F region extends much higher than this,because at great heights the rate of recombination isvery slow, almost balancing the reduced rate of ioniza-tion. The maximum electron concentration is near300 km, and typically about 106 electrons/cm'.

Soft x rays between 10 A and 100 A can penetrate tobetween 100 km and 140 km, forming the E region byionizing all the major constituents. The daytime elec-tron concentration is about 105 electrons/cm'; thisfalls by about a factor of 10 at night.

The plot (Fig. 2) of electron concentration againstheight usually shows a fairly smooth increase from 70-300 km; the "layers" or regions we have mentioned arelargely artifacts of the radar sounding technique usedto study the ionosphere. Minor inflections in the curvehave the effect of making the radio reflections appearto come from a series of discrete layers.


The positive ions corresponding to the free electrons

have been studied by rocketborne mass spectrometers

having no ion source. 0+ is the main ion above 200

km; below this, both 02+ and NO+ are important,especially the latter at night. It should be noted that

the presence of NO+ does not necessarily mean that it

was formed by ionizing NO; a much more likely mech-

anism is one like

N2 + hv N2+ + e, (4)

followed by

N2+ + O NO+ + N. (5)

There are several other possibilities but this one il-

lustrates the general idea.In the D and lower E regions there may be sub-

stantial concentrations of negative ions, especially at

night. Though 0- and °2- can be detected, the majornegative ion is found to be NO2 -.

IV. Temperatures

On rising from the earth's surface into the tropo-

sphere, the temperature falls, because the bulk of solar

radiation is absorbed at and near the ground, and heat

must be conducted up from there (Fig. 1). But even-

tually the temperature rises again to a peak near 50

km, where the heat source is the absorption of the

2000-3000 A region by ozone. There are no further

substantial heat sources up to nearly 100 km, and the

temperature falls to its lowest value at about 90 km.

The actual minimum temperature is not accuratelyknown and may well be variable, but it is not far from

180'K, or -90'C.Nearly all the energy absorbed in forming the iono-

sphere eventually appears as heat, and there is a rapid

rise of temperature above 90 km. The temperature

gradient is close to 60 /km between 110 km and 250

km, and perhaps even to greater heights. Eventually

the temperature becomes constant, because there are

no further energy sources and because the long mean

free path results in a very high thermal conductivity.

Probably the isothermal region extends to several earth

radii. The region of increasing temperature is some-

times called the thermosphere.The temperature of the isothermal region appears to

be about 1500'K at sunspot maximum and 10000 K

or less at sunspot minimum. There may be a diurnal

variation as well. The region above 500-600 km has

special and remarkable properties and is called the

exosphere or geocorona. At these heights a particle

that sets off in an upward direction has a large prob-

ability of leaving the atmosphere temporarily and

going into a gravitational orbit about the earth. Mole-

cules and atoms of oxygen and nitrogen almost never

have enough energy to escape, and will travel in

elliptical orbits until they plunge back into the at-

mosphere, perhaps on the opposite side of the earth

from their point of origin. Although collisions are very

rare in the exosphere, these heavier particles retain a

Maxwell-Boltzmann distribution at a constant tem-perature, and the barometic equation still holds.

The situation is very different for helium and es-

pecially atomic hydrogen. Here many particles have

more than the escape velocity and travel in hyperbolic

orbits. The corresponding returning particles are

missing from the velocity distribution, which is no

longer isotropic. Moreover, the complete velocity

distribution at great heights includes many satelliteorbits, which never strike the thermosphere, and at

several earth radii this component is grossly underpopu-

lated. Eventually, one finds only escaping particles

moving almost radially; the "temperature" of thiscomponent is considerably less than that of the lower

exosphere.Between 80 km and 300 km, and occasionally higher,

useful temperature measurements can be made by spec-

troscopic means. The review by Hunten' still gives a

good idea of the possibilities. Several methods are

practical. The rotational temperature of molecularbands can be measured with moderate spectral resolu-

tion (a few angstroms). Some of these bands arise in

specific airglow layers, but the most useful measure-

ments are made on aurora, using a bright N2 + band at

3914 A (Fig. 4). In favorable cases the height of the

aurora can be measured simultaneously by two-station

photography. The Doppler widths of atomic lines can

be measured by Fabry-Perot interferometers; the red

and green lines of atomic oxygen are especially useful

because of their low transition probability. This en-

sures that the atoms have time to come into transla-tional equilibrium with the atmosphere before they

radiate. Such measurements are made in both aurora

and airglow.

R BRANCH

, - . vvv A-Fig. 4. Measurement of atmospheric temperature from a band

of N2+ in the aurora. The upper tracing shows a sunlit aurora

in the exosphere, at a temperature of 2200'K. The lower is of a

normal aurora at about 300'K. (Vallance Jones and Hunten,"figure reproduced courtesy of Nature.)

170 APPLIED OPTICS / Vol. 3, No. 2 / February 1964

Fig. 5. A map by Fritz in 1881, showing contours of equal probability of occurrence of aurora.

Perhaps the best method of all is to measure the line-width of the resonance lines scattered from an alkalivapor trail released at twilight. The measurement isusually made indirectly by absorbing the radiation in acell containing the same vapor. Sodium has been used,but has disadvantages, including the disturbing effectsof natural atmospheric sodium and the corrosion of cellwindows by sodium vapor. Potassium is thereforeprobably the most practical choice.

Similar methods have been used to measure thetemperature of the exosphere, using the absorption ofLyman-a by atomic hydrogen in a cell containing atungsten filament to maintain the dissociation. The in-strument must be flown to several hundred kilometers toavoid absorption by atmospheric hydrogen.

V. Aurora

The striking phenomenon known as the northernlights or aurora has been observed and recorded forthousands of years, but we are still searching for a de-tailed explanation. The general outlines of the ex-

planation are, however, clear. Auroral displays areconcentrated into auroral zones, though they are seenwith diminishing probability at increasing distances.Figure 5 shows the map of auroral occurrence compiledin the last century by Fritz. The auroral zones followrather closely the two rings at 230 from the earth'sgeomagnetic axis: the axis of the dipole componentof the earth's field. In North America, the auroralzone passes near Fairbanks, Alaska, and grazes thesouthern shore of Hudson Bay. The control by themagnetic field makes it clear that the aurora-producingagent is charged particles. The probability of auroraldisplays closely follows the eleven-year cycle of solaractivity, and individual displays can often be relatedto solar flares one to two days earlier; the chargedparticles, therefore, probably come from the sun.Whether they are, at an intermediate stage, trapped inthe earth's magnetic field is still being debated. Thetrapped particles known to form the Van Allen beltsare of higher energy and lower flux than the aurora-producing particles.


One of the aims of auroral spectroscopy is to dis-

cover the nature, flux, and distributions in energy and

angle of the incoming particles. The spectrum varies

somewhat under different conditions, but normally con-

tains lines and molecular bands of oxygen and nitrogen,

being dominated by bands of N2 and N2+ and the

famous forbidden green and red lines of 0 i. The main

band systems are the following; the approximate wave-

length regions occupied are also shown:

N2N2N2N2+N2+02+

Vegard-Kaplanfirst positivesecond positiveMeinelfirst negativefirst negative

2000-4000 X5800-16000 A3000-4000 A6800-16000 A3600-5200 A5200-6400 A

The Vegard-Kaplan system, like the green and red

lines, arises from a forbidden transition. The MNeinel

bands were discovered first in the aurora, and only

later observed in the laboratory. Other atomic lines

observed arise from permitted and forbidden transitions

of 0 i, 0 II, N i, and N ii.

The presence of these emissions from normal at-

mospheric constituents is hardly surprising. What is

remarkable is the sporadic presence of theBalmer lines of

hydrogen; moreover, they show a large broadening to

the violet when observed along the magnetic field.

This can only mean that the hydrogen atoms are inci-

dent on the atmosphere at high speed. The maximum

Doppler shift does not necessarily give the initial

velocity of the protons; a fast proton will not capture

an electron and radiate until it has slowed to an energy

of about 100 keV.It is tempting to assume that these fast protons are

the agents producing aurora. Detailed study, how-

ever, shows that this cannot be so. First, the Balmer

lines are sometimes absent from the spectra. Second,

even when these lines are strong, the proton flux de-

duced from their intensity is found to be insufficient to

produce the observed intensity of atmospheric emis-

sions.2 It is therefore probable that normal aurora is

produced by incident electrons. Certain intensity

ratios within the spectrum are consistent with this con-

clusion.There is no doubt that when protons are incident

they can excite an aurora-like spectrum. Moreover,

the careful study of the shapes of the Balmer lines has

given important information about the incident pro-

tons. These shapes can only be explained if the pro-

tons have a wide spectrum of initial velocities and a

wide angular distribution. Because they all arrive at

nearly the same time, the immediate source must be

much closer than the sun. Possibly the particles are

trapped in, and then expelled from, the outer part of

the earth's magnetic field, or perhaps they are ac-

celerated locally in electric fields or by a betatron mech-

anism. It is also possible that they are stored in a

magnetic field of solar origin and released when the

magnetized region reaches the earth.

Artificial auroras have been produced by a number of

high-altitude nuclear explosions. It is usually as-

sumed that the main exciting agent is electrons from

i3 decay of the highly radioactive fission and fusion

products from the explosion.

VI. Night Airglow

The night airglow or nightglow is a fairly steady

luminosity arising in the upper atmosphere. It is re-

sponsible for roughly half of the visual brightness of a

dark sky. The most prominent features in the visible

part of the spectrum are the oxygen green and red

lines, and by observation of these alone it is difficult or

impossible to distinguish a bright nightglow from a

faint aurora. However, the spectra are very different

in the ultraviolet and infrared. Some of the nightglow

emissions are rather unexpected for the normal atmos-

pheric mixture of nitrogen and oxygen. In particu-

lar, the sodium D lines appear, with a variable inten-

sity which at times approaches that of the green oxygen

line. Two forbidden band systems of 02 are known:

the Herzberg in the ultraviolet, and the atmospheric

in the infrared. The 0-0 band of the latter is reab-

sorbed in the lower atmosphere, but the 0-1 is able to

penetrate to the ground. Other bands appear to be

present in the ultraviolet, but have not been identified.

There is good evidence for a continuum in the green,

but its source is unknown.By far the brightest emission is the vibration-rota-

tion system of OH (Meinel bands), starting in the

visible but reaching its greatest intensity in the 3- to

4-ji region. The total intensity, if it were visible, would

be comparable with that of a very bright aurora, but

without the fine structure. It is striking that no vibra-

tional level higher than the ninth is excited at an ob-

servable rate.Although electron excitation cannot be ruled out, it

is usually considered that most nightglow emissions are

excited during recombination of ions and association of

oxygen atoms. For example, the three-body reaction

0 + 0 + M - 02 + M

should be able to excite either the oxygen molecule or

the third body M. This reaction should operate best

in the 90-km region, where many of the airglow emis-

sions are observed to arise (the green line, the sodium

lines, and the 02 bands). The 0 i red lines are pro-

duced at a much greater height, and are assumed to be

excited primarily in the dissociative-recombination re-

action

NO+ + e - N + 0*.

The most satisfactory mechanism for the OH band ex-

citation is


(2)

(6)

03 + H °2 +OH* (7)

with the H being regenerated by

OH+O--02 +H. (8)Reaction (7) gives just the right amount of energy to

excite the ninth vibrational level, as observed; it hasalso been studied in the laboratory and found to giverise to the same bands observed in the nightglow.

Nightglow emissions are not completely constant inbrightness, but display slow variations in time andspace, with scales of perhaps a few hours and a thousandkilometers, respectively. In some cases there are alsolarge annual or seasonal changes. Some of the emis-sions vary together, while others are completely uncor-related. Many years of study have been given tothese variations, but no particular pattern has beenrevealed. In only one case has a plausible mechanismbeen presented: reaction (6) seems to correlate wellwith relevant ionospheric measurements of electrondensity.

VII. Twilight and Day Airglow

When the sun is shining on the upper atmosphere,there are a number of additional emissions which donot appear in the nightglow. Historically, many ofthese have been studied intensively during twilight;moreover, the twilight method offers the possibility ofmeasuring the vertical distribution of the emission asthe shadow sweeps through it. The dayglow itselfhas become accessible to observation only in the lastyear or two. The most direct method is to observefrom a rocket, above the overwhelming light from thelower atmosphere (Fig. 6). However, special methods,can also be used at the ground to discriminate againstthis background.

Dayglow and twilight emissions can be excited in twogeneral ways. The first is by direct optical excitationof an atom or molecule by sunlight (resonance scatter-ing or fluorescence); a good example is the sodium Dlines. The second is by chemical or ionic reactions, asin the nightglow, but involving species which exist in

X (A)

Fig. 6. Spectrum of the dayglow observed from a rocket. 0

(Figure reproduced courtesy of Wallace and Nidey and ofJournal of Geophysical Research.)

sufficient concentration only when sunlight is present,or has been present very recently.

In addition to the D lines, the resonance lines ofpotassium and lithium appear. The latter is subject tosporadic enhancement, some apparently natural butsome associated with large thermonuclear explosions,those in the 50-km height region being particularly ef-fective. The H and K lines of ionized calcium havebeen observed on occasion, but are often too faint to bedetected. The sodium, which has been studied most,occupies a layer centered at about 90 km. It is believedthat reactions with atomic oxygen are responsible formaintaining the alkalis in the atomic form in this region.There is some evidence that the sodium abundance isgreater in the dayglow than in twilight, 9 but the ques-tion is not settled.

The first negative bands of N2+ can be detected intwilight with difficulty; the mechanism is againfluorescence under solar radiation, as soon as N2+ ionscan be created. The height of the ionization and theionizing agent are not established. The intensity ismuch greater in the dayglow'0 ; here there is little doubtthat the ions are created by solar extreme ultraviolet,and then are observed by fluorescence.

The red lines of 0 i appear in the evening twilight,showing a slow decay; in the morning, the intensity ismuch less. They are apparently produced by thesame reaction as in the nightglow. Here again, theintensity is much greater in the dayglow; the emissionhas been observed from both the ground" and highaltitudes." It is possible that the analogous doublet ofN at 5200 A is present in the evening with low in-tensity, and in the daytime 0 with considerable in-tensity. The green line of 0 i has occasionally beenfound to be enhanced in twilight and appears to befairly bright in the dayglow. 0 Dissociative recombina-tion of NO+ does not give enough energy, but the cor-responding reaction in 2+ is a possibility.

The infrared atmospheric system of 02 originates in aforbidden transition between the first excited andground states of the molecule. The 0-1 band at 1.58 ucan be observed in the evening twilight, and the 0-0band (which is absorbed in the lower atmosphere) hasbeen measured from high altitudes in the daytime.13

The excitation appears to be by a chemical mecha-nism, but the reactions involved have not been definitelyestablished.

Lyman- a can be observed by rocketborne photo-meters. Although many of the observations have beenmade at night, it appears that this is really a twilighteffect, arising by scattering of solar radiation in the ex-tended outer atmosphere, or geocorona, of the earth.Since the geocorona reaches to several earth radii, itcan readily be observed from the night side of the earth.This airglow radiation is in turn scattered back up by hy-drogen below the rocket for heights greater than 120 km.


In this paper, an attempt has been made to present

enough background material on the physics and chem-

istry of the upper atmosphere to permit an understand-ing of the detailed papers on specific subjects. It

has been necessary to treat many topics in cursory

fashion, and to omit many more. As far as possible,

tentative results have been so labeled, but in some cases

a false impression may have been given that a conclu-

sion is established, or alternative interpretations may

have been ignored for the sake of presenting a simple

picture. The author would like to apologize for this

neglect, and also for omitting reference to the names of

those who have done the original work.

References

1. J. A. Ratcliffe, Physics of the Upper Atmosphere (Academic

Press, New York, 1960).

2. J. W. Chamberlain, Physics of the Aurora and Airglow(Academic Press, New York, 1961).

3. D. R. Bates. The Earth as a Planet, G. P. Kuiper ed. (Univ.of Chicago Press, Chicago, 1954), Chap. 12.

4. S. K. Mitra, The Upper Atmosphere, 2nd ed. (The AsiaticSociety, Calcutta, 1952).

5. D. R. Bates, The Earth and Its Atmosphere (Basic Books,New York, 1958).

6. H. S. W. Massey and R. L. F. Boyd, The Upper Atmosphere(Hutchinson and Co., London, 1958).

7. D. R. Bates, ed., "Theoretical Interpretation of UpperAtmosphere Emissions", Planetary Space Sci. 10 (1963)

[also reprinted as Intern. Astron. Union Symp. No. 18

(1962)].8. D. M. Hunten, Ann. G6ophys. 17, 249 (1961).

9. J. E. Blamont and T. M. Donahue, J. Geophys. Res. 66,1407 (1961).

10. L. Wallace and R. A. Nidey, J. Geophys. Res. (to be pub-lished).

11. J. F. Noxon and R. M. Goody, J. Atmos. Sci. 19, 342 (1962).

12. E. C. Zipf, Jr., and W. G. Fastie, J. Geophys. Res. (to bepublished).

13. J. F. Noxon and A. Vallance Jones, Nature 196, 157 (1962).

photos M. Dreyfus

J. Strong Johns Hopkins, J. N.Howard AFCRL, A. AngstromStockholm, and J. C. Beckman

Beckman and Whitley.

D. Q. Wark Weather Bureau editor of the feature on atmospheric optics in the February 1964 issue, W. Nordberg NASA, A. H. Barrett

MIT, D. Gates NBS-Boulder, H. Grier Eppley, V. Suomi Wisconsin, and R. O'B. Carpenter Geophysics Corporation of America.


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optics of the upper atmosphere

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