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1-1-1976

Optical Radiation from the Atmosphere

Doran J. Baker Utah State University

William R. Pendleton Jr. Utah State University

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Recommended Citation Baker, Doran J. and Pendleton, William R. Jr., "Optical Radiation from the Atmosphere" (1976). Space Dynamics Lab Publications. Paper 2. https://digitalcommons.usu.edu/sdl_pubs/2

This Article is brought to you for free and open access by the Space Dynamics Lab at DigitalCommons@USU. It has been accepted for inclusion in Space Dynamics Lab Publications by an authorized administrator of DigitalCommons@USU. For more information, please contact [email protected]. cÌ E1 Baker, Doran J., and William R. Pendleton Jr. 1976. “Optical Radiation from the Atmosphere.” Proceedings of SPIE 0091: 50–62. doi:10.1117/12.955071.

OPTICAL RADIATION FROM THE ATMOSPHERE Doran J. Baker and William R. Pendleton,Pendleton, Jr.Jr. Electro-Electro-Dynamics Dynamics Laboratories,Laboratories, UtahUtah StateState University Logan, Utah 84322

ABSTRACT The -interfaceinterface region which lies between the meteorological atmosphere of the Earth and "outer" space is a source ofof abundantabundant opticaloptical radiation.radiation. The purpose of thisthis paper isis toto provide the optical instrumentation engineer with a generalized understanding and a summary reference of naturally-naturally -occurring occurring aerospace radiation phenomena.phenomena. The colors of thethe radiationradiation extend over the full optical spectrum fromfrom ultravioletultraviolet throughthrough thethe infrared.infrared. The emissions, observed during both day and night times,times , areare richrich inin lineline andand bandband spectra.spectra. The parameter­parameter- ization of atmosphericatmospheric light by frequency (or photonphoton energy)energy) andand byby spectral radiance is discussed. The sources of the natural light from the gases of the atmosphere are grouped into four categories: (1) airglow mechanisms, (2) thermal processes, (3) scatteringscattering phenom­phenom- ena, and (4)(4) auroral excitations.excitations. An overview of the characteristic spectral occurrences and intensities is given.given. INTRODUCTION The purpose of this paper is to provide a general understanding of the light which comes from the atmosphere ofof thethe Earth.Earth. This atmospheric radiation rangesranges fromfrom thethe farfar ultravio-ultravio­ let through the far infrared.infrared. We includeinclude exoatmosphericexoatmospheri c primaryprimary radiationsradiations suchsuch asas sun-sun­ light, moonlight,moonlight, starlight, planetary light,light, andand zodiacalzodiacal light,light, onlyonly toto thethe extentextent thatthat they are primary sources ofof subsequentlysubsequently radiatedradiated oror scatteredscattered atmosphericatmospheric lightlight energy.energy. For our purposes the definition of the upperupper limitlimit ofof thethe atmosphereatmosphere willwill bebe takentaken asas thethe altitude at which molecules cancan leaveleave thethe atmosphereatmosphere without without undergoingundergoing collisionscollisions alongalong thethe way. In other words,words, thethe molecularmolecular mean mean-free-path - free -path approachesapproaches a scale height.height.

Figure 1 shows the range of the radiantradiant energyenergy spectrumspectrum interpretedinterpreted inin termsterms ofof thethe mole-mole­ cular physical phenomena involved.'involved. 1 We will consider the wavelengths ofof thethe lightlight toto rangerange from about 11 mm for thethe farfar infraredinfrared toto aboutabout 1010 nm for the extreme ultraviolet.ultraviolet. The cate-cate­ gorizations of the subregions ofof thethe lightlight spectrumspectrum havehave aa highhigh degreedegree ofof arbitrarinessarbitrariness and,and, of course, there are no abrupt frequencyfrequency boundaries.boundaries. The visible light, which ranges from red through violet, involves energy transitions of the outer shell electrons ofof thethe atomsatoms andand moleculesmolecules whichwhich makemake upup thethe atmosphere.atmosphere. Ultra-Ultra­ violet light interactions cancan includeinclude photonphoton transitionstransitions withinwithin thethe innerinner shellsshells asas well.well. The infrared spectrum of radiation from the atmosphere, on the other hand, is dominated by energy mechanismsmechanisms associated with the vibrationvibration ofof molecules.molecules. The midmid-infrared -infrared region isis rich with molecular fundamentalfundamental rotationrotation-vibration -vibration bands, whereas many ofof thethe overtonesovertones ofof the bands occur in the nearnear infrared.infrared. Pure rotational spectraspectra areare seenseen inin thethe farfar infrared.infrared. PARAMETERIZATION OF ATMOSPHERIC LIGHTLIGHT

The parameters of atmospheric lightlight areare frequency,frequency, radiance,radiance, andand polarization.polarization. The fre-fre­ quency of light ranges fromfrom aboutabout ;% THzTHz (terahertz)(terahertz) forfor the the farfar infraredinfrared upup to to somesome 2,5002,500 THz for thethe farfar ultraviolet.ultraviolet. The prefix tera, we recall,recall, representsrepresents aa factorfactor ofof 101'.10 12 . Thus, the frequency of thethe opticaloptical radiationradiation portionportion ofof thethe electromagneticelectromagnetic spectrumspectrum coverscovers four orders. Frequency and Photon EnergyEnergy Since light is emitted or absorbed in discrete energy quanta, the frequency of light is also characterized by specifyingspecifying thethe photon energy,energy,

E __E hv,hM, (1)

wherewhere'2

E = photon energy inin eVeV oror joulesjoules h e- Planck constant E 6.626167 xx 10-3'TO' 34 joulejoule-sec -sec E 4.135700 xx 10-'510" 15 eVeV-sec -sec V frequency inin HzHz

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The photon energy ranges from aboutabout 10-310~ 3 eVeV forfor thethe farfar infraredinfrared toto aboutabout 100100 eVeV forfor thethe ex-ex­ treme ultraviolet. The spectrum on the filmfilm ofof aa gratinggrating spectrographspectrograph isis almostalmost linearlinear withwith wavelength,wavelength,

\a = y/vv/v E= cc/nv, /nv, (2)

where

Aa = wavelength inin meters v = speed ofof lightlight -= c/nol\\ inin metermeter/sec /sec oc = speed of lightlight inin vacuumvacuum E= 2.99792458 xx 10810 8 metermeter/sec /sec n = index of refractionrefraction ofof mediummedium Hence, spectra of optical radiationsradiations areare veryvery commonlycommonly plottedplotted withwith wavelengthwavelength asas thethe inde-inde­ pendent variable (abscissa).(abscissa). However, as shown explicitly inin EquationEquation (2),(2), wavelengthwavelength de-de­ pends upon the index of refraction of thethe gasgas inin thethe spectrograph.spectrograph. Commonly used unitsunits ofof measure for the wavelengths ofof lightlight areare

micrometer (pm)(ym) E= 1010" -66 meter 0 nanometer (nm)nm) = 10~10'99 meter sE 10 A angstrom (A) =- 10'1010" 10 meter = 10'"IQ- 4 umym Unlike the grating spectrograph, the spectrum from aa prism spectrograph departs markedlymarkedly from linearity with wavelength. It is, in fact, more nearly linear with inverseinverse wavelength,wavelength, i.e.,i.e.; with wavenumber,wavenumberj

a = 1/AI/A E= v/v\>/v -- nv/c (3) where a = wavenumber inin meter'meter'.1 . Wavenumber, like wavelength, isis dependentdependent uponupon thethe indexindex ofof refractionrefraction ofof thethe gasgas inin thethe spectrograph. Wavenumber, which is the numbernumber ofof completecomplete wavewave cyclescycles inin aa unitunit distance,distance, isis beingbeing usedused increasingly to characterize lightlight spectra.spectra. The use originated because ofof thethe discoverydiscovery byby Hartley, Balmer, Rydberg and others of multiplicative relationshipsrelationships inin spectraspectra thatthat dependdepend upon reciprocal wavelength differences.differences. In other words, wavenumber isis proportionalproportional toto photon energy,

a = nE/hc.r\E/he. (4) Also, the computercomputer-generated -generated "readout" ofof anan interferometerinterferometer-spectrometer -spectrometer isis linearlinear inin wave-wave- number. The most commonly used unit forfor wavenumber isis reciprocalreciprocal centimetercentimeter (cm'').(cm" 1 )- As usedused byby spectroscopists in practice, the cm''cm' 1 unitunit cancan bebe interpretedinterpreted asas anan alternatealternate unitunit forfor photon energy oror forfor "cycles"cycles-per-second" -per- second" frequency. This is because theythey usuallyusually convertconvert their wavenumber values to vacuum wavenumber usingusing

aavac vac r= aair/naira a^r. /TI a^r. (5)

which isis simplysimply a E v/c\>/e (6) avacvae = (6) Thus, vacuum wavenumber isis nothingnothing moremore thanthan frequencyfrequency withwith a a scalescale factor factor of of 1 1/e,/c, wherewhere one hertz is equivalent toto 1/(3l/(3 xx 108)10 8 ) metermeter' -11 oror l/(31/(3 x 1010)10 10 ) cm-1cm" In wavenumber unitsunits light ranges from about 10 cm''cm" 1 in the farfar infraredinfrared toto 10'10 7 cm''cnr 1 An thethe farfar ultraviolet.ultraviolet. A memory prop isis thethe following:following: "1 ympm corresponds both to 10,00010,000 AA andand toto 10,00010,000 cm-1cm" 1 andand isis equivalent to a photon energyenergy ofof aboutabout oneone eV."eV." (To be exact, it is 1.241.24 eV.)eV. ) It is important to note that the spectral resolution ofof aa spectrometerspectrometer expressedexpressed inin wave-wave- number is related to that in wavelength byby

AaAc == -AX/X2-AA/A' (7)

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Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/13/2014 Terms of Use: http://spiedl.org/terms DORANDORANJ. J. BAKER, WILLIAM R.R.PENDLETON, PENDLETON, JR.

where Aa = wavenumber resolution inin meter"meter'1 E -aIR-a//? X = wavelength inin metersmeters AaAX = wavelength resolutionresolution inin metersmeters E X/R\/R R = resolvingresolving powerpower E X/AX,a /Aa, _E -a-a/Aa /Aa The negative sign in these incremental (differential)(differential) equationsequations isis usedused becausebecause increasingincreasing increments of a and X areare oppositeopposite fromfrom oneone another.another. The definition ofof resolutionresolution elementelement is taken as the interval between the half responseresponse pointspoints onon eithereither sideside ofof thethe peakpeak re-re­ sponse.sponse.' 3 The wavelength resolution element AXAX ofof aa monochromatormonochromator isis somewhatsomewhat independentindependent ofof X.X. However, for the interferometerinterferometer itit isis thethe resolutionresolution elementelement AaAa thatthat is is constant constant with with wave wave- - number.

Intensity Another parameter byby which thethe atmosphericatmospheric lightlight isis characterizedcharacterized isis thethe intensity.intensity. Generally atmospheric sourcessources areare extended,extended, i.e.,i.e., theythey havehave significantsignificant spatialspatial extentextent inin contrast with the point sourcessources commoncommon toto astronomy.astronomy. The lightlight intensityintensity isis specifiedspecified byby the radiance

L = 1imlim AO/ASAOA$/ASAft (8)

where L = radiance inin watt m-'sr-1m- 2 sr~ 1 AOA$ = incrementalincremental lightlight fluxflux inin wattswatts AS = incrementalincremental apparentapparent surfacesurface areaarea inin meter'meter 2 MlAft = incrementalincremental solidsolid angleangle inin steradianssteradians If the flux $ is expressed in lumens,lumens, thisthis extendedextended sourcesource entityentity isis calledcalled brightness.brightness. Thus, the radiance or brightness of a source is the light flux emitted per unit area of the source into each unit of solidsolid angleangle "away"away fromfrom thethe source."source." Radiance can be converted from power flux to photon flux units using the relationship

1 photonphoton/sec /sec e= 1.9864751.986475 (1(1/X) /a) xx 10-251Q- 25 watt / Q x a2 22 xx 10''5TO' 25 /X watt. ^(9)y > Another unit for thethe photon radiance whichwhich hashas beenbeen widelywidely adoptedadopted byby atmosphericatmospheric scientistsscientists is the rayleigh defined byby 1"

1i rayleigh == (1(l/4ir) /4rr) x 101010 10 photons sec-Im-'sr-1sec- 1 m- 2 sr- 1 (10)

The Baker formulasformula 5 for conversionconversion fromfrom powerpower radianceradiance LL toto rayleighrayleigh (photon)(photon) radianceradiance RR isis

R = 27rXL2irXL xx 1015,10 15 , (11)(11 ) where L isis inin watt m-2sr-1m" 2 sr~ 1 andand XX isis inin meters.meters. The light flux into a photometer which remotely views an extended source of radiance LL is easily computed.computed. Due to an equivalence theorem;theorem,5 ifif thethe sourcesource cancan bebe assumedassumed toto uniform-uniform­ ly fill the viewing field of the instrument,instrument, thenthen

$(11. = LAQ,LAft, (12)

where § = light1ight fluxflux inin wattswatts L = radiance ofof sourcesource inin wattwatt m-2sr-1m -2 sr" 1 A = area of photometer aperture or collector in metermeter'2 ftS2 = field of viewview ofof photometerphotometer inin steradians.steradians. The ordinate of a spectrum isis usedused toto indicateindicate thethe intensityintensity ofof thethe lightlight atat thethe variousvarious wavelengths or wavenumbers. This power spectral density or spectral radiance,radiance,

L,LXJ \ = lim1im AL/AX (13)

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or

L6 °--= lim1 im AL/AaAL/Aa (14) is derived from the wavelength or wavenumber resolutionresolution ofof thethe spectrometerspectrometer andand isis generallygenerally expressed on a per angstrom, per nanometer, perper micrometer oror onon aa perper reciprocalreciprocal centimetercentimeter basis. For example, photon spectral radianceradiance isis oftenoften expressedexpressed inin rayleighsrayleighs perper angstromangstrom or rayleighs per reciprocal centimeter.centimeter. The rayleigh is a very useful unitunit ofof measuremeasure forfor atmosphericatmospheric light.light. Features of thethe airglow generally are expressed inin rayleighsrayleighs (R),(R), thosethose ofof auroraaurora inin kilorayleighskilorayleighs (kR),(kR), and those of thermal and scattered radiation inin megarayleighs (MR).(MR).

Light Polarization Any incident beam of lightlight cancan bebe specifiedspecified inin termsterms ofof twotwo componentscomponents thatthat areare polarizedpolarized in perpendicular azimuths.azimuths. The emission from a particularparticular molecule hashas oneone polarizationpolarization com-com­ ponent withwith the direction specified byby thethe orientationorientation ofof thethe electricelectric fieldfield vector.vector. How-How­ ever, an ensemble of molecules will havehave thethe azimuthsazimuths ofof thethe oscillatorsoscillators randomlyrandomly distribut-distribut­ ed and thus the net lightlight willwill bebe unpolarized.unpolarized. Elliptical polarization will resultresult fromfrom subsequent scattering andand reflections.reflections. The degree of polarization isis givengiven byby

6 - L L + L . (15) = (Lmax Lmini max +Lmin)m^n where

LLmax == maximum radianceradiance observedobserved whenwhen aa polarizerpolarizer isis rotatedrotated max ^ in the beambeam L . = minimum radianceradiance observedobserved whenwhen aa polarizerpolarizer isis rotatedrotated minm^ n in the beambeam

When R3 == 1 the beam isis saidsaid toto bebe plane polarized;polarized; whenwhen a3=0 = 0 thethe lightlight isis unpolarizedunpolarized (or(or circularly polarized).polarized). SOURCES OF ATMOSPHERIC LIGHT The natural light from thethe atmosphereatmosphere cancan bebe categorizedcategorized intointo fourfour generalgeneral areasareas accord-accord­ ing to the source processes.processes. These categoriescategories are:are: (I) airglow, (II)(II) thermalthermal radiation,radiation, (III) scattered light, and (IV)(IV) aurora.aurora. Due to the interactive nature of thethe atmosphericatmospheric mechanisms involved, exclusive definitionsdefinitions areare inappropriate;inappropriate; however,however, descriptivedescriptive cate-cate­ gories are helpful forfor aa generalgeneral overview.overview. There are six principal emissionemission mechanismsmechanisms leadingleading toto atmosphericatmospheric radiationradiation whichwhich wewe will discuss in terms of the precedingpreceding sourcesource categories:categories:

1. Chemiluminesoenoe-Chemiluminescence - emissionemission duedue toto thethe excitationexcitation of one or more of thethe productsproducts ofof exothermicexothermic chemicalchemical reactions.reactions. 2. Fluorescence- absorptionabsorption of photonsphotons of one energy with subsequent emission ofof photonsphotons ofof lesserlesser energy.energy. 3. Energy transfer- emissionemission due to the excitation of one particle transferred fromfrom anotheranother particleparticle duringduring aa collision.collision. 4. Resonance scattering- absorption and emission of photons with-with­ out a change inin thethe photonphoton energy.energy. 5. Charged particle collisions- emissionemission duedue toto thethe excitationexcitation of atoms or molecules byby fastfast chargedcharged particles.particles. 6. Dissociative recombination- emission due to the excitation of atoms or molecules inin thethe recombinationrecombination ofof thermalthermal ionsions andand electrons oror byby ionion-ion -ion recombination.

Airglow The night airglow, discovered byby NewcombNewcomb inin 1901,61901, 6 isis thethe relativelyrelatively faintfaint nonthermalnonthermal luminescence that results from photochemical reactionsreactions ofof thethe dilutedilute gasesgases ofof thethe upperupper at-at­ mosphere. The airglow isis basically produced by sunlight; it originates during the daytime and persists throughout thethe night.night. Solar ultraviolet energy isis storedstored atat altitudesaltitudes aroundaround and above the mesopause inin dissociated, ionizedionized andand excitedexcited speciesspecies duringduring thethe dayday andand thenthen released atat night by various relaxation processes suchsuch asas chemicalchemical association,association, transfertransfer ofof excitation and ionic recombination (emission(emission mechanisms 1,1, 3 and 6).6). The airglowairglow "mirrors""mirrors" the chemical state of the upper atmosphere, making itit possiblepossible toto remotelyremotely monitormonitor itit inin astonishing detail.detail.

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Since the photochemical processesprocesses thatthat causecause thethe airglowairglow dependdepend criticallycritically uponupon thethe con-con­ ditions of illumination, there areare marked differencesdifferences betweenbetween thethe daytimedaytime andand thethe nighttimenighttime airglow. There are also features peculiarpeculiar toto thethe twilighttwilight airglow,airglow, thethe transitiontransition betweenbetween day and night airglow.airglow. When thethe upperupper atmosphereatmosphere isis illuminatedilluminated byby sunlight,sunlight, itit isis im-im­ possible to completely distinguish scattering from thethe otherother airglowairglow processes.processes. Resonance scattering from certain species is therefore often includedincluded asas partpart ofof thethe daytimedaytime andand twilight airglow. The most intense night airglow originatesoriginates inin aa layerlayer nearnear thethe mesopause.mesopause. This brightbright band can easily be seenseen whenwhen viewedviewed edgeedge-on -on fromfrom rocketsrockets or satellites (Fig.(Fig. 2).2). However, some airglow features are most prevalent at higherhigher altitudes,altitudes, notablynotably atomicatomic hydrogenhydrogen LymanLyman alpha, the atomic oxygen redred lineline andand nitricnitric oxideoxide infraredinfrared bands.bands. Detailed spectral atlases of thethe atmosphericatmospheric nightnight airglowairglow maymay bebe foundfound inin thethe literature.literature. Unattenuated radianceradiance valuesvalues forfor thethe brightestbrightest lineslines andand bandsbands areare summarized summarized in in Tattle Taole 1 for typical cases of nighttime, twilight andand daytimedaytime conditions.conditions. Measurements are usuallyusually referred to the zenith.zenith. The radiance generallygenerally increasesincreases awayaway fromfrom thethe zenithzenith asas

R = Rz secsec e,0, (16)(16]

where R = photon radiance inin rayleighsrayleighs atat angleangle e0 Rz = zenith photonphoton radianceradiance inin rayleighsrayleighs e = angle from zenithzenith inin radiansradians This isis thethe van Rhijn effect'effect 7 whichwhich isis seenseen whenwhen anan opticallyoptically-thin -thin airglowairglow layerlayer isis viewedviewed obliquely. The first airglow feature discovered'discovered 8 waswas thethe greengreen lineline ofof atomicatomic oxygenoxygen atat 55775577 A& whichwhich constitutes about 7%7% ofof thethe clear,clear, moonlessmoonless nighttimenighttime atmosphericatmospheric lightlight seen seen by by a adark dark- - adapted eye.eye. About 15% of the light is attributed to zodiacal light, which is sunlight scattered by interplanetary dust.dust. The remainder of the naturalnatural visualvisual lightlight ofof thethe nightnight sky is due to other airglowairglow speciesspecies (33(33%), %), toto starlightstarlight and to other scatteredscattered light.light. The 55775577-A -A greengreen lineline atat lowlow altitudesaltitudes in the night airglow resultsresults fromfrom aa chemicalchemical re-re- action, namely, the ChapmanChapman reaction'reaction 9

0 + 0 + 0 }02+ 0* 07](17) 0* -> 00 ++ h\>hv

where thethe asteriskasterisk ((*) *) denotesdenotes anan electronically-excitedelectronically- excited state.state. The characteristic shapeshape ofof the altitude profile in whichwhich the airglow forms is calledcalled aa ChapmanChapman layer." A pair of relatively bright atomic oxygen lineslines occuroccur atat 63006300 andand 63646364 A./ This oxygenoxygen red multiplet airglow originatesoriginates withinwithin thethe F-F-region region of the ionosphere andand isis attributedattributed toto dissociative recombination withwith photoelectronsphotoelectrons"ll

OZO + e- ->+ 0* + 00 (18)[IS) 0* } 0 + hvh\> Airglow radiations fromfrom nitrogen,nitrogen, thethe mostmost abundantabundant gasgas ofof thethe terrestrialterrestrial atmosphere,atmosphere, are sparse except for somesome atomicatomic lines.lines. The neutral nitrogen molecule in the electronic ground state has no dipole moment andand thereforetherefore isis a poor radiator. The most intenseintense nightnl*9 nt airglow comes from hydroxyl moleculesmolecules atat anan altitudealtitude ofof aboutabout 8585 km.km. The chemicalchemical reaction12reaction

H ++ 030 3 -> > OH*OH$ + 020 2 (19) OH$->OH+byOH* -> OH + h\> gives rise to thesethese MeinelMeinel rotationrotation-vibration -vibration bands of radiation whichwhich areare widelywidely distri-distri­ buted throughout the visible andand nearnear infraredinfrared portionsportions ofof thethe spectrum.spectrum. The symbolsymbol ((£) *) denotes that thethe hydroxylhydroxyl radicalradical isis inin aa vibrationally-vibrationally-excited excited state.state. Excited oxygen molecules areare formedformed byby aa threethree-body -body reaction inin which atomicatomic oxygenoxygen recombirecombines, nes,

0 + 0 + N2N 2 -> Oi + N2 0* (20) 02 -> 0022 + hv

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This gives rise to the Herzberg electronicelectronic bandsbands inin thethe ultravioletultraviolet andand toto thethe atmosphericatmospheric bands in the red and near infraredinfrared spectralspectral regions.regions. The molecular oxygen emissionsemissions areare thethe brightest luminescent features ofof thethe daytimedaytime airglow.airglow. The emission layerlayer peakspeaks atat aboutabout 60 km, withwith twotwo important infrared bands occurring at 1.271.27 and 1.581.58 um.ym. A transfer ofof excitation mechanismmechanism thatthat isis importantimportant atat aboutabout 100100 kmkm inin thethe airglowairglow isis"13

0* + 020 2 0 + 020 2 (21) 012'of 02 + byh\>

° which gives rise to bands at 76197619 andand 86408640 A.A. Fluorescence is an important processprocess inin excitingexciting somesome emissionsemissions inin thethe daytimedaytime andand twilight airglow.'"airglow. 11* Also, resonanceresonance scatteringscattering byby upper-upper-atmospheric atmospheric gas speciesspecies isis oftenoften included as part of a listing of daytime airglow spectra.spectra. Such features are thethe atomicatomic hydrogen Lyman-aLyman -a lineline atat 12161216 A,A, thethe sodiumsodium multipletmultiplet atat 5890 and 5896 A, heliumhelium atat 10,83010,830 AA and the NO y bands inin thethe ultraviolet.ultraviolet.

Thermal RadiationRadiation The process called thermal emission isis oneone inin whichwhich kinetickinetic energyenergy ofof thethe atmosphericatmospheric constituents is transferred to the radiation field.field. This mattermatter-radiation -radiation interactioninteraction is the reverse of absorption. Thermal emission processes constitute aa partpart ofof thethe theorytheory ofof radiative transfer which involves the energy distribution of thethe qasqas molecules.molecules. This energy is possessed inin fourfour forms:forms: translational, electronic, vibrational, andand rotational.rotational. Since the last three forms are quantized, theythey taketake partpart directlydirectly inin thethe exchangeexchange ofof energyenergy from matter to the radiationradiation field.field. Energy transfer between thethe fourfour modesmodes takestakes placeplace during collisions. As there isis an adjustmentadjustment uponupon everyevery collision,collision, thethe translationaltranslational energies of the molecule can rapidly relax to BoltzmannBoltzmann equilibrium. In equilibriumequilibrium the ratio of the number density of molecules which possesspossess anan energyenergy E1E\ relative to the density of those with energy E2E 2 isis describeddescribed byby anan exponentialexponential BoltzmannBoltzmann distribution,

n(E1) _ gl e(EZ-E')/kT(E 2 -Ei)/kT (22) n(E2)n(E 2 ) g2 where n 2= number density ofof moleculesmolecules inin m-3nr 9g != statistical weight ofof statestate E = energy of statestate inin joulesjoules k = Boltzmann constant 1.38047410 xx 10'2310- 23 joulejoule/°K / °K T _= absolute temperaturetemperature inin °K°K Thus, where E represents thethe kinetickinetic energy,energy, EquationEquation (22)(22) leadsleads toto thethe definitiondefinition ofof aa kinetic temperature even in the absence of totaltotal thermodynamicthermodynamic equilibrium.equilibrium. Similarly, equilibrium within a restricted group of energyenergy levelslevels leadsleads toto aa rotationalrotational temperaturetemperature oror vibrational temperature. The atmosphere can bebe ascribed aa kinetickinetic temperaturetemperature (by(by defini-defini­ tion) at altitudes belowbelow thethe exosphere.exosphere. Airglow species are often inin rotationalrotational equilibriumequilibrium but at the higher altitudes are seldom inin vibrationalvibrational equilibrium.equilibrium. The substantial emission observed fromfrom thethe atmosphereatmosphere throughthrough muchmuch ofof thethe infraredinfrared regionregion is due to (1)(1) the presence ofof polyatomicpolyatomic moleculesmolecules havinghaving strongstrong rotationrotation andand vibration vibration- - rotation bands and (2) the fact that at atmosphericatmospheric temperaturestemperatures thermalthermal processesprocesses cancan excite rotational and vibrational transitions.'stransitions. 15

Thermal emissions fromfrom thethe atmosphereatmosphere areare veryvery brightbright inin thethe nearnear andand mid mid-infrared -infrared re-re­ gions. The emissivity of air is high at the wavelengths where the characteristic bands of HH20,2 0, C0CO22 and 0033 occur, such as 2.7, 4.3, 6.3,6.3, 9.6,9.6, andand 1515 um.ym. TheThe majormajor featuresfeatures ofof typicaltypical spectra of atmospheric thermal radiationsradiations areare summarizedsummarized inin TableTable 1.1. These thermalthermal emittersemitters include methanemethane (CH"),(CH*), nitric acid (HNO3),(HN0 3 ), thethe nitricnitric oxidesoxides (NO,(NO, NO2),N0 2 ), andand 0(630(63 pm).ym). Because ofof the temperature and concentration dependence,dependence, thethe thermalthermal emissionsemissions areare mark-mark­ edly dependent upon altitude.altitude. For instance,instance, nearlynearly allall ofof thethe waterwater vapor vapor emission emission originat originat­ -- e's.es, below below 5 km. The zenith spectralspectral radiance typically drops two orders of magnitude from sea levellevel toto 1010 kmkm andand thenthen another tenfold betweenbetween 1010 andand 3030 km.km. The lower atmosphere isis opticallyoptically-thick -thick toto mostmost ofof thethe thermalthermal radiations,radiations, as distin­distin- guished from optically-thinoptically -thin conditionsconditions forfor mostmost ofof the airglow emissions. Radiative energyenergy is transported byby thethe complicatedcomplicated absorption-absorption-reemission reemission processprocess withwith thethe a a-fold-fold attenuationattenuation

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length being less than severalseveral kilometers.kilometers. Optical depth is given byby

DoD Q - DD = Inln LL/LXox /L Xo (23)

where D = optical depth inin meters L^LÀ = spectral radianceradiance inin wattswatts m-2srm" 2 sr~ -'pm-11 ym- 1 at one location relative toto thatthat atat positionposition "o""o" At altitudes belowbelow aboutabout 7070 kmkm thethe opticallyoptically-thick -thick condition prevailsprevails forfor manymany ofof thethe thermal species.species. The good radiators are goodgood absorbers.absorbers. Below this altitude thethe timetime forfor deactivationdeactiyation by collision isis typicallytypically lessless thanthan thethe radiativeradiative lifetime.lifetime. Local thermodynam-thermodynam- ic equilibrium can then generallygenerally bebe assumed.assumed. Due to the infraredinfrared opacityopacity andand thethe largelarge solid angleangle subtended by the surface, earthshine isis an importantimportant sourcesource ofof infraredinfrared energyenergy to the atmosphere belowbelow thethe menopause.mesopause.

Scattered LightLight Looking overhead, aa clearclear daytimedaytime skysky isis somesome 1010 millionmillion timestimes brighterbrighter thanthan thethe nightnight sky. This is, of course, duedue toto scatteringscattering, ofof sunlight.sunlight. Scattering of lightlight inin thethe atmos-atmos­ phere may be grouped intointo threethree categories:categories: (1) resonantresonant scattering by atomsatoms andand moleculesmolecules (2) RayleighRayleigh scattering by atomsatoms andand moleculesmolecules (3) MieMie scattering byby aerosols.aerosols. Resonant scattering occurs when thethe incidentincident photonsphotons havehave aa frequencyfrequency closéclose toto thatthat ofof aa possible energy transition in one of thethe gasgas atomsatoms oror molecules.molecules. The interactinginteractingquantum quantum causes a transition to a higher excited statestate havinghaving aa shortshort lifetime. The transitiontransition toto the lower state takes place in one step,step, andand thethe processprocess isis calledcalled coherentcoherent scattering. The resonant-scatteringresonant- scattering processesprocesses generallygenerally involve groundground-state -state connectedconnected transitions. For molecules which have only narrownarrow statesstates ofof internalinternal energyenergy andand incidentincident photonsphotons whichwhich have a frequency far from that of a possible transition,transition, aa "simple""simple" scatteringscattering modelmodel cancan bebe used for gas molecules. This Rayleigh theory of molecular scatteringscattering givesgives aa scatteringscattering coefficient thatthat isis proportionalproportional toto thethe fourthfourth-power -power of the frequency,16frequency, 16

8-"sR vv44 -c, I/A 4 . (24) ri 1 /ñ4. (24) Thus, in exceptionally clear weather the spectral brightness distribution of the daytime sky differs from the solar spectrumspectrum inin aa generalgeneral wayway byby aa RayleighRayleigh factorfactor whichwhich isis proportionalproportional to a-4.A"\ In turbid atmosphere thethe proportionalityproportionality isis vn\> n wherewhere nn isis generallygenerally lessless thanthan 4.4. A typical daytime spectrum of thethe skysky isis veryvery complicated.complicated. The actual colorcolor ofof thethe skysky depends both upon the solarsolar zenithzenith angleangle andand thethe directiondirection ofof thethe lineline ofof sight.sight. Rayleigh scattering, multiple scattering, selective attenuationattenuation ofof sunlightsunlight byby thethe atmosphericatmospheric gases,gases, and scattered earth albedo allall mustmust bebe takentaken intointo account.account. Spatially, thethe radianceradiance variesvaries about one order of magnitude overover thethe daytimedaytime sky.sky. It is brightest inin thethe aureoleaureole aroundaround the sun and at the horizon; the minimum occurs relatively near the zenith on the solar meridian opposite thethe sun.sun. The Rayleigh-Rayleigh-scattered scattered lightlight hashas aa highhigh degreedegree of optical polarization. The maximum polarization occurs on the solar meridian at aa pointpoint 90°90° awayaway fromfrom thethe sun.sun. The degreedegree ofof polarization (Eq.(Eq. 15) of the sky generally lies betweenbetween 0.50.5 andand 0.70.7 andand doesdoes notnot exceedexceed about 0.85 even on an exceptionally clear dayday with thethe sunsun atat thethe horizon.horizon. There areare threethree angles alongalong thethe meridianmeridian wherewhere thethe polarizationpolarization dropsdrops toto zero.zero."17 Scattered moonlight resembles sunlightsunlight butbut isis veryvery muchmuch weaker.weaker. For a full moon atat nightnight the sky brightness isis aboutabout 22 x 10-610~ 6 that of solar daytime;daytime; forfor aa newnew moonmoon thethe ratioratio dropsdrops to 1010" -99 . The sky brightness due to a high full moon corresponds to that of solarsolar twilight at a solar depression angle.(belowangle.(below thethe horizon)horizon) ofof aboutabout 1010°. °. The night skysky brightnessbrightness duedue to the sun isis significantsignificant toto aa solarsolar depressiondepression angleangle ofof aboutabout 1818°. °. The contribution duedue toto a full moon can bebe neglectedneglected atat aa depressiondepression angleangle ofof greatergreater thanthan about about 7 7°.°. The scattering of lightlight fromfrom "large""large" particlesparticles (>0.1( >0.1 urn) pm) isis modeledmodeled by the complicated Mie theory.theory. In fact, the Rayleigh theory is the small particle special case of the Mie theory.theory.'616 In general,general, the scattering from the large particles varies as vv2,2 , i.e., the scattering isis "whiter.""whiter." The scattering coefficientcoefficient isis lessless thanthan forfor thethe smallsmall particles,particles, and there is a strong forwardforward component.component. These so-so-called called aerosolaerosol particlesparticles areare waterwater droplets, ice crystals, dust, microorganisms, etc., and are a very active optical component of the atmosphere (rainbows,(rainbows, aureoles,aureoles, haloes,haloes, etc.).etc.). Most of thethe aerosolsaerosols areare concentratedconcentrated

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in the height range belowbelow aboutabout 44 km,km, butbut highhigh-altitude -altitude aerosols areare alsoalso presentpresent andand givegive rise to phenomena such as the noctilucentnoetilucent clouds whichwhich occuroccur atat aboutabout 7575 km.km. These luminousluminous clouds makemake their appearance betweenbetween thethe twilighttwilight archarch atat thethe horizonhorizon andand thethe darkeningdarkening night skysky overhead.overhead."18 The zodiacal light isis exoatmosphericexoatmospheric scatteredscattered sunlightsunlight duedue toto interplanetaryinterplanetary dust.dust. The gegenschein is particularly brightbright zodiacalzodiacal lightlight thatthat isis seenseen atat thethe antisolarantisolar direction.direction.

Aurora The aurora borealis in the north and the aurora australis in the south generally appear as faint luminous phenomena duringduring thethe night.night. However, spectacular displays maymay bebe seenseen atat times in certain locations freefree fromfrom scatteredscattered citycity lights.lights. The auroras appearappear inin thethe skysky most frequently in two zones aroundaround thethe magneticmagnetic polespoles ofof thethe Earth.Earth. A wide range of spatial forms and structuresstructures areare seen.seen. General auroral form categories are:are: (1) relatively stable bands or arcs, (2) rays, (3) irregular patches, and (4) large homogeneous areas.areas."19 The auroral light results from the injection of fast, charged particles, principally solar electrons and protons, intointo thethe Earth'sEarth's atmosphere.'atmosphere. 8 The first spectralspectral studiesstudies ofof the aurora were made by Angstrom inin 1867.201867. 20 The optical spectrumspectrum isis characterizedcharacterized byby num-num­ erous emission lines and bandsbands fromfrom neutralneutral andand singlysingly ionizedionized atomicatomic andand molecularmolecular nitrogennitrogen and oxygen.21oxygen. 21 Hydrogen emission lines are regular features of proton auroras; sodium and helium lines are occasionallyoccasionally observed.observed. Table 3 summarizes thethe charactercharacter ofof auroralauroral emission species.species. The most frequently observed auroralauroral emissionsemissions areare thethe atomicatomic oxygenoxygen auroralauroral greengreen lineline at X5577A5577 and the red lineline multiplet atat X6300/64.A6300/64. The excitation ofof thethe greengreen lineline isis byby ^ direct electron excitation ofof 0,0, electronelectron dissociationdissociation ofof 020 2 andand byby energyenergy transfertransfer fromfrom N2.N 2 . The altitude of the emission isis generallygenerally betweenbetween 100100 andand 160160 kmkm wherewhere thethe energyenergy deposition from the auroral particles takestakes place.place. The oxygen excitation statestate whichwhich resultsresults inin thethe red line is longlong-lived -lived (110(110 sec)sec) and is de-de-excited excited byby NN2;2 ; thereforetherefore the red auroraaurora originat-originat­ es from high altitudes. The mechanisms involveinvolve transfertransfer fromfrom NN*, *, secondary electrons (<(< 250250 km) and thermal electrons (>(> 250250 km).km). The radiance of thethe 0(5577 A)A) lineline isis usedused asas thethe standard for categorizing thethe auroralauroral intensity.intensity. The InternationalInternational BrightnessBrightness CoefficientCoefficient (IBC)(IBC)2222 ofof anan auroraaurora is called I,I, II, III, or IV according to whether thethe photonphoton radianceradiance of the green line in kilorayleighski1orayleighs isis 1,1, 10,10, 100,100, oror 1000,1000, respectively.respectively. The ionization of molecular nitrogen leavesleaves manymany ofof thethe molecularmolecular ionsions inin thethe upperupper statestate of the first negative bands,bands, N2 + e--} N$*NZ* + e- + e- * (25) 02-*N;" ++ NjNt + byhv This leads to emission in bands includingincluding thethe 3914,3914, 42784278 andand 47094709 A. The ratioratio betweenbetween the total rate of ionization and the emissionemission ofof X3914X3914 photonsphotons isis 1818 atat 100100 km.km. Due to thethe promptness with which the radiation isis emitted,emitted, eithereither thethe NZN 2 firstfirst negativenegative X3914X3914 oror thethe X4278 band is an even betterbetter auroralauroral diagnosticdiagnostic monitormonitor thanthan isis thethe relativelyrelatively slowly- slowly-emitt­ emitt- ed X5577 greengreen line.line. Emissions of neutral molecularmolecular nitrogennitrogen inin thethe VegardVegard-Kaplan -Kaplan (ultraviolet),(ultraviolet), firstfirst positive (red-(red-infrared), infrared), andand secondsecond positive (violet-(violet-ultraviolet) ultraviolet) bands resultresult fromfrom excita-excita­ tion by secondary electrons2'electrons 23

N2N 2 + e- -> N?N2 + e- (26) N N2 + hv Secondary electrons also produce excitedexcited statesstates ofof molecularmolecular oxygenoxygen whichwhich cancan radiateradiate inin thethe red and infrared24infrared 24 020 2 + e-e~ +-> 020 2 + e- (27](27) 020 2 +-. 0022 ++ h\>hv Balmer lines result from collisionalcollisional excitationexcitation ofof auroralauroral H and also chargecharge exchange,exchange, e.g.,e.g.,

HH++ + 112N 2 -» H* + Nt (28) H* + H + byh\) The emission lines are Ha at 65636563 AA andand H5H$ atat 48614861 A.A.

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SPECTRAL ATLASES A number of good spectralspectral atlasesatlases ofof atmosphericatmospheric emissionemission featuresfeatures areare availableavailable inin thethe literature. For airglow the reader is referred to the reviews and atlases of ChamberlainChamberlain"18 and of Krassovsky et aí.25al. 25 Additional visiblevisible spectral referencesreferences areare byby Ingham26Ingham 26 andand Broadfoot and Kendall.27Kendall. 27 In the infrared the altas of Vallance Jones13Jones 13 andand thethe spectraspectra ofof Connes and Gush,Gush,2828 Gush and Buijs,Buijs,2929 Baker etet aZ.,aZ.,2"21f BunnBunn andand Gush,Gush,3030 andand MurcrayMurcray"31 areare helpful. Table 11 gives a generalized working estimateestimate summarysummary ofof atmosphericatmospheric radiation.radiation. The authors are grateful forfor thethe assistanceassistance ofof LorettaLoretta Clyde,Clyde, KathyKathy Bayn,Bayn, KathyKathy Baker,Baker, Carla Lewis and Glen Allred.Allred. The review by Ralph EmbryEmbry isis alsoalso acknowledged.acknowledged.

REFERENCES

1. D.J.D, J. Baker and W.L. Brown,Brown, "Presentation"Presentation ofof Spectra,"Spectra," App.App. Optics,Optics, Vol.Vol. 5,5, 1966,1966, p. 1333. 2. E.R.E, R. Cohen and B.N.B.N. Taylor,Taylor, J.J. PhysicalPhysical ChemicalChemical ReferenceReference Data,Data, Vol.Vol. 2,2, 1961,1961 , p.p. 715. 3. J,J. Strong, ConceptsConcepts ofof ClassicalClassical Optics,Optics, Freeman,Freeman, SanSan Francisco,Francisco, 1959,1959, pp.pp. 210 210-212. -212. 4. D.J.D, J. Baker and G.J.G.J. Romick,Romick, "Interpretation"Interpretation ofof thethe UnitUnit 'Rayleigh''Rayleigh 1 inin TermsTerms ofof ColumnColumn Emission Rate or Apparent Radiance ExpressedExpressed inin InternationalInternational SystemsSystems Units,"Units, 1 App.App. Optics,Optics, Vol. 15, 1976.1976. 5. D.J.D, J. Baker, "Rayleigh,"Rayleigh, thethe UnitUnit ofof LightLight Radiance,"Radiance," App.App. Optics,Optics, Vol.Vol. 13,13, 1974,1974, p.2160.p.2160. 6. S.S, Newcomb, "An"An InternationalInternational ReviewReview ofof SpectrsocopySpectrsocopy andand AstronomicalAstronomical Physics,"Physics," Astrophysics J., Vol. 14,14, 1901,1901, p.p. 297.297. 7. P.J. van Rhijn, "On"On thethe BrightnessBrightness ofof thethe SkySky atat NightNight andand thethe TotalTotal AmountAmount ofof Star-Star­ light," Astrophysics J., Vol.Vol. 50,50,. p.p. 356. 8. J.W. Chamberlain, Physics ofof thethe AuroraAurora andand Airglow,Airglow, AcademicAcademic Press,Press, NewNew York,York, 1961.1961. 9. S. Chapman, "Some"Some PhenomenaPhenomena ofof thethe UpperUpper Atmosphere,"Atmosphere," (Bakerian(Bakerian Lecture).Lecture). Proc. Roy.Roy. Soc., Vol.Vol. A132,A132, 1931a,1931a, pp.pp. 353353-374. -374. 10. S. Chapman, "Absorption andand DissociativeDissociative oror IonizingIonizing EffectsEffects ofof MonochromaticMonochromatic Radia-Radia­ tion in an Atmosphere onon aa RotatingRotating Earth,"Earth," Proc.Proc. Phys.Phys. Soc.,Soc., Vol.Vol. 43,43, 1931b,1931b, pp.pp. 2626-45. -45. 11. D.R. Bates and H.S. Massey, "The"The BasicBasic ReactionsReactions inin thethe UpperUpper Atmosphere.Atmosphere. H.II. The Theory of Recombination inin thethe IonizedIonized Layers,"Layers," Proc.Proc. Roy.Roy. Soc.,Soc., Vol.Vol 192,192, 1947a,1947a, pp.pp. 1-16,1 -16. 12. D.R. Bates and M. Nicolet, "The"The PhotochemistryPhotochemistry ofof AtmosphericAtmospheric WaterWater Vapor,"Vapor," J.J. Geophys. Res., Vol.Vol. 55,55, 1950b,1950b, pp.pp. 301301-327. -327. 13. A. ValianceVallance Jones,Jones, "The"The InfraredInfrared SpectrumSpectrum ofof thethe Airglow,"Airglow," SpaceSpace Sci.Sci. Rev.,Vol.Rev., Vol. 15,15, 1973, pp.pp. 355355-400. -400. 14. D.M. Hunten, "Airglow-" Ai rgl ow-Introducti Introduction on andand Review,"Review," The Radiating Atmosphere, B.M.B.M. McCormac (ed.),(ed.), D.D. ReidelReidel Publ.Pub! . Co.,Co., Dordrecht,Dordrecht, Holland,Holland, 1971.1971. 15. J.T. Houghton, "Infrared"Infrared EmissionEmission fromfrom thethe StratosphereStratosphere andand Mesosphere,"Mesosphere, " Proc.Proc. Roy.Roy. Soc.Soc., , Vol.Vol . 288,288, 1965,1965, p.p. 545. 16.16 . R.IR.M.4. Goody, Atmospheric Radiation, ClarendonClarendon Press,Press, Oxford,Oxford, 1964,1964, p.p. 290.290. 1717.. G.G.V.1/. Rozenberg, Twilight, A Study inin Atmospheric Optics,Optics, PlenumPlenum Press,Press, NewNew York,York, 1966, p.p. 1!15. 1818.. R.K.R.!,K. Soberman, "Noctilucent Clouds,"Clouds," Scientific American,American, Vol.Vol. 208,208, p.p. 50. 1919.. A. Omholt, The OpticalOptical Aurora,Aurora, SpringerSpri nger-Verl -Verlag, ag , Berlin, 1971. 2020.. S. Chapman, "History of Aurora Airglow,"Airglow," Aurora andand Airglow,Airglow, ReinholdReinhold PublishingPublishing Corp.,Corp. , New York,York, 1967,1967, p.p. 15. 2121.. A. Vallance Jones, "Auroral"Auroral Spectroscopy,"Spectroscopy ," SpaceSpace Sci.Sci. Rev.,Rev., Vol.Vol. 11,11, 1971,1971, p.p. 776.776 2222.. M. Walt, Auroral Phenomena, StanfordStanford UniversityUniversity Press,Press, Stanford,Stanford, 1965,1965, p.p. 40.40. 2323.. R.C.R. I. Whitten and I.G.I.G. Poppoff,Poppoff, Fundamentals ofof Aeronomy,Aeronomy, Wiley,Wiley, NewNew York,York, 1971,1971, p.3.p 3. 2424.. D. Baker, A. Steed, R. HuppiHuppi andand W. Pendleton,Pendleton, Jr.,Jr., "Near"Near InfraredInfrared SpectrumSpectrum of an AuroAurora,""a," J. Geophys. Res, Vol. 81, 19761976 (in(in publication).publication). 2525. o V.V.I.I. Krassovsky, N.N. ShefovShefov andand V.I.V.I. Yarin,Yarin, "Atlas"Atlas ofof thethe AirglowAirglow SpectrumSpectrum 3000-3000- 12400 A," Planetary; SpaceSpace Sci.,Sci., Vol.Vol. 9,9, 1962,1962, pp.pp. 883883-915. -915. 2626.. M.!M.F.F. Ingham, "The Spectrum ofof thethe Airglow,"Airglow," ScientificScientific American,Ameriean, Vol.Vol. 226,226, 1972,1972, pp. 7878-85. -85. 0 2727.. A.!A.L.L. Broadfoot andand K.R.K.R. Kendall,Kendall "The"The AirglowAirglow Spectrum,Spectrum, 31003100-10,000 -10,000 A,"A," J.J. Geophys.Geophys. Res.,Res. , Vol.Vol . 73,73, 1968,1968, pp.pp. 426426-429. -429. 2828.. J. Connes and H.PH.P. Gush, "Spectroscopic du Ciel Nocturne dans 1l'InfrarougeInfrarouge par Transformation de Fourier,"Fourier " J. Phys. Radium, Vol.Vol. 20,20, 1959,1959, p.p. 915.915. 29. H.P. Gush and H.L.H.L Buijs,Buijs, "The"The NearNear InfraredInfrared SpectrumSpectrum ofof thethe NightNight AirglowAirglow ObservedObserved from High Altitude," Can. J. Phys., Vol.Vol. 42,42, 1964,1964, p.p. 1037.1037. 30. F.E. Bunn and H.P.H.P Gush,Gush, "Spectrum"Spectrum ofof thethe AirglowAirglow BetweenBetween 33 andand 44 Microns,"Microns Can.Can. J.J. Phys., Vol. 50,50, 1972,1972, p.p. 213213 (also(also "Between"Between 44 andand 88 Microns,"Microns," Vol.Vol. 48,48, 1970,1970, p.p. 98).98). 31. D.G. Murcray et ai.,al., "Atomspheric"Atomspheric EmissionEmission atat HighHigh Altitudes,"Altitudes," FinalFinal ReportReport AFCRL-AFCRL- 7272-0353, -0353, UniversityUniversity ofof Denver, JuneJune 1972.1972, 32. J. Hennes and L. Dunkleman,Dunkleman, "Photographic"Photographic ObservationsObservations ofof NightglowNightglow fromfrom Rockets,"Rockets, J. Geophys. Res., Vol.Vol. 71,71, 1966,1966, p.p. 755.755.

58 /SPIE/ SPIE Vol.Vol. 9191 MethodsMethods forfor AtmosphericA tmospheric Radiometry (1976)

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FREQUENCY (THz)(THz) IO7 .OOI.001 1/4I/4 I 1010 JOO100 250 1000 25002500 2.5 xx 104IO 4 107

1GH.I GHz * 1 I 1 1 1 ' 1 WAVENUMBER (cm"(cm)1 ) .04.04 A IO 4004OO 40OO4000 104IO4 25k l05IO5 106IO6 ^ 4 xx 108IO e 1 \ T 1 v RADIO WAVES MICROWAVES FAR INFRAREDINFRARED MID NEAR VISIBLE ULTRAVIOLET XX-RAYS -RAYS GAMMAGAM MA- -RAYS RAYS INFRAREDINFRARED IR

Rotation Rotation-Rotction- \ 750075OQ - Inner -Shell-Shell VibrationVi brat ion c o0 \ o Electron Fundamentals 0 A 40004OOO AA Transitions l«ay fiD N§ 1 S2 OuterOuter-Shell -Shell c o .1'> Electron áo ° TransitionsTransitions o o o lOOO A 'I 1000 A 100IOO AA I/41/4 AA 1 At 11 I/*1/4 ImIm 25p.25/i 2.5p.2.5/i lµl/i O.1p.0.1/1 IOnlOn .025,O25 nn WAVELENGTH (m)(m)

. 1 I .I iI I I I I AJ 5µ .001 .01.01 05D5 O.I0.1 0:50.5 II 10IO 100 50k PHOTON ENERGYENERGY (eV) (eV)

Fig. 1. Radiant energyen e rgy spectrums pec t r urn interpretedi nt e r p r e t ed ini n termst e rm s ofof gasgas emissions.em i s s i o ns.

Fig. 2. Airglow layerlayer seenseen edgeedge-on -on from rocket atat 9393 km.3zkm. 32

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Table 1.1. Working-Working-estimate estimate summary of typical atmospheric radiation features.features. W(Aelorngm) Rayleigh Radiance (R)(R) inin thethe ZenithZenith Wavelengtha Species Identification^Identificationb (A or pm) Day Twilight Night IBC III Aurora

304 A He+He 22P2 2 P 5-10 {750}c{750}c 584 He 2p'P°(22p 1 P°(2 uv)uv) h-l2-1 k {750}{750} <^ 55 834 0+ 2s2p" "P(1TO uv)uv) 0.5k {200} 15{~130-215}15{-130-215} 5k (est.)(est.) 955-1438 NN22 b'IIu(B-H)b x nu (B-H) 989 0 3s3s'3D°(5r3 D°(5 uv)uv) ~lk (est.)(est.) 1026 H 32P(4)32 P(L3 ) 10 {200}d{200}d 1027 0 3d3D°(43d 3 D°(4 uv)uv) 1085 N+ 2s2p32s2p 3 3D°(13 D°(1 uv)uv) lkIk (est.)(est.) 1135 N 2s2p" 4P(2"P(2 uv)uv) 1152 0 3s3s''D°(6fl D°(6 uv) ~lk-lk (est.)(est.) 1200 N 3s3s"P(14 P(l uv)uv) 0.4 {180} 5k 1216 H 22P(La,)2 2 P(La ) 14k {1000} 4k [102-105][102 -10 5 ] 1300-1500 NN22 aa'IIlng g(L-B-H)(L-B-H) 7k {140}{140} ~100k-100k 1304 0 3s3s3S°(23 S°(2 uv) 7.5k {190} 25k 1356 0 3s5S°(13s 5S°(l uv)uv) 0.35k {140} 5k 1493, 1744 N 3s3s2P(42 P(4 uv, 9 uv)uv) ~1.5k-1.5k 1900-2300 NO AA2E+(y2Z+ (y bands)bands) Iklk [70-150] 2000-4000 NN22 AA3E-tj(V-K)3 zJ(V-K) 45k (var.)(var.) 2600-3800 0022 AA3Eú(Herzberg3zJ(Herzberg I)I) < 1.5k [90][90] 2800-5000 NN22 C3IIu(2P)C 3nu (2P) 0.9k [>[> 130]130] 90k {(250)@3371{ ( ^ )f°3371 2972 0 'S [2F] 0.15k 20 12 5k 3466 N 22PP [2F] 2.4k (var.)(var.) 3500-5500 NtNZ BB2Eú(lN)2Z+(1N) 2k [150][150] 0.2-0.5 [300][300] < 11 150k 3727 0+0+ 22DD [!F]&3p[1F]&30S°(3)lfS 0 (3) 3736 0+ 30S°(3)3p 4 S°(3) 3p 3 P°(2) 3889 He 3p3P°(2) 11 [> 400] 3914 N+NZ B2Eú[1NB2Z+[1N (0,0)](0,0)] 100k 3934 CaCa++ 4p2P4p2 P ÷+ 4s2S4s 2 S 3947 0 4p5P(3)4p 5 P(3) -O.lk-O.Ik 3967 0+0+ 3p2P°(6)3p 2 P°(6) 4041 NN++ 4f3G(39)4f 3 G(39) 0.3k 4076 00++ 3d3d"F(10)4 F(10) -0.5k 4278 NjN2 BB24.-42Ejj [IN [1N (0,1)](0,1)] 30k 4340 H 552(S,P,D)2 (S,P,D) (Hy)(Hy)

4368 0 4p4p3P(5)3 P(5) 1 0.3k 4418 00++ 3p3p2D°(5)2 D°(5) 0.2k 4623 N+ 3p3p3P(5)3 P(5) 0.3k 4649 0+ Sp^D^l)3p"D°(1) -0.5k~0.5k

4861 H 442(S,P,D)2 (S,P,D) (Hß)(H3 ) 1 {200} -O.lk-0.1k (H(H aurora)aurora) 5000-6500 N0NO22 continuum ~1'1 per A 26k HIj(1,0)@5609 ,0)05609 5000-9000 otOZ b^q(lN)b"E5(1N) 26k 5005 NN++ 3d3d3F°(19)3 F°(19) 0.6k 600kí(0,0)@l.11um 0.505-,-'2.50.505--2.5 \mum N2 A 2IIu(Meinel)nu (Meinel) 10k 600k{{01110k60)il.llym 5200 N 2?D°D° [1F][IF] 90 [200-250] 10 << 11 0.3k 5300-6300 OH XX2II(Ov=62n(Az;=6 seq.) 0.2k [85][85]

60 /SPIE Vol.Vol. 9191 MethodsMethods for Atmospheric Radiometry (1976)

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Table 1.1. (cont.) WavelengthWavelengthaa Species Identificationl'Identification* Rayleigh Radiance (R)(R) inin thethe ZenithZenith (A or um)ym) Day Twilight Night IBC III Aurora 0.53--70.53-7 umym NN22 BB3II9(1P)3ng (iP) 10k [150] 900k900k30k |(0>0)01.05ym )@1.05 um 5577 0 >S'S [3F] 3k [90-300] 0.4k 0.25k 100k 5679 N+ 5679 N+ 3p3D(3)3p 3 D(3) 0.7k 5876 He 3d3D(11)3d 3 D(ll) 5890-6 Na 3p2P3p2 P ->÷ 3s2S(D-lines)3s 2 S(D-lines) 30k [92] 1-4kl-4k [92][92] 0.02-0.15 [92][92] < lkIk 6157 6157 0 4d4d5D°(10)5 D°(10) ~0.1 6200-7700 OH XX2II(Av=52n(A?;=5 seq.) 2k [85][85] 6300-64 6300-64 0 'D [1F][IF] 2-20k [250-300][250-300] lkIk 0.01-0.0.01-0.5k 5k 7k b'Z+fAt) 6350-9000 0022 b'E9(At) 0.3M [40-120] -lk~lk 1.3M 6563 6563 H 332(S,P,D)(Ha)2 (S,P,D)(Ha ) 3 [200] ~0.3k-0.3k 2p 2 P°(l) 6708 Li 2p2P°(1) 0.01-lk0.01-1k [90][90] 0.20k [90][90] 0+ 2po (- 2F:] 7319-30 0+ 2P° [2F] < lkIk 0.4 0.75-1.01 ympm OH X2II(Av=4)X 2n(A^=4) 12k [85] 7619 0022 b'E9b l Z+ [At(0,0)][At(0,0)] 300k [40-120] 6k -1M~1M 4p 2 7699 K 4p2PP ->+ 4s2S4s 2 S 40 7774 7774 0 3p5P(1)3p 5 P(D -1.6k -0.2k~0.2k 7k 8212 3p 4 P°(2) 8212 N 3p4P0(2) 3k 8447 8447 0 3p3P(4)3p 3 P(4) -1.1k-l.lk 15k 8618 3p2 P°(8) 8618 N 3p2P°(8) 4k 8645 02 b l Z+ 8645 02 b'E9 [At (0,1)](0,1)] 10k [40-120] Iklk [80] 60k 8692 3p"D°(l) 8692 N 3p4D°(1) Ilkllk 9400 HH202 0 (201) B1B! 0.98-1.4 pmym OH XX2II(Av=3)2 IT(Az;=3) 90k [85] ~1 -1 N0NO22 continuum 10M 1.04 2 P°[3F] 1.04 N 2P°[3F] 35k (est.)(est.) 1.06-1.60 0022 a^IRa'Ag(IR At)At) 20M [60-70][60-70] 5M ~85k:85k [90 km] ~1M (var.)(var.) 2p 3 P°(l) 1.083 He 2p3P°(1) 0.1-3k [500][500] 1.129 0 3d3D°3d 3 D° 1.130 0 4s4s5S°(7)5 S°(7) 1.14 H20H20 (HI)(111) B1B! 1.27 1.27 OZ02 aa'Ag[IR1Ag [IR At (0,0)](0,0)] .20M~20M .5M*5M ',80k~80k ~1M1M (var.)(var.) 1.316 4s 3 S° -> 3p 3 P 1.316 0 4s3S° ± 3p3P 2k 1.38 HH202 0 (101) B1B! 1.4-2.3 OH XX2II(Av=2)2n(Az;=2) 0.55M 1.58 0022 a'Ag [IR[IR At (0,1)](0,1)] 300k [60-70][60-70] 80k 4k 1.88 HH202 0 (001) 131B! 2.0 C0CO22 (12°1) 2.65-2.80 HH202 0 (IOO)A!(100)A, & (100)B1(IOO)B! 35k {9}{9} 2.7-3.5 NO XX21I(Av=2)2n(At>=2) 2.8-4.5 OH xX2II(Av=1)2n(A^=i) 0.35M [85] 3.3 CH4 (ooonooi)(v(00011001)(v3(f2))3 (/2 )) J2M [85] 4.3 (2M [85] 4.3 CO2C02 (00°l)(v(00°1)(v3)3 ) nor'I160M [12][12] 4.5 N20N 2 0 (00°l)(v(00°1)(v3)3 ) 4.8 CO2C0 2 (ll'O)(11'0) 0.2M [85][85] 4.9-6.0 NO xX2II(Av=1)2n(A^=i) 4.7M {84}

SPIE Vol. 91 Methods forfor AAtmospheric tmospheric Radiometry (1976) / 61

Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/13/2014 Terms of Use: http://spiedl.org/terms DORAN J. BAKER, WILLIAM R.R. PENDLETON,PENDLETON, JR.JR.

Table 1.1. (cont.) WavelengthWveor pm)a Rayleigh Radiance (R) in the Zenith Species IdentificationIdentificationb25 — Rayleigh Radiance (R) in the Zenith (A or ym) Day Twilight Night IBC III Aurora 6.2 pmym N0NO22 vv3(b1)3 (bi) 6.36,3 HH2O20 (010) AlA! 1800M [12] 7.67,6 CH4 (00000111(00000111) )(v(v4(f2))4 (/2 )) 7.8 N20N 20 (10 0°0)(vl)0)(V!) 2500M [12][12] 9.4 CO2C02 (00°1)-(02°0)(00 °1)- (02 °0) 9.6 030 3 (001)(001)(V3(b1)) (v 3 (2?i)) 10M {80} 16M (II)(II) 10.3 CO2C02 (00(00°1 °1)- )-(10°0) (10 °0) 19k {80} 11.3 HNO3HN0 3 1500M {8}{8} 13.8 CO2C02 (10°0)-(01(10 °0)- (0110) 1 0) 0.35M {80}{80} 0.7M (II)(II) 1237M {80} 15.0 CO2C02 (01(0110)(v2)1 0)(v 2 ) I237M {80} 142,000M|42 S OOOM {8}{8} 17.0 N20N2 0 (Ol'OMv.)(01'0)(v2) 0.34M {80}{80} 19.2 H2OH20 rotational 7k {7}{7} 63 0 33P2P2 --3P13 Pi 1.7M {120}{120} 147 0 33P1Pi- -3P03 Po

(a) Wavelengths givengiven forfor atomicatomic featuresfeatures areare commonlycommonly-used -used values forfor thethe multiplets.multiplets. The molecularmolecular-band -band wavelengthswavelengths areare alsoalso commonlycommonly-used -used values.values. Spectral intervalsintervals areare in-in­ dicated for band systems.systems. (b) IdentiIdentifications: f ications : (1) The designationdesignation ofof thethe upperupper statestate forfor aa transitiontransition isis followedfollowed by by the the multi multi- - plet number On(in parentheses),parentheses ), oror otherother suitablesuitable designationdesignation ofof thethe transition.transition. The multiplet numbersnumbers followfollow thethe widely-widely-adopted adopted assignmentsassignments ofof Mrs.Mrs. MooreMoore-Si -Sitterly tterly inin her NBS compilation ofof spectralspectral data.data. In cases where the transition isis notnot in-in­ dicated, it may bebe assumedassumed thatthat thethe lowerlower levellevel isis thethe groundground state.state. (2) Key to abbreviations: 11 uv E ultraviolet multiplet No.No. 1 in NBS compilations.compilations. (L(La,a , Lg)Lß) E Lyman alpha and beta emissions of atomicatomic hydrogen.hydrogen. L-B-HL -B -H ELyman-Lyman-Birge-Hopf Birge -Hopfield ield (a(a -»• X) band systemsystem ofof N2.N 2 . V-KV -KEVegard-Vegard-Kaplan Kaplan (A 4-•> X)X) bandband system ofof N2.Na­ y-bandsy -bands E GammaGamma bandband systemsystem (A(A 4.-+• X) ofof NO.NO. Herzberg I s A ->- XX systemsystem ofof 02.0 2 . IP1P E First positivepositive systemsystem (B(B -> A) ofof N2.N 2 . 2P s Second positive system (C(C •> B) of N2.N 2 . IF1F - Forbidden multiplet No.No. 1 in NBS compilations. IN1N E First negative system of Nt (B(B }-> X)X) andand OZ0 2 (b(b -> a). (H(H,,ft , Hß)H^) E Balmer alpha and beta emissions ofof atomicatomic hydrogen.hydrogen. Meinel = Meinel band systemsystem (A(A } X)X) ofof N|.N. At = Atmospheric systemsystem (b(b 3 X)X) ofof 02.0 2 . IR At E Infrared atmosphericatmospheric systemsystem (a(a ->} X)X) ofof 0O.2 . (Q)(c) 5-105 -10 {750}{750} indicatesindicates aa zenithzenith intensity ofof 55-10 -10 RR fromfrom anan observingobserving altitude ofof 750 km.km. An entry suchsuch asas lkIk [70[70-150] -150] indicatesindicates a total zenith intensityintensity ofof 1000R1000R resultingresulting from an emitting region predominantly betweenbetween 7070 andand 150150 km.km. An entryentry suchsuch asas 2k2k [85][85] indicates a total zenith intensity associated with anan emittingemitting regionregion havinghaving aa character-character­ istic altitude ofof 8585 km.km.

(d) A number ofof thethe entriesentries inin thisthis tabletable areare fromfrom Hunten.1°Hunten. 11*

62 //SPIE SPIE Vol.Vol. 9191 MethodsMethods for AtmosphericA tmospheric Radiometry (1976)

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