Utah State University DigitalCommons@USU Space Dynamics Lab Publications Space Dynamics Lab 1-1-1976 Optical Radiation from the Atmosphere Doran J. Baker Utah State University William R. Pendleton Jr. Utah State University Follow this and additional works at: https://digitalcommons.usu.edu/sdl_pubs 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 withoutwithout 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 thethe farfar infraredinfrared upup toto 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 50 /SPIE Vol.Vol. 9191 MethodsMethods forfor AtmosphericAtmospheric Radiometry (1976) Downloaded From: http://proceedings.spiedigitallibrary.org/ on 10/13/2014 Terms of Use: http://spiedl.org/terms OPTICAL RADIATION FROMFROM THETHE ATMOSPHEREATMOSPHERE 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