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The solar electromagnetic radiation environment
Solar Ener~,.y.Vol. 12.pp. ]97-203. PergamonPress.1968. Printedin Great Britain
THE SOLAR ELECTROMAGNETIC R A D I A T I O N ENVIRONMENT* H A R R I E T H. MALITSON~: (Received 21 February 1968~
Abstract--Observations of the spectral distribution of solar electromagnetic radiation have been made from wavelengths shorter than 1 A (I 0-acre) to wavelengths longer than 100 m (104cm) by many different techniques, each of which is applicable over only a smaJl part of the total range. Most of the energy emitted by the sun (98 per cent) lies between 3000 and 40,000 A. A large fraction of the solar radiation falling between these wavelength limits manages to penetrate the Earth's atmosphere, so that it can be studied from ground level. By observing the amount of solar energy reaching the ground at many times during the day, an extrapolation can be made that gives a reasonably accurate value of the energy received at the top of the atmosp h e r e - the solar constant. No radiation from the Sun at wavelengths below approximately 2900 ,~. has been detected at ground level, and very little radiation in the i.r. at wavelengths greater than 30-40.000 ,~ can come through the atmosphere. Therefore. observations in these spectral regions must be made from rockets or satellites. Balloons and certain aircraft can also be used to advantage, especially in the i.r. In addition to the usual difficulties involved in the carrying out of space experiments, there are those due to the lack of adequate laboratory standards and the variability of the Sun in the extreme u.v. and X-ray region. Nevertheless, the uncertainties in information about the spectral distribution of solar radiant flux are smaller than the departures from the solar irradiance curve of the flux from the best high energy solar simulators available. R 6 s u m 6 - D e s observations de la distribution spectrale de la radiation solaire 61ectromagn6tique, ont 6td fares en partant de longueurs d'onde inf6rieures ~ 1A (10 -s cm)jusqu'/~ des Iongueurs d'onde sup6rieures 100 m ( 104 cm), par diff6rentes m6thodes, chacune s'applicant/t une faible part seulement de la totalit6 de la gamme. La plus grande pattie de l'6nergie 6raise par le soleil (98 pour cent) se trouve entre 3000 et 40,000 A. Une fraction importante de la radiation solaire qui tombe entre ces limites de Iongueurs d'onde, r6ussit/~ pdn6trer dans ratmosph6re terrestre, de telle sorte qu'on peut 1"6tudier au sol. En observant la quantit6 d'6nergie solaire qui atteind le sol fi diff6rents moments de la journde, on peut faire une extrapolation qui donne une valeur sutSsamment pr6cise de l'6nergie revue au sommet de la zone atmosph6rique-la constante solaire. A des longueurs d'onde inf6rieures ~t environ 2900 A. il n'a pas 6t6 possible de d6tecter de radiation au sol et, seule, une tr6s petite radiation dans I'infra-rouge/~ des Iongueurs d'onde sup6rieures h 30-40.000 A peut traverser ratmosph6re. Par cons6quent, les obervations dans ces r6gions spectrales doivent ,~tre faites par fusees ou satellites. Les ballons et certains avions peuvent aussi 6tre utilises, surtout dans la zone infrarouge. En plus des difticult6s habituelles que l'on rencontre au cours des experiences spatiales, il y a celles dues au manque de normes de laboratoire ad6quates et ~ la variabilit6 du soleil dans la zone extr6me de l'ultraviolet et des rayons X. N6anmoins, les incertitudes rencontr6es quand on recueille des informations concernant la distribution spectrale de flux solaire radiant, sont inf6rieures/l celles rencontrees h partir de la courbe d'irradiance solaire, du flux des meilleurs simulateurs solaire de haute 6nergie disponibles. Resumen-- Se han hecho observaciones sobre la distribuci6n espectral de la radiaci6n electromagn6tica solar con longitudes de onda desde menores de I ,~ (l 0 -s cm) hasta mayores de 100m ( 104 cm), mediante empleo de muy diversas t6cnicas, cada una de las cuales corresponde a s61o una pequefia parte de la gama total. La mayor parte de la energia emitida por el sol (un 98 pot ciento), se halla comprendida entre 3000 y 40,000 ,~. U ne porcion importante de la radiaci6n situada entre dichos limites de longitud de onda consique penetrar la atmdsfera terrestre, Io que permite su estudio a nivel del sue{o. Mediante observaci6n de la cantidad de energia solar que incide sobre el sue{o a muchas horas del dia, es posible hacer una extrapolaci6n que indique con bastante exactitud el valor de la energia recibida en la parle superior de la atmosfera, es decir, la llamada constante solar. No se ha detectado a nivel del suelo radiaci6n solar con longitud de onda menor de 2900 A. aproximadamente, ni puede atravesar la atm6sfera gran cantidad de radiaci6n infrarroja con longitudes de onda inferiores a 30-40.000 A. Pot Io tanto, la observaci6n de estas regiones espectrales debe ser hecha desde * 1967 Solar Energy Society Conference paper. t Laboratory for Space Sciences, NASA-Goddard Space Flight Center, Greenbelt. Maryland, U.S.A. 197
198
H.H.
MALITSON
cohetes o satglites. Tambien se logran resultados satisfactorios empleando globos y determinados tipos de avi6n, especialmente en lo que respecta a la regi6n infrarroja. Ademb,s de las dificultades que normalmente han de afrontarse en la realizaci6n de experimentos espaciales, est:~fi las motivadas por falta de adecuades normas de laboratorio y por la variabilidad del sol en la regi6n extrema de rayos ultra-violeta y rayos X. Sin embargo, los elementos de duda en la informaci6n sobre distribuci6n del flujo radiaaere solar son menores que las divergencias, respecto a la curva de irradiaci6n solar, del flujo presentado por los mejores simuladores solares de gran capacidad disponibles.
INTRODUCTION
A KNOWLEDGE of the nature and effects of solar electromagnetic radiation is essential for the design of space vehicles and their instrumentation. This radiation is a very important factor in the thermal equilibrium of spacecraft, and can cause changes in the properties of both organic and inorganic materials leading to degradation and possible failure of the mission. The following review of the present state of knowledge of the solar electromagnetic radiation environment outside the Earth's atmosphere is presented for the information of those who may be involved in spacecraft or space experiment design. The curve in Fig. 1 represents the distribution with wavelength of the Sun's radiant energy falling on one square centimeter at the top of the Earth's atmosphere when the Earth is at its mean distance from the Sun, one astronomical unit or 1.5 x 10:acm. The so-called solar constant, or the total energy at all wavelengths received from the Sun at the top of the atmosphere, has a value of 2.00 ___0.04 cal/cm 2 min. The solar constant would equal the total area under the curve, if the curve were plotted on a linear scale and extended from zero to infinity in wavelength. Solar radiation covers the entire electromagnetic spectrum from waves shorter than 1 A or 10-8 cm to those longer than 100 m (104 cm). Roughly 98 per cent of the energy in the solar spectrum lies between 3000 and 40,000 A(4 ~m). These wavelength limits define one of two spectral "windows" where the Earth's atmosphere is transparent to radiation (the other is the radio window, which will not be discussed here). VISIBLE
WINDOW:
3000A-4/~m
The designation "visible" is somewhat misleading, since the human eye is sensitive only to radiation of wavelengths about 4000-8000 A. Thus this window includes some radiation in the near u.v. and near i.r. When viewed through a spectroscope, the solar spectrum consists of a bright continuous band overlain by thousands of dark absorption lines, called Fraunhofer lines. The continuum arises in the hotter photospheric levels of the Sun, and the lines come from absorption of the continuum at certain discrete frequencies by atoms of various elements in the higher, cooler photospheric layers. In Fig. 1, the effect of the strongest of the dark lines is barely apparent in the slight irregularity of the solar curve near its peak. which falls at 4700 ,~ in the visible. The classical method[l, 2] for determining the solar irradiance in the visible is due to Langley. Briefly. it consists of making measurements, over a certain wavelength range, of solar energy received at many times during the day so that the Sun is at various altitudes above the horizon. The values obtained are then extrapolated to intensity outside the Earth's atmosphere by means of the relation I = loe -~''
( 1)
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199
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Fig. I. T h e solar electromagnetic radiation spectrum. Solid lines represent m e a s u r e m e n t s : dotted lines, estimates. F = reference [ I 0] (rocket m e a s u r e m e n t s ) . N . & A . = reference [ 11 ] (estimated from ionospheric data). O S O - 1 = data from first orbiting solar observatory. W. A. White. Private c o m m u n i c a t i o n 11964). U K - 1 = reference[9] !Ariel I satellite date).
(
200
H. H, M A L I T S O N
solar radiation through the atmosphere when the Sun is at a certain elevation h. to the path length for the Sun in the zenith, corrected for the difference in atmospheric pressure at the observing site and at sea level), 1 = measured intensity when the Sun is at elevation h and I0 = intensity at the top of the Earth's atmosphere. When log/ is plotted against m, a straight line results under ideal conditions. However. conditions are always less than ideal, even when the measuring instrument is placed at a location where the atmosphere is very clear and stable. The variability of the atmosphere is the major source of error. Others are stray light in the instrument. which is particularly troublesome near the ends of the wavelength range where the solar intensity is low; and radiation from around the Sun when there is haze. The relative values obtained by Dunkelman and Scolnik[2] were considered by them to be accurate t o _ 3 per cent except near the ends of the wavelength range, where the estimated error rose to ± 6 per cent. Converting the relative values to absolute values by comparing the measured illuminance of the standard lamp used for calibration with the CIE standard luminosity curve, involved an additional error of ± 10 per cent. Johnson [ 1], from a study of the Smithsonian solar-constant work and rocket u.v. measurements, concluded that the absolute scale of Dunkelman and Scoinik should be raised by 9 per cent. The curve given here includes this revision. For wavelengths from 6500 ,~ to 1-2 tzm, Johnson used the 1940 data of Moon[3], which are probably still the best available. From 1.2 to 2.4/xm, Moon had found that a 6000°K gray body curve fitted the observations well. Beyond 2.4/~m, the molecular absorption bands of such atmospheric constituents as oxygen, water vapor and carbon dioxide absorb so strongly that a continuous spectrum cannot be observed from the Earth. I N F R A R E D R E G I O N : 4~tm TO 100g.m A N D UP
The i.r. spectrum also has the appearance of a continuum crossed by dark lines, but in addition to the atomic lines, there are molecular bands made up of tens or hundreds of individual lines. Actually, this description is based on observations in the near i.r. at 1 or 2/zm. Solar spectrograms for the middle and far i.r. are not yet in existence, and will have to await further development of detectors. The most reliable measurements of the absolute intensity of solar radiation in the i.r. are recent ones from balloons [4, 13], although there is one ground-based measurement[5] in a narrow window at 11.1/zm. The results indicate that the i.r. radiation of the Sun is less intense than one would expect from extrapolation of the visible radiation to longer wavelengths. At the peak of solar emission at 4700 A, the radiation is characteristic of a 6100°K blackbody; at 4/~m, the blackbody temperature has fallen to about 5600°K, at 11.1/zm it is roughly 5000°K, and 100/~m it has decreased to the neighborhood of 4300°K. U L T R A V I O L E T A N D X-RAYS: 3000 ,~ DOWN
Below 2900 X, no radiation from the Sun has been detected at ground level. Atmospheric ozone in a layer 10--40 km high absorbs down to about 2200 A, where molecular oxygen above 75 km takes over and does most of the absorbing down to 900 ,~. Below this wavelength, many atmospheric constituents absorb all radiation before it gets within 150 km of the Earth's surface. Rockets and satellites, therefore, must be used to obtain data from 2900 ,~ down. Since October 1946, when the Naval Research Laboratory obtained the first solar
The solar electromagnetic radiation environment
201
spectrum down to about 2100 ,~ from a captured V-2 rocket, there has been much effort directed toward obtaining an exact knowledge of the short wavelength component of solar radiation. Several excellent review articles exist[14-16], giving the present status of solar u.v. and X-ray work, and there will be no attempt to give a complete discussion here. Starting at 3000 f~, the spectrum is again a bright continuum with many dark lines across it. As the wavelength decreases, the lines become more closely packed and the continuum falls off rapidly in brightness. Consequently, the spectral irradiance decreases very rapidly indeed (Table l). There is an especially abrupt decrease in continuum intensity at 2085/~ so that the Fraunhofer lines are very weak and shallow. Bright lines begin to appear superimposed on the weak continuum. At wavelengths shorter than 1700 A, the Fraunhofer lines are gone, the continuum is extremely weak, and bright lines are responsible for most of the emission. The bright lines arise from elements in the chromosphere of the Sun, which is considerably hotter than the photosphere Table I. Fraction of solar constant below indicated wavelength [ 15] Wavelength tAt 7000 4000 3000 2400 1900 1600 1000
Fraction 0.5 0.1 0.01 0.001 0.0001 0.00001 0.000001
although it lies above it. In the far u.v. and X-ray portions of the spectrum, many of the bright lines arise in the corona of the Sun, which is higher than the chromosphere and has a very high temperature, about 1.5 × 106°K. The most reliable results to date[6, 16] on the energy distribution in the solar spectrum from 3000-1027 A are presented in Table 2. The accuracy of these measurements has been estimated as _ l 0 per cent to 2000 ,~ and better than-_4-_20 per cent from 2000 to 1400,~. Below 1400/~ there may be errors as great as 40 per cent. These errors arise in large part from difficulties in the calibration of flight instruments due to a lack of absolute laboratory standards in the u.v. It should be realized, however, that even perfectly calibrated instruments will give different values depending on the acitivity of the Sun at the time. In the visible, the Sun does not change its output by more than 1 or 2 per cent. However, at wavelengths shorter than about 1300 A, more and more variability occurs, and increases of several orders of magnitude in a few seconds are possible at wavelengths shorter than 10 ,~. Such increases, called X-ray bursts, occur in connection with solar flares and related activity, and consist of strong continuum radiation from nonthermal processes in the solar atmosphere. Some X-ray bursts of varying intensity and spectral distribution are shown on the left side of Fig. 1. SOLAR SIMULATION
In order to test materials for space applications, it is necessary to expose them to a laboratory simulation of the space environment. The simulation of solar radiation S.E. Vol. 12No. 2-E
202
H.H.
MALITSON
Table 2. Intensity of the solar spectrum (continuum and lines) extraterrestrially at 1 astronomical unit over the indicated wavelength ranges 16, 16] Wavelength range (A)
Energy flux density (ergs/cm 2 sec)
30O0-29O0 2900-2800 2800-2700 2700-2600 1 2625-2525 ~ note overlap 2525-2425 2425-2325 2325-2225 2225 -2125 2125-2025 2025-1925 1925-1825 1825-1775 1775-1325 1325-1027
6300 3400 2200 20OO 1260 770 660 710 550 235 125 69 19 32 5.7
involves the attainment of the correct spectral distribution along with very high intensities, often over large beam cross sections. No existing light source has the right spectral distribution, so compromises must be made according to the purpose of the test. There are sources that will give the same integrated power as the Sun over a wide spectral band, and there are others that will match the solar spectral distribution fairly well in narrow regions. Reference[17] is recommended as a source of information on existing light sources, detectors, and filters for the middle u.v. CONCLUSION
For the future, improvements in i.r. detectors and u.v. absolute standards are urgently needed. Constant monitoring of the solar u.v. and X-ray flux will be possible through the increased use of satellites. This will no doubt be a most valuable source of information, both for those of us who study the Sun itself and for those who are interested in precise definition of the space environment. REFERENCES [ 1] F.S. Johnson, The solar constant. J. Met. 11, 431 (1954). [2] L. Dunkelman and R. Scolnik, Solar spectral irradiance and vertical atmospheric attenuation in the visible and ultraviolet. J. opt. Soc. Am. 49,356 (1959). [3 ] P. Moon, Proposed standard solar-radiation curves for engineering use. J. Franklin inst. 230, 583 (1940). [4] F. H. Murcray, D. J. Murcray and W. J. Williams, The spectral radiance o f the sun from 4/~ to 5/.t. Appl. Optics 3, 1373 (1964). [5] F. Saiedy and R. M. Goody, The solar emission intensity at i 1/z. Roy. Astron. Soc. 119 213 (1959). [6] C. R. Detwiler. D. L. Garrett. J. D. Purcell and R. Tousey. The intensity distribution in the ultraviolet solar spectrum. Ann, G~ophys. 17. 263 ( 1961 ). [7l H: E. Hinteregger. Telemetering monochromator measurements of extreme ultraviolet radiation. In Space Astrophysics (Edited by W. Liller}, pp. 74-95. McGraw-Hill, New York ( 1961 ). [8] H. Zirin. L. A. Hall, and H. E. Hinteregger, Analysis of the solar emission spectrum from 1300 to 250 A as observed in August 1961. In Space Research IlL Proceedings of the Third International Space Science Symposium, Washington. D.C.. 1962 (Edited by W. Priester}, pp. 760-771. Interscience. New York (1963).
The solar electromagnetic radiation environment
203
[9] P. J. Bowen, K. Norman. et al. Measurements of the solar spectrum in the wavelength band 4-14 A. Proc. R. Soc. 281. 538 (I 964). [10] H. Friedman, Solarradiation.Astronautics 7, 14 (1962). [11] M. Nicolet and A. C. Aikin, The formation of the D region in the ionosphere. J. Geophys. Re3. 65. 1469 (1960). [ 12] R. E. Bourdeau, S. Chandra and W. M. Neupert, Time correlation of extreme ultraviolet radiation and thermospheric temperature. J. Geophys. Res. 69, 4531 ( ! 964). [ 13] R. Beer, Decrement of the solar continuum in the far infrared. Nature 209, 1226 (1966). [14] R. Tousey, The radiation from the Sun. In The Middle Ultraviolet: Its Science and Technology/Edited by A. E. S. Green), pp. 1-39. Wiley, New York (1966). [15] R. Rousey. The extreme ultraviolet spectrum of the Sun. Space Sci. Rev. 2.3 (1963). [16] H. E. Hinteregger. Absolute intensity measurements in the extreme ultraviolet spectrum of solar radiation. Space Sci. Rev. 4. 461 (1965). [ 17] J. Hennes and L. Dunkelman, Ultraviolet technology. In The M;,ddle Ultraviolet: Its Science and Technology (Edited by A. E. S. Green), pp. 304-377. Wiley, New York t 1966).
THE SOLAR ELECTROMAGNETIC R A D I A T I O N ENVIRONMENT* H A R R I E T H. MALITSON~: (Received 21 February 1968~
Abstract--Observations of the spectral distribution of solar electromagnetic radiation have been made from wavelengths shorter than 1 A (I 0-acre) to wavelengths longer than 100 m (104cm) by many different techniques, each of which is applicable over only a smaJl part of the total range. Most of the energy emitted by the sun (98 per cent) lies between 3000 and 40,000 A. A large fraction of the solar radiation falling between these wavelength limits manages to penetrate the Earth's atmosphere, so that it can be studied from ground level. By observing the amount of solar energy reaching the ground at many times during the day, an extrapolation can be made that gives a reasonably accurate value of the energy received at the top of the atmosp h e r e - the solar constant. No radiation from the Sun at wavelengths below approximately 2900 ,~. has been detected at ground level, and very little radiation in the i.r. at wavelengths greater than 30-40.000 ,~ can come through the atmosphere. Therefore. observations in these spectral regions must be made from rockets or satellites. Balloons and certain aircraft can also be used to advantage, especially in the i.r. In addition to the usual difficulties involved in the carrying out of space experiments, there are those due to the lack of adequate laboratory standards and the variability of the Sun in the extreme u.v. and X-ray region. Nevertheless, the uncertainties in information about the spectral distribution of solar radiant flux are smaller than the departures from the solar irradiance curve of the flux from the best high energy solar simulators available. R 6 s u m 6 - D e s observations de la distribution spectrale de la radiation solaire 61ectromagn6tique, ont 6td fares en partant de longueurs d'onde inf6rieures ~ 1A (10 -s cm)jusqu'/~ des Iongueurs d'onde sup6rieures 100 m ( 104 cm), par diff6rentes m6thodes, chacune s'applicant/t une faible part seulement de la totalit6 de la gamme. La plus grande pattie de l'6nergie 6raise par le soleil (98 pour cent) se trouve entre 3000 et 40,000 A. Une fraction importante de la radiation solaire qui tombe entre ces limites de Iongueurs d'onde, r6ussit/~ pdn6trer dans ratmosph6re terrestre, de telle sorte qu'on peut 1"6tudier au sol. En observant la quantit6 d'6nergie solaire qui atteind le sol fi diff6rents moments de la journde, on peut faire une extrapolation qui donne une valeur sutSsamment pr6cise de l'6nergie revue au sommet de la zone atmosph6rique-la constante solaire. A des longueurs d'onde inf6rieures ~t environ 2900 A. il n'a pas 6t6 possible de d6tecter de radiation au sol et, seule, une tr6s petite radiation dans I'infra-rouge/~ des Iongueurs d'onde sup6rieures h 30-40.000 A peut traverser ratmosph6re. Par cons6quent, les obervations dans ces r6gions spectrales doivent ,~tre faites par fusees ou satellites. Les ballons et certains avions peuvent aussi 6tre utilises, surtout dans la zone infrarouge. En plus des difticult6s habituelles que l'on rencontre au cours des experiences spatiales, il y a celles dues au manque de normes de laboratoire ad6quates et ~ la variabilit6 du soleil dans la zone extr6me de l'ultraviolet et des rayons X. N6anmoins, les incertitudes rencontr6es quand on recueille des informations concernant la distribution spectrale de flux solaire radiant, sont inf6rieures/l celles rencontrees h partir de la courbe d'irradiance solaire, du flux des meilleurs simulateurs solaire de haute 6nergie disponibles. Resumen-- Se han hecho observaciones sobre la distribuci6n espectral de la radiaci6n electromagn6tica solar con longitudes de onda desde menores de I ,~ (l 0 -s cm) hasta mayores de 100m ( 104 cm), mediante empleo de muy diversas t6cnicas, cada una de las cuales corresponde a s61o una pequefia parte de la gama total. La mayor parte de la energia emitida por el sol (un 98 pot ciento), se halla comprendida entre 3000 y 40,000 ,~. U ne porcion importante de la radiaci6n situada entre dichos limites de longitud de onda consique penetrar la atmdsfera terrestre, Io que permite su estudio a nivel del sue{o. Mediante observaci6n de la cantidad de energia solar que incide sobre el sue{o a muchas horas del dia, es posible hacer una extrapolaci6n que indique con bastante exactitud el valor de la energia recibida en la parle superior de la atmosfera, es decir, la llamada constante solar. No se ha detectado a nivel del suelo radiaci6n solar con longitud de onda menor de 2900 A. aproximadamente, ni puede atravesar la atm6sfera gran cantidad de radiaci6n infrarroja con longitudes de onda inferiores a 30-40.000 A. Pot Io tanto, la observaci6n de estas regiones espectrales debe ser hecha desde * 1967 Solar Energy Society Conference paper. t Laboratory for Space Sciences, NASA-Goddard Space Flight Center, Greenbelt. Maryland, U.S.A. 197
198
H.H.
MALITSON
cohetes o satglites. Tambien se logran resultados satisfactorios empleando globos y determinados tipos de avi6n, especialmente en lo que respecta a la regi6n infrarroja. Ademb,s de las dificultades que normalmente han de afrontarse en la realizaci6n de experimentos espaciales, est:~fi las motivadas por falta de adecuades normas de laboratorio y por la variabilidad del sol en la regi6n extrema de rayos ultra-violeta y rayos X. Sin embargo, los elementos de duda en la informaci6n sobre distribuci6n del flujo radiaaere solar son menores que las divergencias, respecto a la curva de irradiaci6n solar, del flujo presentado por los mejores simuladores solares de gran capacidad disponibles.
INTRODUCTION
A KNOWLEDGE of the nature and effects of solar electromagnetic radiation is essential for the design of space vehicles and their instrumentation. This radiation is a very important factor in the thermal equilibrium of spacecraft, and can cause changes in the properties of both organic and inorganic materials leading to degradation and possible failure of the mission. The following review of the present state of knowledge of the solar electromagnetic radiation environment outside the Earth's atmosphere is presented for the information of those who may be involved in spacecraft or space experiment design. The curve in Fig. 1 represents the distribution with wavelength of the Sun's radiant energy falling on one square centimeter at the top of the Earth's atmosphere when the Earth is at its mean distance from the Sun, one astronomical unit or 1.5 x 10:acm. The so-called solar constant, or the total energy at all wavelengths received from the Sun at the top of the atmosphere, has a value of 2.00 ___0.04 cal/cm 2 min. The solar constant would equal the total area under the curve, if the curve were plotted on a linear scale and extended from zero to infinity in wavelength. Solar radiation covers the entire electromagnetic spectrum from waves shorter than 1 A or 10-8 cm to those longer than 100 m (104 cm). Roughly 98 per cent of the energy in the solar spectrum lies between 3000 and 40,000 A(4 ~m). These wavelength limits define one of two spectral "windows" where the Earth's atmosphere is transparent to radiation (the other is the radio window, which will not be discussed here). VISIBLE
WINDOW:
3000A-4/~m
The designation "visible" is somewhat misleading, since the human eye is sensitive only to radiation of wavelengths about 4000-8000 A. Thus this window includes some radiation in the near u.v. and near i.r. When viewed through a spectroscope, the solar spectrum consists of a bright continuous band overlain by thousands of dark absorption lines, called Fraunhofer lines. The continuum arises in the hotter photospheric levels of the Sun, and the lines come from absorption of the continuum at certain discrete frequencies by atoms of various elements in the higher, cooler photospheric layers. In Fig. 1, the effect of the strongest of the dark lines is barely apparent in the slight irregularity of the solar curve near its peak. which falls at 4700 ,~ in the visible. The classical method[l, 2] for determining the solar irradiance in the visible is due to Langley. Briefly. it consists of making measurements, over a certain wavelength range, of solar energy received at many times during the day so that the Sun is at various altitudes above the horizon. The values obtained are then extrapolated to intensity outside the Earth's atmosphere by means of the relation I = loe -~''
( 1)
where o" -- coefficient of attenuation, m = absolute air mass (--- ratio of path length of
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2 o~(~3~
? .......... .....F.............................. l
Fig. I. T h e solar electromagnetic radiation spectrum. Solid lines represent m e a s u r e m e n t s : dotted lines, estimates. F = reference [ I 0] (rocket m e a s u r e m e n t s ) . N . & A . = reference [ 11 ] (estimated from ionospheric data). O S O - 1 = data from first orbiting solar observatory. W. A. White. Private c o m m u n i c a t i o n 11964). U K - 1 = reference[9] !Ariel I satellite date).
(
200
H. H, M A L I T S O N
solar radiation through the atmosphere when the Sun is at a certain elevation h. to the path length for the Sun in the zenith, corrected for the difference in atmospheric pressure at the observing site and at sea level), 1 = measured intensity when the Sun is at elevation h and I0 = intensity at the top of the Earth's atmosphere. When log/ is plotted against m, a straight line results under ideal conditions. However. conditions are always less than ideal, even when the measuring instrument is placed at a location where the atmosphere is very clear and stable. The variability of the atmosphere is the major source of error. Others are stray light in the instrument. which is particularly troublesome near the ends of the wavelength range where the solar intensity is low; and radiation from around the Sun when there is haze. The relative values obtained by Dunkelman and Scolnik[2] were considered by them to be accurate t o _ 3 per cent except near the ends of the wavelength range, where the estimated error rose to ± 6 per cent. Converting the relative values to absolute values by comparing the measured illuminance of the standard lamp used for calibration with the CIE standard luminosity curve, involved an additional error of ± 10 per cent. Johnson [ 1], from a study of the Smithsonian solar-constant work and rocket u.v. measurements, concluded that the absolute scale of Dunkelman and Scoinik should be raised by 9 per cent. The curve given here includes this revision. For wavelengths from 6500 ,~ to 1-2 tzm, Johnson used the 1940 data of Moon[3], which are probably still the best available. From 1.2 to 2.4/xm, Moon had found that a 6000°K gray body curve fitted the observations well. Beyond 2.4/~m, the molecular absorption bands of such atmospheric constituents as oxygen, water vapor and carbon dioxide absorb so strongly that a continuous spectrum cannot be observed from the Earth. I N F R A R E D R E G I O N : 4~tm TO 100g.m A N D UP
The i.r. spectrum also has the appearance of a continuum crossed by dark lines, but in addition to the atomic lines, there are molecular bands made up of tens or hundreds of individual lines. Actually, this description is based on observations in the near i.r. at 1 or 2/zm. Solar spectrograms for the middle and far i.r. are not yet in existence, and will have to await further development of detectors. The most reliable measurements of the absolute intensity of solar radiation in the i.r. are recent ones from balloons [4, 13], although there is one ground-based measurement[5] in a narrow window at 11.1/zm. The results indicate that the i.r. radiation of the Sun is less intense than one would expect from extrapolation of the visible radiation to longer wavelengths. At the peak of solar emission at 4700 A, the radiation is characteristic of a 6100°K blackbody; at 4/~m, the blackbody temperature has fallen to about 5600°K, at 11.1/zm it is roughly 5000°K, and 100/~m it has decreased to the neighborhood of 4300°K. U L T R A V I O L E T A N D X-RAYS: 3000 ,~ DOWN
Below 2900 X, no radiation from the Sun has been detected at ground level. Atmospheric ozone in a layer 10--40 km high absorbs down to about 2200 A, where molecular oxygen above 75 km takes over and does most of the absorbing down to 900 ,~. Below this wavelength, many atmospheric constituents absorb all radiation before it gets within 150 km of the Earth's surface. Rockets and satellites, therefore, must be used to obtain data from 2900 ,~ down. Since October 1946, when the Naval Research Laboratory obtained the first solar
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spectrum down to about 2100 ,~ from a captured V-2 rocket, there has been much effort directed toward obtaining an exact knowledge of the short wavelength component of solar radiation. Several excellent review articles exist[14-16], giving the present status of solar u.v. and X-ray work, and there will be no attempt to give a complete discussion here. Starting at 3000 f~, the spectrum is again a bright continuum with many dark lines across it. As the wavelength decreases, the lines become more closely packed and the continuum falls off rapidly in brightness. Consequently, the spectral irradiance decreases very rapidly indeed (Table l). There is an especially abrupt decrease in continuum intensity at 2085/~ so that the Fraunhofer lines are very weak and shallow. Bright lines begin to appear superimposed on the weak continuum. At wavelengths shorter than 1700 A, the Fraunhofer lines are gone, the continuum is extremely weak, and bright lines are responsible for most of the emission. The bright lines arise from elements in the chromosphere of the Sun, which is considerably hotter than the photosphere Table I. Fraction of solar constant below indicated wavelength [ 15] Wavelength tAt 7000 4000 3000 2400 1900 1600 1000
Fraction 0.5 0.1 0.01 0.001 0.0001 0.00001 0.000001
although it lies above it. In the far u.v. and X-ray portions of the spectrum, many of the bright lines arise in the corona of the Sun, which is higher than the chromosphere and has a very high temperature, about 1.5 × 106°K. The most reliable results to date[6, 16] on the energy distribution in the solar spectrum from 3000-1027 A are presented in Table 2. The accuracy of these measurements has been estimated as _ l 0 per cent to 2000 ,~ and better than-_4-_20 per cent from 2000 to 1400,~. Below 1400/~ there may be errors as great as 40 per cent. These errors arise in large part from difficulties in the calibration of flight instruments due to a lack of absolute laboratory standards in the u.v. It should be realized, however, that even perfectly calibrated instruments will give different values depending on the acitivity of the Sun at the time. In the visible, the Sun does not change its output by more than 1 or 2 per cent. However, at wavelengths shorter than about 1300 A, more and more variability occurs, and increases of several orders of magnitude in a few seconds are possible at wavelengths shorter than 10 ,~. Such increases, called X-ray bursts, occur in connection with solar flares and related activity, and consist of strong continuum radiation from nonthermal processes in the solar atmosphere. Some X-ray bursts of varying intensity and spectral distribution are shown on the left side of Fig. 1. SOLAR SIMULATION
In order to test materials for space applications, it is necessary to expose them to a laboratory simulation of the space environment. The simulation of solar radiation S.E. Vol. 12No. 2-E
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Table 2. Intensity of the solar spectrum (continuum and lines) extraterrestrially at 1 astronomical unit over the indicated wavelength ranges 16, 16] Wavelength range (A)
Energy flux density (ergs/cm 2 sec)
30O0-29O0 2900-2800 2800-2700 2700-2600 1 2625-2525 ~ note overlap 2525-2425 2425-2325 2325-2225 2225 -2125 2125-2025 2025-1925 1925-1825 1825-1775 1775-1325 1325-1027
6300 3400 2200 20OO 1260 770 660 710 550 235 125 69 19 32 5.7
involves the attainment of the correct spectral distribution along with very high intensities, often over large beam cross sections. No existing light source has the right spectral distribution, so compromises must be made according to the purpose of the test. There are sources that will give the same integrated power as the Sun over a wide spectral band, and there are others that will match the solar spectral distribution fairly well in narrow regions. Reference[17] is recommended as a source of information on existing light sources, detectors, and filters for the middle u.v. CONCLUSION
For the future, improvements in i.r. detectors and u.v. absolute standards are urgently needed. Constant monitoring of the solar u.v. and X-ray flux will be possible through the increased use of satellites. This will no doubt be a most valuable source of information, both for those of us who study the Sun itself and for those who are interested in precise definition of the space environment. REFERENCES [ 1] F.S. Johnson, The solar constant. J. Met. 11, 431 (1954). [2] L. Dunkelman and R. Scolnik, Solar spectral irradiance and vertical atmospheric attenuation in the visible and ultraviolet. J. opt. Soc. Am. 49,356 (1959). [3 ] P. Moon, Proposed standard solar-radiation curves for engineering use. J. Franklin inst. 230, 583 (1940). [4] F. H. Murcray, D. J. Murcray and W. J. Williams, The spectral radiance o f the sun from 4/~ to 5/.t. Appl. Optics 3, 1373 (1964). [5] F. Saiedy and R. M. Goody, The solar emission intensity at i 1/z. Roy. Astron. Soc. 119 213 (1959). [6] C. R. Detwiler. D. L. Garrett. J. D. Purcell and R. Tousey. The intensity distribution in the ultraviolet solar spectrum. Ann, G~ophys. 17. 263 ( 1961 ). [7l H: E. Hinteregger. Telemetering monochromator measurements of extreme ultraviolet radiation. In Space Astrophysics (Edited by W. Liller}, pp. 74-95. McGraw-Hill, New York ( 1961 ). [8] H. Zirin. L. A. Hall, and H. E. Hinteregger, Analysis of the solar emission spectrum from 1300 to 250 A as observed in August 1961. In Space Research IlL Proceedings of the Third International Space Science Symposium, Washington. D.C.. 1962 (Edited by W. Priester}, pp. 760-771. Interscience. New York (1963).
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[9] P. J. Bowen, K. Norman. et al. Measurements of the solar spectrum in the wavelength band 4-14 A. Proc. R. Soc. 281. 538 (I 964). [10] H. Friedman, Solarradiation.Astronautics 7, 14 (1962). [11] M. Nicolet and A. C. Aikin, The formation of the D region in the ionosphere. J. Geophys. Re3. 65. 1469 (1960). [ 12] R. E. Bourdeau, S. Chandra and W. M. Neupert, Time correlation of extreme ultraviolet radiation and thermospheric temperature. J. Geophys. Res. 69, 4531 ( ! 964). [ 13] R. Beer, Decrement of the solar continuum in the far infrared. Nature 209, 1226 (1966). [14] R. Tousey, The radiation from the Sun. In The Middle Ultraviolet: Its Science and Technology/Edited by A. E. S. Green), pp. 1-39. Wiley, New York (1966). [15] R. Rousey. The extreme ultraviolet spectrum of the Sun. Space Sci. Rev. 2.3 (1963). [16] H. E. Hinteregger. Absolute intensity measurements in the extreme ultraviolet spectrum of solar radiation. Space Sci. Rev. 4. 461 (1965). [ 17] J. Hennes and L. Dunkelman, Ultraviolet technology. In The M;,ddle Ultraviolet: Its Science and Technology (Edited by A. E. S. Green), pp. 304-377. Wiley, New York t 1966).