AERO NOMICA ACTA

Texte intégral

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I N S T I T U T D ' A E R O N O M I E S P A T I A L E D E B E L G I Q U E 3 - Avenue Circulaire

B • 1180 BRUXELLES

A E R O N O M I C A ACTA

A - N° 239 - 1981

Balloon observations of solar ultraviolet irradiance at solar minimum

by

P.C. SIMON, R. PASTIELS and D. NEVEJANS

B E L G I S C H I N S T I T U U T V O O R R U I M T E - A E R 0 N O M I E 3 - Rinplaan

B • 1180 BRUSSEL

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FOREWORD

The paper "Balloon observations of solar irradiance at solar minimum" is accepted for publication in Planetary and Space Science, 29, 1981.

A V A N T - P R O P O S

L'article "Balloon observations of solar irradiance at soiar minimum" sera publié dans Planetary and Space Science, 29, 1981.

VOORWOORD

Het artikel "Balloon observations of solar irradiance at solar minimum" zal verschijnen in Planetary and Space Science, 29, 1981.

VORWORT

Die Arbeit "Balloon observations of solar irradiance at solar minimum" wird in Planetary and Space Science, 29, 1981 herausgegeben werden.

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BALLOON OBSERVATIONS OF SOLAR U L T R A V I O L E T IRRADIANCE AT SOLAR MINIMUM

b y

Paul C. SIMON, Roger PASTIELS and Dennis NEVEJANS

A b s t r a c t

Balloon o b s e r v a t i o n s of solar i r r a d i a n c e between 200 and 240 nm have been p e r f o r m e d in 1976 and 1977 c o r r e s p o n d i n g to minimum c o n d i t i o n s of solar a c t i v i t y . U l t r a v i o l e t spectra have been r e c o r d e d f o r d i f f e r e n t z e n i t h angles at an a l t i t u d e of 41 km b y means of a s p e c t r o - meter w i t h a spectral bandpass of 0 . 4 nm. Solar i r r a d i a n c e s at 1 A . U . c o n f i r m p r e v i o u s values obtained b y balloon. T h e y are compared w i t h o t h e r measurements and discussed in term of possible l o n g - t e r m v a r i a b i l i t y .

Résumé

Des o b s e r v a t i o n s d u f l u x solaire e n t r e 200 et 240 nm ont été réaliseés à p a r t i r de ballons s t r a t o s p h é r i q u e s en 1976 et 1977 q u i c o r r e s p o n d e n t à des périodes d ' a c t i v i t é solaire minimum. Les spectres u l t r a v i o l e t s o n t été obtenus p o u r d i f f é r e n t s angles z é n i t h a u x à une a l t i t u d e de 41 km au moyen d ' u n spectromètre ayant une bande passante de 0 . 4 nm. Les v a l e u r s de f l u x solaire à 1 U . A . c o n f i r m e n t les r é s u l t a t s obtenus précédemment par ballon. Elles sont comparées avec les a u t r e s o b s e r v a t i o n s et discutées en f o n c t i o n d ' u n e éventuelle v a r i a t i o n à long terme dans ce domaine de l o n g u e u r d ' o n d e .

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Samenvatting

In de loop van 1976 en 1977 w e r d , met b e h u l p van een ballon- e x p e r i m e n t en g e d u r e n d e een periode van kalme zon, de z o n n e f l u x gemeten voor g o l f l e n g t e n gelegen tussen 200 en 240 nm. Op een hoogte van 41 km en bij v e r s c h i l l e n d e zenithale afstanden werden spectra opgenomen door een u l t r a v i o l e t spectrometer met een doorlaatband van 0 , 4 nm. De gemeten f l u x op 1 A . E . van de zon b e v e s t i g t eerder gedane ballonwaarnemingen. De f l u x w a a r d e w o r d t v e r g e l e k e n met andere metingen en ook w o r d t z i j n eventuele v e r a n d e r i n g op lange t e r m i j n b e h a n d e l d , althans voor d i t g o l f l e n g t e g e b i e d .

Zusammenfassung

Die S o n n e n s t r a h l u n g zwischen 200 u n d 240 nm ist in 1976 u n d 1977, w ä h r e n d das Sonnen A k t i v i t ä t s Minimum, mit L u f t b a l l o n e n gemessen w o r d e n . UV spectra s i n d f ü r v e r s c h i e d e n e Zenit Wickeln von einer Höhe 41 km, mit einen Spectrometer ( 0 . 4 nm* " b a n d p a s s " ) beobachtet w o r d e n . Diese Messungen bestöctigen f r ü h e r e L u f t b a l l o n Messungen der gesamten S o n n e n s t r a h l u n g . Diese Messungen sind auch mit anderen v e r g l i c h e n worden um lang zeitige V a r i a t i o n e n heraus zu b r i n g e n .

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I N T R O D U C T I O N

The photochemical processes, the temperature and the dynamics in the stratosphere are directly driven by the absorption of solar ultraviolet irradiance with a wavelength greater than 180 nm. The interval 200-240 nm which corresponds to the Herzberg continuum of molecular oxygen, is particularly important for the ozone budget in the stratosphere. Photodissociation of the molecular oxygens produces oxygen atoms which are the primary source of ozone. Other trace species like halocarbons, N

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0 and H N O j are also photodissociated in this wavelength interval. Hence, an accurate knowledge of solar irradiance from 200 to 240 nm is required for photochemical calculations.

In addition, its possible long-term variability with the solar activity cycle has to be investigated in order to distinguish any natural variability of ozone in the upper stratosphere from any other phenomena.

The solar spectrum in the 200-240 nm wavelength interval is characterized by Fraunhofer absorption lines and by the large discontinuity near 210 nm corresponding to the Al I absorption edge.

Irradiance values measured in this spectral range have been previously discussed by Simon (1978) and the data reported by Simon (1974a) have been generally adopted up to 230 nm for modelling purposes while those of Broadfoot (1972) were used beyond this wavelength. Recent measur- ements of Mount et al. (1980) and Heath (1980) give systematically lower values than those of Broadfoot (1972). Also, the data of Heath (1980) are between 3 and 12 percent lower than those of Simon (1974a) and 25 percent higher than those of Mount et al. (1980). Consequently new measurements are needed in order to obtain more accurate values of solar irradiance between 200 and 240 nm.

The question of variability in this wavelength range is not fully resolved. According to recent observations made from Nimbus 7 (Heath, 1980) variations with solar rotation are between 2 and 4 percent

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but long-term variations would be still masked by the accuracy and the precision in available observations.

The purpose of this work is to report reliable balloon observa- tions of solar irradiance between 200 and 240 nm performed in 1976 and 1977 for minimum and low solar activity conditions.

The new results will be compared with earlier measurements and will be discussed in terms of possible long-term variations for which only an upper limit is estimated.

OBSERVATIONS

The observations of the ultraviolet solar spectrum have been performed by means of an Ebert-Fastie spectrometer of 25 cm focal length. This instrument has been flown in 1972 and 1973 and is extensively described in a previous paper (Simon, 1974b). The only difference is that the spectral bandpass has been set at 0.4 nm instead of 0.6 nm.

The absolute calibration of the spectrometer has been performed before and after each flight. Several deuterium lamps referenced to the National Bureau of Standards spectral irradiance scale have been used for wavelengths below 300 nm. An average of calibration results has been taken. The reproducibility of calibration is within ± 4 percent over the entire wavelength range. As the absolute value is known with an uncertainty of ± 6 percent the final calibration is given with an accuracy of ± 10 percent and a precision of ± 4 percent. The spectrometer has been integrated in a stabilized gondola, pointing to the sun with an accuracy of the order of 10 arc min. Both flights were performed from the "Centre National d

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Etudes Spatiales"

launching site situated in Gap (44°33'N

/

6°05'E). The first flight took place on 1 July 1976, the second on 7 July 1977. The gondola , which had a weight of about 300 kg, was carried by Zodiac balloons of

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350.000 m and reached a floating altitude of about 41 km. The observa-

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tion conditions are summarized in Table 1.

D A T A R E D U C T I O N

In situ measurements of solar irradiance between 200 and 240 nm performed at a ceiling altitude of 41 km by means of balloon- borne spectrometer must be corrected for. the residual absorption by molecular oxygen and ozone in order to obtain the corresponding extra- terrestrial irradiance values. As observations are made for different solar zenith angles, measured solar fluxes can be easily extrapolated to zero air mass. T h i s can be done only if the floating altitude of the balloon is held rigorously constant during the observing period in order to keep, for all recorded spectra, the same column content for molecular oxygen and ozone above the gondola. A second method consists for correcting the observed spectra by taking into account the residual atmospheric absorption. Molecular oxygen content can be deduced from pressure measurement and ozone content can be determined from absorbed solar spectra recorded between 270 and 285 nm by the same solar spectrometer. Small changes within 8 percent in ozone content at the ceiling altitude have been measured with a precision better than 1 percent. These changes correspond to altitude variations less than 340 m at 41 km. Consequently, if the small altitude variations of the gondola are neglected, extrapolation to zero air mass introduced large errors in the extraterrestrial irradiance values when the ozone optical depth is greater than 2.

The data obtained in 1973 and published by Simon (1974a) have been reanalysed using the second method. The new values are in good agreement, within ± 5 percent, with the previous values except for wavelengths beyond 225 nm due to higher ozone optical depth. They have been extended up to 235 nm and are given in Table 2 for comparison purposes.

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Table 1 : Conditions of observation.

Date Altitude Sec x 0 0

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(km) (cm ) (cm )

July 1, 1976 41.6 1 . 3 5 - 2 . 0 4 1.22 x 1 02 2 2.3 x I01 July 7, 1977 41.5 1.62 - 1.91 1.22 x 1 02 2 2.4 x 101

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Table 2a: Solar irradiance (h .cm .s ) obtained by balloon integrated over 5 nra intervals.

Wavelength interval (nm) 1972/1973 1976 1977

200 - 205 205 - 210 210 - 215 215 - 220 220 - 225 225 - 230 230 - 235 235 - 240

4.11 x 10 7.34

17.7 21.6 30.5 27. 1*

29. 4*

12 4.75 x 10 7.80 18.5 22.4 30.9 27. 1 27.9 26.3

12 4.93 x

8 . 0 0 18.3 22. 3 30.4 27.2 28.3 27.7

* Revised values.

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T a b l e 2 b : S o l a r i r r a d i a n c e ( h .cm . s ) i n t e g r a t e d o v e r i n t e r v a l s o f 5 0 0 c m "1'

W a v e l e n g t h i n t e r v a l Wavenumber i n t e r v a l 1 9 7 6 1977 (nm) (cm

2 0 0 . 0 - 2 0 2 . . 0 5 0 , 0 0 0 - 4 9 , 5 0 0 1, . 6 8 x 10 1 . 79

2 0 2 . 0 - 2 0 4 . . 1 4 9 , 5 0 0 - 4 9 , 0 0 0 2 . 0 6 2 . 12

2 0 4 . 1 - 2 0 6 . . 2 4 9 , 0 0 0 - 4 8 , 5 0 0 2, . 3 8 2 . 4 3

2 0 6 . 2 - 2 0 8 . . 3 4 8 , 5 0 0 - 4 8 , 0 0 0 2 , . 7 4 2 . 8 4

2 0 8 . 3 - 2 1 0 . .5 4 8 , 0 0 0 - 4 7 , 5 0 0 5 . . 2 0 5 . 3 0

2 1 0 . 5 - 2 1 2 . . 8 4 7 , 5 0 0 - 4 7 , 0 0 0 8 . . 0 5 7, . 9 5

2 1 2 , 8 - 2 1 5 , 0 4 7 , 0 0 0 - 4 6 , 5 0 0 8 , , 9 8 8 > 8 2

2 1 5 . 0 - 2 1 7 . . 4 4 6 , 5 0 0 - 4 6 , 0 0 0 9 . . 2 6

1 01 3

9 . 2 7 2 1 7 . 4 - 2 1 9 . . 8 4 6 , 0 0 0 - 4 5 , 5 0 0 1. . 2 1 x 1 01 3 1, . 19

2 1 9 . 8 - 2 2 2 . . 2 4 5 , 5 0 0 - 4 5 , 0 0 0 1. . 2 1 1 , . 2 0

2 2 2 . 2 - 2 2 4 . .7 4 5 , 0 0 0 - 4 4 , 5 0 0 1. . 7 6 1, . 7 4

2 2 4 . 7 - 2 2 7 . . 3 4 4 , 5 0 0 - 4 4 , 0 0 0 1. . 4 2 1. . 4 1

2 2 7 . 3 - 2 2 9 . ,9 4 4 , 0 0 0 - 4 3 , 5 0 0 1. , 4 6 1. . 4 7

2 2 9 . 9 - 2 3 2 . . 6 4 3 , 5 0 0 - 4 3 , 0 0 0 1. . 6 5 1. . 6 7

2 3 2 . 6 - 2 3 5 . .3 4 3 , 0 0 0 - 4 2 , 5 0 0 1. ,37 1. , 4 0

2 3 5 . 3 - 2 3 8 . . 1 4 2 , 5 0 0 - 4 2 , 0 0 0 1. . 5 8 1 . . 6 6

2 3 8 . 1 - 2 4 1 . .0 4 2 , 0 0 0 - 4 1 , 5 0 0 1. , 3 0 1. , 3 8

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The measurements performed in 1976 and 1977 have also been corrected for the residual absorption : final irradiance values correspond to an average over 20 corrected spectra. A standard deviation of the order of 1 to 2 percent has been obtained between 210 and 230 nm. From 230 to 240 nm, it increases up to 5 percent because of higher optical depth. Discussion on ozone measurements will be given elsewhere (Simon and Peetermans, 1981). The final accuracy on irradiance values from 200 to 240 nm is ± 15 percent.

R E S U L T S A N D D I S C U S S I O N S

Irradiance values deduced from the two balloon flights are given in Table 2 for different spectral intervals. Results from the 1976 flight are also reported on figure 1 with the other published measur- ements. The agreement between the two balloon measurements is within 6 percent. Figure 2 presents the ratios of irradiance values integrated over 5 nm from 200 to 240 nm taking as a reference the data obtained on 1 July, 1976. The 5 nm interval has been chosen in order to reduce as much as possible the effect of the different spectrometer resolutions on irradiance values. The agreement with the previous balloon observations (Simon, 1974a) is within ± 5 percent between 210 and 240 nm but the new irradiance values are 13 percent higher around 200 nm. Heath (1980) proposed lower data obtained from the Nimbus 7 satellite but they become in close agreement with the 1976 values around 235 nm. Measurement of Mount et al. (1980) are systematically lower by 30 percent than those presented in this work while those of Broadfoot (1972) are 20 to 30 percent higher. The disagreement with published values for wavelengths below 210 nm (for instance with data of Samain and Simon, 1976, and B r u e c k n e r et al., 1976) is important.

It is difficult to deduce quantitatively any long-term variability of solar irradiance around 210 nm from these observations.

The measurements performed by balloon in 1972, 1973, 1976 and 1977

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WAVELENGTH ( n m )

Fig. 1.- Observed solar spectral irradiance between 200 and 240 nm. Data of Samain and Simon (1976) and Mount et^ a1. (1980) have been averaged over 0.4 nm for comparison purposes.. Data of Heath (1980) are not

represented here because they were obtained with a bandpass of 1 nm.

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1.4

CD

1.2

CD

>> 1

D

£ 0.8 ro i < cr

0.6

_ r

— B70

S72/73 S 7 7

H 7 8 r •

" J

MRT79

J I I L J I J L 200 205 210 215 220 225 230 235 240

WAVELENGTH ( n m )

Fig. 2.- Ratio of solar irradiance measurements integrated over 5 nm intervals from 200 to 240 nm in comparison with the data obtained on 1 July 1976.

The labeling is as follows : B70 - Broadfoot (1972), S72/73 - Simon (1974a), H78 - Heath (1980), MRT 79 -Mount et al . (1980), S77 - this work, July 7, 1977.

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a r e i n close a g r e e m e n t b e t w e e n 210 a n d 235 nm a n d c o r r e s p o n d t o low a n d minimum a c t i v i t y c o n d i t i o n s o f t h e S u n . U n f o r t u n a t e l y , t h e l a r g e d i s c r e p a n c i e s b e t w e e n m e a s u r e m e n t s o f B r o a d f o o t ( 1 9 7 2 ) a n d M o u n t et al.

( 1 9 8 0 ) b o t h o b t a i n e d f o r maximum c o n d i t i o n s o f s o l a r a c t i v i t y r e s p e c t i v e l y f o r solar c y c l e s 20 a n d 21 d o n o t s o l v e t h e q u e s t i o n of t h e p o s s i b l e v a r i a b i l i t y w i t h t h e s o l a r c y c l e in t h i s w a v e l e n g t h r a n g e . On t h e o t h e r h a n d , d a t a o b t a i n e d b y H e a t h ( 1 9 8 0 ) in N o v e m b e r 1978 f o r i n t e r m e d i a t e a c t i v i t y level a r e o n l y 10 p e r c e n t lower a r o u n d 210 nm t h a n t h o s e o b t a i n e d at minimum c o n d i t i o n s i n 1976, i . e . w i t h i n t h e i r q u o t e d a c c u r a c i e s ( 1 0 - 15 p e r c e n t ) . C o n s e q u e n t l y , t h e s y s t e m a t i c d i v e r g e n c e s in a v a i l a b l e o b s e r v a t i o n s a r e p r o b a b l y d u e t o e x p e r i m e n t a l e r r o r s . T h e lack o f c a l i b r a t i o n i n t e r - c o m p a r i s o n b e t w e e n t h e d i f f e r e n t i n s t r u m e n t s u s e d f o r t h e v a r i o u s o b s e r v a t i o n s makes i m p o s s i b l e a n y q u a n t i t a t i v e d e t e r m i n a t i o n of l o n g - t e r m v a r i a b i l i t y f r o m . 1970 t o 1979 w h i c h is s t i l l masked b y t h e a c c u r a c y o f each m e a s u r e m e n t . O n l y an u p p e r l i m i t o f 20 p e r c e n t can be t e n t a t i v e l y p r o p o s e d i n t h e 210-240 nm w a v e l e n g t h i n t e r v a l b u t s u c h v a r i a b i l i t y c o u l d also be n e g l i g a b l e .

I n c o n c l u s i o n , v a l u e s p r o p o s e d b y Simon (1974a) a r e c o n f i r m e d b e t w e e n 210 a n d 240 nm b y t h e new balloon o b s e r v a t i o n s a n d b y t h e s a t e l l i t e m e a s u r e m e n t s p r e s e n t e d b y H e a t h ( 1 9 8 0 ) w i t h i n t h e i r q u o t e d a c c u r a c i e s . I t a p p e a r s c l e a r l y t h a t v a l u e s o f B r o a d f o o t ( 1 9 7 2 ) g e n e r a l y a d o p t e d b e y o n d 230 nm f o r p h o t o d i s s o c i a t i o n r a t e c a l c u l a t i o n s a r e too h i g h a n d s h o u l d be r e d u c e d on t h e basis o f t h e new m e a s u r - e m e n t s . V a l u e s o f M o u n t e t a l . ( 1 9 8 0 ) need t o be c o n f i r m e d because t h e y a r e s y s t e m a t i c a l l y lower t h a n o t h e r a v a i l a b l e m e a s u r e m e n t s . F u r t h e r o b s e r v a t i o n s w i t h a p r e c i s i o n b e t t e r t h a n 5 p e r c e n t a r e b a d l y needed i n o r d e r t o d e t e r m i n e t h e p o s s i b l e l o n g - t e r m v a r i a b i l i t y b e y o n d 210 n m .

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REFERENCES

BROADFOOT, A . L. (1972). T h e solar s p e c t r u m 2100 - 3200 A . A s t r o - p h y s . J. 173, 681.

BRUECKNER, G . E . , B A R T O E , J . D . F . , KJELDSETH MOE, O. and VAN HOOSIER, M . E . (1976). A b s o l u t e solar u l t r a v i o l e t i n t e n s i t i e s and t h e i r v a r i a t i o n s w i t h solar a c t i v i t y . Part I : t h e wave- l e n g t h region 1750 - 2100 A . A s t r o p h y s . J . 209, 935.

H E A T H , D . F . (1980). Spatial and temporal v a r i a b i l i t y of ozone as seen f r o m space, in Proceedings of t h e NATO advanced s t u d y i n s t i t u t e on atmospheric ozone ( E d . A . C . A i k i n ) , p . 45.

MOUNT, G . H . , ROTTMAN, G . J . and T H I M O T H Y , J . G . (1980). The solar s p e c t r a l i r r a d i a n c e 1200 - 2550 A at solar maximum.

J. Geophys. Res. 85, 4271.

S A M A I N , D. and SIMON, P . C . (1976). Solar f l u x d e t e r m i n a t i o n in the s p e c t r a l range 150 - 210 nm. Solar P h y s . 49, 33.

P . C . (1974a). Balloon measurements of solar f l u x e s between 1960 and 2300 A , in Proceedings of the t h i r d conference on t h e climatic impact assessment p r o g r a m ( E d s . A . J . B r o d e r i c k and T . M . H a r d ) , p . 137.

P . C . ( 1 9 7 4 b ) . O b s e r v a t i o n de I ' a b s o r p t i o n d u rayonnement u l t r a v i o l e t solaire par ballons s t r a t o s p h e r i q u e s . B u l l . Acad.

Roy. B e l g . , CI. Sci. 60, 617.

P . C . (1978). I r r a d i a t i o n solar f l u x measurements between 120 and 400 nm. C u r r e n t position and f u t u r e needs. Planet. Space Sci. 26, 355.

P . C . and PEETERMANS, W. (1981). Ozone t r e n d in the u p p e r s t r a t o s p h e r e : r e s u l t s from solar U . V . a b s o r p t i o n measur- ements from 1976 to 1980, 3th S c i e n t i f i c Assembly of IAMAP, H a m b u r g .

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SIMON,

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