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 B R U X E L L E S
A E R O N O M I C A A C T A
A - N° 241 - 1981
Balloon observations of solar ultraviolet irradiance during solar cycle 21
by
P C. SIMON, R. PASTIELS, D. NEVEJANS and D. GILLOTAY
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 O N O M I E 3 • Ringlaan
B • 1180 BRUSSEL
FOREWORD
T h i s paper has been presented at the Symposium on the Solar Constant and the Spectral Distribution of Solar Irradiance which was held in Hamburg ( F R G ) on A u g u s t 17-18, 1981. It will be published m the proceedings of the conference.
A V A N T - P R O P O S
Cet article résume une communication présentée au Symposium sur " T h e Solar Constant and the Spectral Distribution of Solar Irradiance" qui s'est tenu à Hambourg ( R F A ) le 17 et 18 août 1981. n sera publié dans les compte-rendus de la conférence.
VOORWOORD
Dit a r t i k e l ' ' w e r d voorgedragen op het "Symposium on The Solar Constant and the Spectral Distribution of Solar Irradiance" dat werd gehouden te Hamburg ( B R D ) van 17 tot 18 augustus 1981. Het zal gepubliceerd worden in de verslagen van deze conferentie.
VORWORT
Diese Arbeit wurde in Hamburg ( D B R ) zum Symposium " T h e Solar Constant and the Spectral Distribution of Solar Irradiance" (17.
zum 19. A u g u s t 1981) aufgetragen. Sie wird in den Vorträgen der Konferenz heraugegeben werden.
B A L L O O N O B S E R V A T I O N S OF S O L A R U L T R A V I O L E T I R R A D I A N C E D U R I N G S O L A R C Y C L E 21
by
P . C . S I M O N , R. P A S T I E L S , D. N E V E J A N S and D. G I L L O T A Y
A b s t r a c t
Balloon borne spectrometric observations were made d u r i n g the period 1976 to 1980 around 41 km altitude in order to measure solar irradiance from 200 to 240 nm and from 270 to 330 nm. Extraterrestrial solar irradiance values were deduced from the measurements.
Quantitative determinations of solar variabilities are limited by the reproductibility of these observations around 210 nm. Comparison with other observations will be made and possible long-term irradiance v a r i a - tions will be d i s c u s s e d .
Résumé
~ Des observations du flux solaire entre 200 et 240 nm et entre 270 et 330 nm ont été faites à l'aide de ballons stratosphériques à 41 km d'altitude de 1976 à 1980. Les flux solaires extraterrestres ont été déduits de ces mesures. La détermination quantitative des variations solaires autour de 210 nm est limitée par la reproductibilité des mesures. Les observations effectuées par d'autres laboratoires sont comparées avec les résultats de ce travail et les éventuelles variations à long terme sont discutées.
Samenvatting
Gedurende de periode 1976 tot en met 1980 werden aan boord van stratosferische ballons spectroscopische waarnemingen v e r r i c h t omtrent de zonneflux in de golflengtegebieden 200 tot 240 en 270 to 330 nm. Uit deze metingen op 41 km hoogte werden de waarden van de zonneflux buiten de aardatmosfeer afgeleid. De reproduceerbaarheid van deze metingen beperkt de quantitatieve bepaling van veranderingen in de zonneflux rond 210 nm. Verder worden de meetresultaten vergeleken met andere waarnemingen en worden de fluxvariaties op lange termijn behandeld.
Zusammenfassung
Beobachtungen der Sonnenstrahlung zwischen 200 und 240 nm und zwischen 270 und 330 nm wurden mit Hilfe stratosphärischen Ballonen auf einer Höhe von 41 km während der Periode 1976-1980 gemacht. Die Sonnenausthalungen ohne atmosphärischen Absorption wurden aus diesen Messungen gerechnet. Die quatitativen Sonnenvariationen in der Nähe 210 nm sind d u r c h die Wiedererzeug barkeit der Messungen beschränkt.
Vergleichungen mit anderen Beobachtungen sind d u r c h g e f ü h r t und die möglichen, langzeitlichen Variationen sind diskutiert.
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I N T R O D U C T I O N
D u r i n g solar cycle 20, t h e o n l y measurement of solar i r r a d i a n c e c o v e r i n g t h e w a v e l e n g t h range c o r r e s p o n d i n g to t h e H a r t l e y bands of ozone was p e r f o r m e d b y Broadfoot (1972) in June 1970 by means of a r o c k e t - b o r n e s p e c t r o m e t e r . I r r a d i a n c e values f r o m 210 to 320 nm were p u b l i s h e d w i t h a q u o t e d accuracy of ± 10 p e r c e n t and a bandpass of 0 . 3 nm. O b s e r v a t i o n s at several d i s c r e t e wavelengths were made in t h e same w a v e l e n g t h range b y means of t h e BUV spectrometer on board the Nimbus 4 satellite and were p u b l i s h e d b y Heath (1973).
A l t h o u g h t h e bandpass of the l a t t e r i n s t r u m e n t (1 nm) made an accurate comparison w i t h Broadfoot's data d i f f i c u l t , t h e agreement seemed reasonably good. On t h e o t h e r h a n d , balloon o b s e r v a t i o n s c o v e r i n g t h e range 196-230 nm b y means of a spectrometer w i t h a bandpass of 0 . 6 nm were r e p o r t e d b y Simon (1974a) g i v i n g i r r a d i a n c e values 30 to 15 p e r c e n t lower t h a n those p u b l i s h e d b y Broadfoot (1972). T h e q u o t e d accuracy is ± 20 p e r c e n t . T h e same b a l l o o n - b o r n e spectrometer y i e l d e d i r r a d i a n c e data beyond 284 nm (Simon, 1975) w i t h an accuracy of ± 15 p e r c e n t and t h e same bandpass. T h e agreement w i t h Broadfoot (1972) is good up to 305 nm. It should be mentionned t h a t Broadfoot (1972) quoted l a r g e r u n c e r t a i n t i e s between 300 and 320 nm. Beyond 300 nm, o t h e r measurements were made b y A r v e s e n et al. (1969) f r o m a C o n v a i r Jet „ A i r c r a f t y i e l d i n g a quoted accuracy of 25 p e r c e n t at 300 nm.
Disagreements reaching 40 p e r c e n t e x i s t between t h e data of A r v e s e n et al. (1969) and those of Broadfoot (1972) in t h e w a v e l e n g t h i n t e r v a l
1
300-320 nm, v e r y i m p o r t a n t f o r t h e formation of O D from t h e p h o t o - dissociation of Og. Values of Simon (1975) have been w i d e l y accepted as being more reliable in t h i s i n t e r v a l . On t h e o t h e r h a n d , values of A r v e s e n et al. (1969) s u f f e r f r o m u n c e r t a i n t i e s due to change in the s p e c t r a l i r r a d i a n c e scale of t h e NBS in 1973. C o n s e q u e n t l y , f o r aeronomical p u r p o s e s , t h e data of Simon (1974) were g e n e r a l l y adopted from 200 to 230 nm, those of Broadfoot (1972) from 230 to 285 nm,
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those of Simon (1975) f r o m 285 to 330 nm and those of A r v e s e n et al.
(1969) ajusted on t h e i r r a d i a t i o n f l u x values obtained b y Labs and Neckel (1970) b e y o n d 330 nm ( c f r . D e l a b o u d i n i è r e et a l . , 1978).
Since 1976, c o r r e s p o n d i n g to the b e g i n n i n g of solar cycle 21, several measurements have been p e r f o r m e d in o r d e r to i m p r o v e o u r knowledge of solar f l u x e s b e y o n d 200 nm. T h e y include o b s e r v a t i o n b y means of t h e Nimbus 7 satellite ( H e a t h , 1980), r o c k e t s ( M o u n t et a l . , 1980; Mount and Rottman, 1981) and s t r a t o s p h e r i c balloons (Simon et a l . , 1981).
T h e p u r p o s e of t h i s w o r k is to r e p o r t b r i e f l y new o b s e r v a - tions of solar u l t r a v i o l e t i r r a d i a n c e in t h e w a v e l e n g t h i n t e r v a l s 200- 240 nm and 270-330 nm, p e r f o r m e d b y balloon f l i g h t s o c c u r i n g d u r i n g t h e . r i s i n g phase of solar cycle 21, namely between 1976 and 1980.
These new r e s u l t s will be compared w i t h e a r l i e r measurements and will be discussed in t e r m s of a possible l o n g - t e r m v a r i a t i o n of t h e solar o u t p u t .
O B S E R V A T I O N S
T h e o b s e r v a t i o n s of t h e u l t r a v i o l e t solar s p e c t r u m have been p e r f o r m e d b y means of an E b e r t - F a s t i e s p e c t r o m e t e r of 25 cm focal l e n g t h . T h i s i n s t r u m e n t has been flown in 1972 and 1973 and is e x t e n s i v e l y d e s c r i b e d in a p r e v i o u s paper (Simon, 1974b).
T h e absolute c a l i b r a t i o n of t h e spectrometer has been p e r f o r m e d b e f o r e and a f t e r each f l i g h t . Two d i f f e r e n t t y p e s of c a l i b r a - t i o n sources have been u s e d , namely d e u t e r i u m and q u a r t z - h a l o g e n lamps. B o t h are r e f e r e n c e d to t h e National B u r e a u of S t a n d a r d s s p e c t r a l i r r a d i a n c e scale. These two t r a n s f e r s t a n d a r d s o v e r l a p between 250 and 320 nm. In f a c t , b e y o n d 280 nm, t h e spectrometer signal o b t a i n e d f r o m
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deuterium lamps decreases rapidly and only the quartz-halogen lamps give higher signal-to-noise ratios. In addition, the latter standards allow the adjustement of the deuterium irradiance scale by comparison in the overlap wavelength interval, in order to take into account any change due to aging of deuterium lamps d u r i n g five years of measurement.
Several lamps of each type have been used and for each flight an average of the calibration results has been adopted. In the 200-240 nm wavelength interval, the reproducebility of calibration is within ± 4 percent. A s 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. Because in the 28U-330 nm wavelength interval a reproducebility within ± 2 percent and lower uncertainties for the irradiance scale of quartz-halogen sources (± 3 percent) are obtained one reaches an accuracy of ± 5 percent for the final calibration.
Three flights were performed from Gap (44°27'N, 6°02'E) on 1 July 1976, 7 July 1977 and 24 June 1980. The 4th flight took place from A i r e - s u r - l ' A d o u r (43°42'N, 0°15'W) on 14 September 1979. The
3
gondola of about 300 kg was carried by Zodiac balloons of 350.000 m launched by the "Centre National d'Etudes Spatiales" ( F r a n c e ) . The ceiling altitude was about 41 km for each flight.
D A T A R E D U C T I O N
For the 4 flights, the observed solar spectra have been corrected for the residual atmospheric absorption at the floating altitude deduced from simultaneous pressure and ozone measurements. T h i s method of data reduction is discussed more extensively in a previous paper (Simon et al., 1981). The extraterrestrial irradiance values are
a v e r a g e d o v e r about 20 corrected s p e c t r a . T h e final a c c u r a c y on the solar irradiance v a l u e s is ± 15 percent from 200 to 240 nm and ± 10 percent from 270 to 330 nm. T h e c o r r e s p o n d i n g p r e c i s i o n is respectively of the o r d e r of ± 6 percent and ± 4 percent.
R E S U L T S A N D D I S C U S S I O N
F i g u r e 1 p r e s e n t s the ratios of irradiance v a l u e s integrated o v e r 5 nm from 210 to 240 nm t a k i n g as a reference the data obtained b y Simon et al. (1981) on 1 J u l y 1976. T h e 5 nm interval has been c h o s e n in o r d e r to reduce as much as possible the effect of the different spectrometer resolutions on irradiance v a l u e s . T h e data of Simon (1974a) are not r e p r e s e n t e d for the sake of clarity. T h e y agree within ± 5 percent with 1976 v a l u e s in this wavelength r a n g e .
Systematic d i v e r g e n c e s clearly appear between most of the measurements. T h e y are p r o b a b l y due to experimental e r r o r s . Differences between all the balloon f l i g h t s do not exceed 12 percent.
T h e two last f l i g h t s are in v e r y close agreement, within 5 percent, with the data obtained b y Heath (1980). T h e y confirm that v a l u e s of Broadfoot (1972) have to be lowered b y at least 25 percent in this w a v e l e n g t h r a n g e . T h e lowest v a l u e s of Mount et al. (1980) do not seem reliable because they are not confirmed b y the last rocket flight reported b y M o u n t and Rottman ( 1 9 8 1 ) , u s i n g a similar spectrometer.
F i g u r e 2 r e p r e s e n t s the same comparison for integrated i r r a d i a n c e v a l u e s o v e r 5 nm between 270 and 330 nm, t a k i n g as a reference the data obtained b y balloon on 14 September 1979. T h e agreement between all the o b s e r v a t i o n s is rather good (± 10 p e r c e n t ) up to 295 nm. B e y o n d this w a v e l e n g t h , the data of M o u n t a n d Rottman (1981) become significantly lower than the o t h e r s , as those of Broadfoot (1972) do b e y o n d 305 nm. Balloon o b s e r v a t i o n s also exhibit larger
u
to <T>
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—> D
^ 1
<b
cr
<
cr 0.8 0.6
B70
_ T •
I I T —
MR 80
i ^ S 7 7
1 >H78
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— " — I
N5 7 9
< MRT79 -
i
210 220 230
WAVELENGTH (nm) 240
Fig. 1.- Observed solar irradiance in the spectral range 210-240 nm expressed as ratios relative to the balloon observation on 1 July 1976. Values are integrated over 5 nm intervals. The labeling is as follows : Broadfoot (1972); MR80 : Mount and Rottman (1981); S77, S79, S80 : This work;
H78 : Heath (1980), MRT79 : Mount et al., (1980).
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921.2
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<
er
—r 1 T i T • T I 1 1 I " 1
S77
I
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—r 1 T i
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" 1
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H78 - • 1
1 . 1 , 1 S80
« . i B70 _
M R 80 870
H78 S77 S80 M R 80
i i i i i i i i i i i
270 280 290 300 310 320 330 WAVELENGTH (nm)
F i g . 2.- Observed solar irradiance in the spectral range 270-330 nm expressed as ratios relative to the balloon observation on 14 September 1979. Values are integrated over 5 nm intervals. The labeling is as follows : B70 : Broadfoot (1972); MR80 : M o u n t and Rottman (1981); S 7 7 , S80 : This w o r k ; H 78 : Heath (1980); MRT79 : Mount et a l . , (1980).
discrepancies, up to 15 percent, at longer wavelengths nevertheless the agreement with the values obtained by Heath (1980) is within 5 percent for the 1979 balloon observation.
Up to now, only two new measurements are available between 240 and 270 nm, namely those of Heath (1980) and those of Mount and Rottman (1981). Both are systematically lower in this wavelength interval than those of Broadfoot (1972). T h e y diverge by 15 percent between 240 and 250 nm and agree quite well around 270 nm.
Consequently, Broadfoot's values generally used from 230 to 300 nm in stratospheric aeronomy should be lowered on the basis of the various balloon data and ot the Nimbus 7 observation.
It is difficult to deduce quantitatively any long-term variabi- lity of solar irradiance between 210 and 240 nm lower than 10-15 percent from these balloon observations. T h e y cover the entire rising phase of solar cycle 21 but their disagreements do not exceed 12 percent, i.e. within their, quoted precision (± 6 percent). U n - fortunately, the data of Mount et al. (1980) obtained at solar maximum do not solve the problem because they are not reliable (Mount and Rottman, 1981). On the other hand, the data obtained by Heath (1980) in November 1978 for an intermediate activity level are only 10 percent lower than those obtained by balloon in 1976 from 210 and 240 nm, i.e.
withiiT their quoted accuracies (10 - 15 percent). Consequently, the systematic divergences in available observations are probably due to experimental e r r o r s . The long-term variability beyond 210 nm is still masked by the accuracy and the precision of each measurement. S u c h variability could be negligible.
In conclusion, from the new measurements performed since 1976, solar irradiance values are probably known with an accuracy of
± 15 percent around 210 nm and ± 5 percent at 300 nm. Further
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 ± 2 . 5 p e r c e n t are s t i l l 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 nm.
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R E F E R E N C E S
A R V E S E N , J . C . , G R I F F I N , R . N . , J r . a n d P E A R S O N , B . D . , J r . : 1969, A p p l . O p t i c s , 8, 2215.
B R O A D F O O T , A . L. : 1972, A s t r o p h y s . J . , 173, 881.
D E L A B O U D I N I E R E , J . P . , D O N N E L L Y , R . F . , H I N T E R E G G E R , H . , S C H M I D T K E , G. a n d S I M O N , P . C . : 1978, C O S P A R T e c h n i q u e Manual No 7.
H E A T H , D . F . : 1980, in A i k i n ( e d . ) , P r o c e e d i n g s of the N A T O A d v a n c e s S t u d y Institute on A t m o s p h e r i c Ozone, Federal A v i a t i o n A d m i - n i s t r a t i o n , p. 45.
L A B S , D . and N E C K E L , H. : 1970, Solar P h y s i c s , 15, 79.
M O U N T , G . H . and R O T T M A N , G . J . : 1981, J. G e o p h y s . R e s . , 86, 9193.
M O U N T , G . H . , R O T T M A N , G . J . a n d T I M O T H Y , J . G . : 1980, J. G e o p h y s . R e s . , 85, 4271.
S I M O N , P . C . : 1974a, in B r o d e r i c k and H a r d ( e d s . ) , P r o c e e d i n g s of the T h i r d C o n f e r e n c e on the Climatic Impact A s s e s s m e n t
P r o g r a m , p. 137.
S I M O N , P . C . : 1974b, B u l l . A c a d . R o y . , C I . S c i . , 60, 617.
S I M O N , P . C . : 1975, B u l l . A c a d . R o y . B e l g . , C I . S c i . , 61, 339.
S I M O N , P . C . , P A S T I E L S , R. and N E V E J A N S , D . : 1981, Planetary Space S c i . , 29, in p r e s s .
