AERONOMICA 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 - A v e n u e C i r c u l a i r e B • 1 1 8 0 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 ° 2 1 1 - 1 9 8 0

O b s e r v a t i o n of t h e s o l a r u l t r a v i o l e t r a d i a t i o n

by

P . C . S I M O N

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 I M O M I E 3 - Ringlaan

B - 1180 B R U S S E L

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F O R E W O R D

T h i s paper has been presented at the N A T O Advanced S t u d y Institute on Atmospheric Ozone which was held at Albufeiras. (Portugal) from October 2 to October 13, 197.9. It will be published in the p r o - ceedings of the conference.

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

Cet article résume une communication présentée au " N A T O Advanced S t u d y Institute nn Atmospheric Ozone" qui s'est tenu à Albufeiras (Portugal) du 2 au 13 octobre 1979. Il sera publié dans les compte-rendus de la conférence.

V O O R W O O R D

Dit artikel werd voorgedragen op het " N A T O Advanced S t u d y Institute on Atmospheric Ozone" dat werd gehouden te Albufeiras (Portugal) van 2 tot 13 oktober 197.9. Het zal gepubliceerd worden in de verslagen van deze conferentie.

V O R W O R T

Diese Artikel wurde d u r c h die " N A T O Advanced S t u d y

Institute on Atmospheric Ozone" aufgetragen, die sich in Albufeiras

(Portugal) enthalten hat von den 2 bis den 13 Oktober 1979. Es werd in

die Vorträge der Konferenz publiciert.

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O B S E R V A T I O N OF T H E S O L A R U L T R A V I O L E T R A D I A T I O N

by

P . C . S I M O N

Abstract

Ultraviolet solar irradiance observations at Lyman a and between 125 and 400 nm are reviewed arid discussed. The uncertainties between the different measurements are pointed out and the d i s - crepancies are discussed in terms of solar variability, showing the needs of new observations in order to determine accurately the absolute value of solar ultraviolet irradiation fluxes and its possible variation with the 11-year solar cycle.

Résumé

Les différentes observations du flux solaire ultraviolet à

Lyman a et entre 125 et 400 nm sont présentées et discutées. La

précision des mesures et les désaccords entre les observations récentes

sont analysés en tenant compte des variations éventuelles dues à

l'activité du soleil, montrant la nécessité de nouvelles mesures pour

déterminer avec précision la valeur absolue du flux solaire dans

l'ultraviolet et sa variation avec le cycle de 11 ans.

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Samenvatting

De v e r s c h i l l e n d e waarnemingen van de u l t r a v i o l e t t e z o n n e f l u x bij Lyman a en t u s s e n 175 en 400 nm w o r d e n v o o r g e s t e l d en b e s p r o k e n . De n a u w k e u r i g h e i d van de metingen en het g e b r e k aan overeenstemming tussen de recente waarnemingen worden geanalyseerd met b e t r e k k i n g t o t de v e r a n d e r i n g e n v e r b o n d e n aan de z o n n e a c t i v i t e i t . De noodzaak van nieuwe metingen voor de n a u w k e u r i g e bepaling van de absolute waarde van de z o n n e f l u x in het u l t r a v i o l e t en z i j n v e r a n d e r i n g in f u n k t i e van de e l f j a r i g e c y c l u s w o r d t aangetoond.

Zusammenfassung

Die verschiedene Beobachtungen des U l t r a v i o l e t t e Sonnen- s t r a h l u n g zwischen 175 und 400 nm sowie f ü r Lyman a sind besprochen w o r d e n . Die Präzision der Messungen u n d der U n t e r s c h i e d zwischen die v e r s c h i e d e n e Messungen ist mit die Schwankungen d e r S o n n e n a k t i v i t ä t v e r g l i c h e n w o r d e n . Diese A r b e i t zeigt dass neue Messungen mit höhere Präzisionen in die U - V Wellenlänge n ö t i g s i n d , um die 11-Jahre C y c l u s der S o n n e n a k t i v i t ä t zu s t u d i e r e n .

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

The photodissociation processes of atmospheric constituents in the stratosphere and in the mesosphere are initiated by the solar radiation of wavelengths greater than 175 nm. A s discussed extensively by Nicolet (1979), the penetration of solar irradiances in this wavelength range is driven by the absorption of molecular o x y g e n and of ozone, and by the Rayleigh scattering of light. Radiations of wavelengths below 175 nm are absorbed in the thermosphere, except for the Lyman a emission line which penetrates deeply into the mesosphere, leading to the formation of the ionospheric D - r e g i o n , and for the X - r a y s of wavelengths less than 1 nm. In order to determine the most reliable solar irradiation flux values to be used in photochemical models of the middle atmosphere, an accurate knowledge of the solar irradiances in the ultraviolet is re- quired. Consequently, the available measurements must be critically discussed and compared.

Only full disc observation are pertinent for aeronomic p u r - poses. Observations made with a spatial resolution on the solar disc cannot be considered because of the center-to-limb variation of the solar radiation. For instance, flux measurements made at the center of the disc for wavelengths beyond 152 nm represent only an upper limit for the corresponding irradiance values because of the limb-barkening occuring in this part of the solar spectrum (Samain et al, 1975).

Among the most serious difficulties in determining the correct

value of the solar irradiance are the uncertainties associated with each

observation and the lack of intercomparison of the calibration techniques

used by the different experimenters. On the other hand, when com-

paring different sets of data, the possible variability of the solar fluxes

with the 27-day rotation period of the sun and with the 11-year activity

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cycle must be t a k e n into a c c o u n t . V a r i a t i o n related to solar f l a r e s can be neglected in t h e considered w a v e l e n g t h r a n g e , e x c e p t f o r t h e Lyman a emission line f o r w h i c h an enchancement of t h e o r d e r of 16% has been o b s e r v e d b y Heath (1969) f o r a 3B f l a r e and c o n f i r m e d b y Hall (1971) foh o t h e r f l a r e o b s e r v a t i o n s .

T h e p u r p o s e of t h i s w o r k is to p r e s e n t t h e r e c e n t u l t r a v i o l e t solar i r r a d i a n c e measurements related to t h e middle atmosphere aeronomy, i n c l u d i n g the Lyman a solar l i n e . T h e u n c e r t a i n t i e s associated w i t h t h e d i f f e r e n t experimental t e c h n i q u e s of o b s e r v a t i o n will be p o i n t e d o u t and t h e discrepancies between t h e data will be t e n t a t i v e l y discussed in terms of solar v a r i a b i l i t y .

I I . O B S E R V A T I O N A L DATA

T h e Lyman a emission line

T h e f i r s t measurement of Lyman a i r r a d i a n c e was p e r f o r m e d in 1949 b y Friedman et a t . , (1951) u s i n g a r o c k e t - b o r n e photon c o u n t e r and

10 11 y i e l d o n l y an o r d e r of m a g n i t u d e , between 3.7 x 10 and 3 . 7 x 10

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h v . s e c cm , f o r t h e i n t e n s i t y of t h i s v e r y s t r o n g c h r o m o s p h e r i c l i n e . Since t h a t time, many measurements, u s i n g v a r i o u s o b s e r v a t i o n a l t e c h n i -

11 - 1 - 2

q u e s , and leading to a conventional value of 3 x 10 h v . s e c .cm have been made. For most of t h e o b s e r v a t i o n s , the quoted a c c u r a c y is of t h e o r d e r of ± 30%. T h a t means t h a t t h e i r r a d i a n c e of Lyman a should be

11 - 1 - 2

i n c l u d e d between 2 . 1 and 3.9 x 10 h v . s e c .cm

V a r i a t i o n s of t h i s solar line w i t h t h e 11-year cycle and w i t h t h e 2 7 - d a y rotational p e r i o d of t h e s u n , have to be considered and seem c o n f i r m e d b y o b s e r v a t i o n s p e r f o r m e d d u r i n g t h e solar cycles 19 and 20.

B u t such v a r i a t i o n s are of the same o r d e r as t h e absolute accuracy

claimed b y t h e d i f f e r e n t e x p e r i m e n t e r s . Hence, a main problem is to

determine if these o b s e r v e d v a r i a t i o n s are due to c a l i b r a t i o n u n c e r t a i n -

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t i e s , i n s t r u m e n t a l d e g r a d a t i o n o r c o r r e s p o n d to a real p h y s i c a l change of t h e solar o u t p u t at t h i s w a v e l e n g t h . T h e r e s u l t s o b t a i n e d d u r i n g t h e solar cycle 19 are summarized in table 1. These show a decrease f r o m solar maximum c o n d i t i o n s in 1960 to 1963, except f o r t h e value of H i n t e r e g g e r (1961) w h i c h appears to be r a t h e r low in comparison w i t h t h e o t h e r o b s e r v a t i o n s . D u r i n g t h e same solar c y c l e , Weeks (1967) made an analysis of t h e r e s u l t s obtained b y means of ion chambers o v e r t h e i n t e r v a l between 1955 and 1966. His conclusion is t h a t t h e v a r i a t i o n of t h e Lyman a i r r a d i a n c e appears to be of t h e o r d e r of 40% o v e r t h e 11-year solar c y c l e .

F i g u r e 1 shows t h e r e s u l t s of measurements p e r f o r m e d d u r i n g solar cycle 20. T h e most reliable measurements of Lyman a v a r i a t i o n are those obtained b y means of t h e OSO 5 satellite ( V i d a l ~ M a d j a r , 1975).

T h e e x p e r i m e n t , w h i c h e x h i b i t s an i n s t r u m e n t a l s e n s i t i v i t y d e g r a d a t i o n of o n l y 10% per y e a r , leads t o a v a r i a t i o n of t h e o r d e r of 40%. Empirical relations r e l a t i n g t h e v a r i a b i l i t y of Lyman a i r r a d i a n c e w i t h t h e 10.7 cm solar f l u x have been proposed b y V i d a l - M a d j a r (1975).

V a r i a t i o n s related to t h e 2 7 - d a y rotational p e r i o d of the sun were measured f o r t h e f i r s t time b y K r e p l i n et al. (1962) who deduced from SR 1 satellite o b s e r v a t i o n s , v a r i a t i o n s less t h a n 18% in J u l y 1960.

Since t h a t time, a maximum of solar v a r i a b i l i t y of t h e o r d e r of 30% has been proposed on t h e basis of the o b s e r v a t i o n s of Hall and H i n t e r e g g e r ( 1 9 7 0 ) , Heath (1973), Woodgate et al. (1973) and V i d a l - M a d j a r (1975).

N e v e r t h e l e s s , due to c a l i b r a t i o n and i n s t r u m e n t a l p r o b l e m s , any q u a n t i t a t i v e conclusion c o n c e r n i n g t h e solar Lyman a v a r i a t i o n s may be s u b j e c t to d o u b t . Table 2 p r e s e n t s a comparison of t h e OSO 5 data w i t h r o c k e t f l i g h t o b s e r v a t i o n s , showing t h a t most of t h e data f r o m r o c k e t measurements are in good agreement w i t h the OSO 5 r e s u l t s (less t h a n 10%). C o n s e q u e n t l y , t h e 30% v a r i a t i o n w i t h t h e 27-day r o t a t i o n a l p e r i o d of t h e sun and t h e f a c t o r of 2 v a r i a t i o n w i t h t h e 11-year cycle

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CM I

<r> I

o E

o a» </>

.c Hi a z <

Q <

cr cr

10 12

10 11

10 io

" 4 ,

" %

b f , 67 8 10 11 o o

21 o

^ ^ V r ^ Y » o V^ B

o 20 22

13 1 6

Ly a

_L ± _L _L J_ 1 OCTOBER OCTOBER OCTOBER OCTOBER OCTOBER

1966 1968 1970 1972 1974 SMOOTHED SUNSPOT NUMBER, CYCLE 20

- 10

10

cr

UJ CD ZD Z h- o CL CO z 3.

1/) Q HI h- X o o z (ƒ)

Fig. 1.- Comparison of Lyman a solar irradiation flux measurements during the solar cycle 20. The smoothed sunspot numbers are also shown.

References: 1, 2, 3, 4, 5, 6 and 9, Weeks (1967); 7, Fossi et al.

1969); 8, Bruner and Parker (1969); 10, Hinteregger (1970); 11, Bruner (private communication); 12, Woodgate et al. (1973); 13, Smith (1972);

14, Dickinson (1972); 15, Higgins (1976); 16, Ackerman and Simon (1973); 17, Prinz (1974); 18, Heroux et al. (1974); 19 and 20, Rottman (1974); 21, Oshio (private communication); 22, Heroux (private communi- cation); 23, Rottman (private communication); A, Vidal-Madjar (1975);

B, Vidal-Madjar (private communication) (after Delaboudinière et al.,

1977).

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TABLE 1.- Observation of the Lyman alpha irradiance during solar cycle 19.

Date .„11 . -1 -2

10 h sec cm Instrumentation Reference

Oct 18, 1955 3,5 Ion Chamber Byram et al (1956) Oct 21, 1955 2,4 Ion Chamber Id.

Nov 4, 1955 5,5 - Ion Chamber Id.

Dec 13, 1955 1,8 Spectrograph Miller et al (195.6) Jul 20, 1956 3,7 Ion Chamber Chubb et al (1957) Jul 25, 1956 4,1 Ion Chamber Id.

Jul 20, 1957 3,7 Ion Chamber Byram et al unpu- blished data

Aug 6, 1957 2,1 Spectrograph Aboud et al (1959) Mar 23, 1958 3,9 Ion Chamber Byram et al unpu-

blished data Jul 21, 1959 3,7 Ion Chamber Purcell and

Tousey (1960) Jan 19, 1960 3,7. Ion Chamber Hinteregger (1960) Apr 19, 1960 3,7 Ion Chamber Detwiler et al

(1961)

Aug 20, 1960 3,4 Photoelectric Efremov et al (1962:

Aug 23, 1960 2,0 Monochromator Hinteregger (1961)

Aug 23, 1961 3,1 Monochromator Hall et al (1963)

Dec 12, 1963 2,7 Monochromator Hall et al (1965)

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TABLE 2.- Lyman alpha irradiance : comparison of the 0S0 5 data with rocket flight observations (after Vidal-Mad.jar, 1977)

0S0 5 Rocket Reference for 11 -1 -2

(10 h .sec .cm ) data rocket data

M a r 7

>

1 9 7 0

. 4,41 - 6% Dickinson (1972)

N o v

9.

1 9 7 1

* 2,8 + 26% Higgins (1976)

Feb 28, 1972 * 3,0 - 30% Ackerman and Simon (1973) Jul 10, 1972 3,1 + 8 % Prinz (1974)

Aug 23, 1972 2,99 - 3% Heroux et al (1974)

Dec 13, 1972 2,88 + 10% , Rottman (1974)

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d e d u c e d from the OSO 5 satellite, s h o u l d be r e a l i s t i c b u t need e x p e - rimental confirmation d u r i n g the following solar c y c l e s .

T h e 175-240 nm w a v e l e n g t h i n t e r v a l

T h i s w a v e l e n g t h i n t e r v a l is mainly related to the p h o t o - d i s s o c i a t i o n of molecular o x y g e n in the mesosphere and in the u p p e r s t r a t o s p h e r e .

T h e s o u r c e of the solar s p e c t r u m in t h i s w a v e l e n g t h i n t e r v a l lies in a t r a n s i t i o n region of the solar atmosphere, between the photo- s p h e r e and the c h r o m o s p h e r e . A b s o r p t i o n lines w h i c h dominate the solar s p e c t r u m in the v i s i b l e and in the near u l t r a v i o l e t d i s a p p e a r p r o g r e s - s i v e l y in the v a c u u m u l t r a v i o l e t r a n g e in w h i c h emission lines of chromo- s p h e r i c o r i g i n become p r e d o m i n a n t a r o u n d 150 nm. T h i s is i l l u s t r a t e d in f i g u r e s 1 and 2 p u b l i s h e d b y Samain and Simon (1976) .

U n f o r t u n a t e l y , the solar i r r a d i a t i o n f l u x e s are not s u f f i c i e n t l y

well known, e s p e c i a l l y between 175 and 200 nm, c o r r e s p o n d i n g to t h e

S c h u m a n n - R u n g e a b s o r p t i o n band system of molecular o x y g e n . T h e f i r s t

measurement of solar i r r a d i a n c e in t h i s w a v e l e n g t h i n t e r v a l were made

b y Detwiler et al. (1961) in 1960 b y means of a r o c k e t - b o r n e s p e c t r o -

g r a p h , b u t t h e i r r e s u l t s appear too h i g h , b y a f a c t o r of 3, in

comparison with the more r e c e n t o b s e r v a t i o n s . T h e o n l y measurements

c o v e r i n g all the 175-200 nm w a v e l e n g t h i n t e r v a l are those of Samain and

Simon (1976) and those of B r u e c k n e r et al. ( 1 9 7 6 ) . ' T h e y m e a s u r e d ,

r e s p e c t i v e l y , the solar radiance at the c e n t e r of the d i s k and at a

selected area of the q u i e t s u n u s i n g p h o t o g r a p h i c r e c o r d i n g t e c h n i q u e s .

T h e s e radiances were c o n v e r t e d into solar i r r a d i a n c e u s i n g the c e n t e r -

to-limb v a r i a t i o n s measured b y Samain and Simon (1976). O t h e r v a l u e s

c o v e r i n g , p a r t i a l l y , the same w a v e l e n g t h range have been obtained b y

Rottman (1974), Simon (1974) and H e r o u x and S w i r b a l u s (1976). A l l t h i s

r e s u l t s are g i v e n in f i g u r e 2 f o r c o m p a r i s o n . T h e agreement between

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10 12

E o

! ie

11

c.

O

O <

CC er

SIMON (1974) ROTTMAN (1974)

S A M A I N & SIMON (1976) BRUECKNER et al (1976) HEROUX & SWIRBALUS (1976) H - 5

10 10 170 175 180 185

WAVELENGTH ( n m )

190 195 200

Fig. 2.- Comparison of ultraviolet solar irradiation fluxes reported by various experimenters from 170 to 200 nm.

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these authors is good at 175 nm but beyond this wavelength, dis- agreement, reaching a factor of two, can occur for given wavelengths.

The agreement becomes better, within 20%, around 200 nm for the observations of Simon (1974), Samain and Simon (1976) and Brueckner et al. (1976). It should be pointed out that solar irradiances determined by Samain and Simon (1976) and by Brueckner et al. (1976) are not direct measurements of the full disc. They used limb-darkening data in order to determine irradiance values from radiance data based on photo- graphic spectra for which calibration procedures are more compli- cated. This method introduces certain additionnal errors, leading to less accurate results. On the other hand, both instruments used by Rottman (1974) and by Heroux and Swirbalus (1976) have photoelectric detectors with a solar blind Csl photocathode for which the quantum yield ex- hibits a rapid decay for wavelengths greater than 185 nm. This is illustrated in figure 1 published by Heroux and Swirbalus (1976).

T h u s , accuracies of solar irradiation flux measurements beyond this wavelength are not as good as those at lower wavelengths because the laboratory determination of the spectral sensitivity could have more important experimental uncertainties. In conclusion, one may consider that, between 180 and 190 nm, the data of Samain and Simon (1976) represent the upper limit for the solar irradiance while the data of Heroux and Swirbalus (1976) could give a lower limit. Consequently, the solar irradiation flux is only known with a precision of + 25% in this wavelength range and a mean value between the different observations should be introduced in the photodissociation calculations.

Beyond 200 nm, the recently published measurements of

Broadfoot (1972), Simon (1974) and Simon et al. (1980) should be only

considered in deducing the solar irradiation flux values. They are given

in figure 3 and 4 for comparison. The previous observations of

Ackerman et al. (1971), obtained by means of a balloon-borne spectro-

meter, are 40% higher than those obtained by Simon (1974) who used a

spectrometer integrated in a stabilized gondola. This allowing direct

pointing of the instrument toward the sun.

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Fig. 3.- Comparison of ultraviolet solar irradiation fluxes reported by various experimenters from 200 to 230 nm. The measurements made by Ackerman et al (1971) and Simon et al. (1980) are note represented here for the sake of clarity.

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BROADFOOT (1972) SIMON et al (1980)

(July 7, 1977)

21 I I I I '

230 235 2A0 245 250 255 260 WAVELENGTH (nm)

4.- Comparison of ultraviolet solar irradiation, fluxes reported by various experimenters from 230 to 260 nm.

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T h e measurement of solar i r r a d i a t i o n f l u x e s from b a l l o o n - b a s e d platforms must, o b v i o u s l y , be c o r r e c t e d f o r the r e s i d u a l -absorption b y molecular o x y g e n and ozone because of the f l o a t i n g a l t i t u d e , of the o r d e r of 40 km, reached b y a s t r a t o s p h e r i c balloon. O b s e r v a t i o n s are made f o r d i f f e r e n t solar z e n i t h angles and e x t r a t e r r e s t r i a l solar i r r a d i a n c e s can be d e d u c e d b y e x t r a p o l a t i o n of measured solar f l u x e s to zero air mass. T h i s can be done o n l y if the f l o a t i n g a l t i t u d e of the balloon is held r i g o u r o u s - ly c o n s t a n t in o r d e r to keep, f o r all r e c o r d e d s p e c t r a , the same column c o n t e n t f o r molecular o x y g e n and ozone above the gondola. A second method . c o n s i s t s f o r c o r r e c t i n g the o b s e r v e d s p e c t r a b y t a k i n g into account the real s t r a t o s p h e r i c a b s o r p t i o n . For t h i s p u r p o s e , a c c u r a t e determination of molecular o x y g e n and ozone c o n c e n t r a t i o n should be c a r r i e d out d u r i n g the solar f l u x o b s e r v a t i o n s , on the same gondola.

Both r e d u c t i o n methods have been used f o r the data obtained in 1973 b y Simon (1974), g i v i n g a good agreement, w i t h i n ± 5%, between the r e s u l t s obtained t h r o u g h these two d i f f e r e n t w a y s . T h i s e x c e p t f o r w a v e l e n g t h s b e y o n d 225 nm where the i n f l u e n c e of the a l t i t u d e v a r i a t i o n is v e r y important on solar i r r a d i a n c e values d e d u c e d b y e x t r a p o l a t i o n to zero air mass, due to the h i g h e r ozone a b s o r p t i o n c r o s s section above 225 nm.

Molecular o x y g e n c o n t e n t was d e t e r m i n e d t h r o u g h p r e s s u r e measurements and ozone column d e n s i t y was measured b y means of the solar s p e c t r a r e c o r d e d between 270 and 285 nm b y the same solar s p e c t r o m e t e r .

New balloon o b s e r v a t i o n s have been p e r f o r m e d in 1976 and in 1977, g i v i n g solar i r r a d i a n c e v a l u e s between 210 and 240 nm with an a c c u r a c y of ± 15% (Simon et a l . , 1980). T h e comparison with the p r e - v i o u s balloon o b s e r v a t i o n s p u b l i s h e d b y Simon (1974) and with the r o c k e t measurement p u b l i s h e d b y B r o a d f o o t (1972) is shown on f i g u r e 5. T h e two r e c e n t balloon o b s e r v a t i o n s are, in g e n e r a l , in v e r y good a g r e e m e n t , b e t t e r t h a n ± 5%, e x c e p t b e y o n d 235 nm. T h e y g i v e h i g h e r v a l u e s t h a n those p u b l i s h e d in 1974, with a maximum d i s c r e p a n c y of 9% f o r the 1976's f l i g h t and of 5% f o r the 1977's f l i g h t , o v e r the 210-235 nm w a v e l e n g t h

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i o 1 V

t 1 0 13

10 12 210

1

I

B R O A D F O O T (1972 ) S I M O N (1974) S I M O N et al (1980)

(Duly 1,1976) S I M O N et al (1980)

(July 7,1977)

220 230 1

WAVELENGTH (nm)

240

Fig. 5.- Comparison of ultraviolet solar irradiation flux integrated over 5 nm, obtained by balloon and rocket experiments between 210 and 240 nm.

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interval. Broadfoot's results, given with an accuracy of ± 10%, are systematically higher by a factor varying from 23% to 12% for wave- lengths increasing from 200 to 240 nm. Such disagreement could be due to different calibration techniques or could be interpreted in terms of solar variability with the 11-year cycle and will be discussed later.

The 240 - 400 nm wavelength interval

This wavelength interval mainly related to the photodissociation of ozone has been investigated by many observators involving rocket borne spectrometers measuring solar irradiance between 210 and 320 nm (Broadfoot, 1972), satellite measurements at 12 discrete wavelengths between 255 and 340 nm (Heath, 1973), balloon-borne observation be- tween 270 and 350 nm (Simon, 1975 and Simon et al.,, 1980), aircraft measurements performed by Arvesen et al. (1969) from 300 nm up to the infrared and ground-based observations by Labs and Meckel (1970) between 330 nm and 1250 nm. The only measurement covering the wave- length range between 240 and 280 nm was published by Broadfoot (1972) who quoted a calibration accuracy of ± 10%. The agreement with results of Heath (1973) as rather good. These latter measurements of solar irradiances have been made with a double ultraviolet spectrometer on board the Nimbus 4 satellite, launch in April 1970. The final accuracy is limited by the accuracy of the standard source used, namely ± 8% at 250 nm, ± 4 % at 290 and ± 3% beyond 300 nm.

Figure 6, 7 and 8 presents measurements available beyond

280 nm. Discrepancies of the order of 25%, but reaching 40% at given

wavelengths, appears is the 300-320 nm wavelength interval between

Broadfoot (1972) and Arvesen et al. (1969). It should be pointed out

that Broadfoot (1972) claimed an error larger than ± 10% for only wave-

lengths beyond 300 nm while Arvesen et al. (1969) estimate their error

to be ± 25% at 300 nm, ± 6 % at 320 nm and ± 3 . 2 % at 400 nm.. Conse-

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Fig. 6.- Comparison of ultraviolet solar irradiation fluxes reported by various experimenters from 280 to 310 nm.

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Ui O z <

Q <

<r t r

ARVESEN et al ( 1 9 6 9 )

— • — LABS & NECKEl (1970) BROADFOOT ( 1 9 7 2 ) S I M O N ( 1 9 7 5 )

10 13| 310 315 320 325 WAVELENGTH (nm)

330 335 340

Fig. 7.- Comparison of ultraviolet solar irradiation fluxes reported by various experimenters from 310 to 340 nm.

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10 15

E c

E O

O a> u

««10 ->

•C

ÜJ u z <

O <

cc <r

-1 — •

"

ARVESEN et at (1969)

— L A B S & NECKEl (1970) SIMON (1975)

345 350 355

WAVELENGTH (nm)

360 365 370

Fig. 8.- Comparison of ultraviolet solar irradiation fluxes reported by various experimenters from 340 to 370 nm.

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quentl'y, data between 300 and 320 nm, taken from these two obser- vations, should be considered very carefully. On the other hand, the absolute radiometric scale used for calibration by Arvesen et al. (1969) suffers from uncertainties due to changes in the spectral irradiance scale of the National Bureau of Standards in 1973 (Kosthowski, 1974).

For all these reasons, data published by Simon (1975), which are in good agreement with Broadfoot (1972) below 300 nm give useful solar irradiance in this wavelength region. In addition, new balloon obser- vations performed in 1976 and 1977 (Simon et al., 1980) give solar irradiance within ± 10%, in agreement with the results obtained in 1972 and 1973 (figure 9). The absolute accuracy is of the order of ± 10% and the good reproducibility of the different balloon experiments allows to conclude that the solar irradiance around 300 nm is actually known within 10%.

Other observations have been performed from a Convair Jet aircraft belonging to NASA by Thekaekara et al. (1969). The published irradiance values, integrated over 5 nm, are, in fact, an average between data coming from four different instruments. These are : a single and a double monochromator, a filter radiometer and an inter- ferometer. Other irradiance values beyond 300 nm, with the wavelength interval of 0.1 nm, have also been published by Thekaekara (1974) but, unfortunately, an error in the wavelength scale makes it difficult to make a correct comparison with the other measurements (Simon, 1975).

Very accurate ground-based observations have been carried out by Labs and Neckel (1970) from a high mountain observatory for wavelengths beyond' 330 nm. Irradiance values were deduced from

radiance measurements near the disc center and converted into ir-

radiation fluxes using the limb-darkening data of David and Elste

(1962). Absolute calibration was referenced to the International Practical

Temperature Scale-1968. The near ultraviolet and visible irradiance

values given by Labs and Neckel (1970) for 10 nm intervals could be

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10 15 E o

Ui u z

S io cc <r

M

BALLOON MEASUREMENT ÖF SOLAR IRRADIANCE

SIMON (19755 SIMÖN et ai (i960) (July 1, 1976) SiMON et at (I960) (July 7, 1977)

280 290 3ÓÖ 310 32Ó JL

WAVELENGTH (hm) 330 340 350

Fig. 9i- Comparison óf ultraviolet solar irradiation flux integrated over 5 nm, obtained by balloon experiménts between 280 and 35Ó nui.

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used in order to adjust the data of Arvesen et al (1969), obtained with a higher resolution, into a correct radiometric scale for wavelength above 330. Nevertheless, it should be taken into account that the data of Labs and Neckel (1970), which extents up to 1250 nm, leads to a solar constant value of 1358 W.cm which is 0.9% lower than the recent _2 solar constant measurements obtained by Willson and Hickey (1977),

. . - 2

giving a value of 1370 W.m .

Variability with the 27-day rotional period

Except in the case of the Lyman-o emission line, already discussed, very few investigations on this subject have been made experimentally. Only two reliable observations are available : firstly those published by Heath (1973) based on broad-band sensors on board of the satellites Nimbus 3 and 4, which were launched, respectively, in April 1969 and April 1970, and, secondly, those published by Hinteregger et al. (1977) who observed ultraviolet flux variations during the 1974-1976 period by means of a spectrometer on board of the satellite Atmospheric Explorer C. An additional observation has been performed by Prag and Morse (1970) but results have been obtained for only one solar rotation, leading to a variation of more than 50% for the broad wavelength interval 160-210 nm. This value seems too high by comparison with data obtained by Heath (1973) and Hinteregger et al.

(1977) given in figure 10. For these authors, the irradiance variation beyond 175 nm, related to the 27-day rotational period, is less than 10%

and are decreasing with increasing wavelength to be of the order of 1%

around 300 nm. The value for wavelengths between 175 and 210 nm is

also confirmed by Brueckner et al. (1976) on the basis of plage

radiance measurements. Consequently, it appears that solar variation

around 200 nm with its 27-day rotational period is of the same order of

magnitude as the variability due to the semi-annual change of the

sun-earth distance, corresponding to a ± 3.3% variation for the total

solar irradiance.

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100

10

<r

27-DAY SOLAR FLUX VARIATIONS

o s

t

L y a

O • WOODGATE et al (1973) O HEATH (1973)

• VIDAL-MADJAR (1975) HINTEREGGER et al (1977)

-

100 200

WAVELENGTH ( n m )

300

10.- Solar flux variations related to the solar rotational period from Lyman a to 300 nm. The values of Hinteregger et al. (1977) correspond to the Carrington rotation n° 1615 (June 1974).

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Variability with the 11-year cycle

The solar irradiation flux variability with the 11-year cycle is v e r y poorly known for all the solar spectrum. Long-term variability of the extreme ultraviolet flux with the solar cycle is quite evident but, even in this wavelength range, the data available do not suffice to determine, quantitatively, any variations d u r i n g the last solar cycle (Schmidtke, 1978). The situation is worse in the wavelength range related to the middle atmosphere photodissociation processes. The in- adequate time coverage of reliable data d u r i n g the solar cycle 20 and the e r r o r s associated with each available measurement do not permit quantitative conclusions of solar variability with the 11-year cycle as it was stated by Simon (1978) and Delaboudiniere et al. (1978). T h i s is illustrated by figures 11 and 12.

Nevertheless, quantitative variation of the solar irradiance between 175 and 380 nm has been proposed recently by Heath and Thekaekara (1977) on the basis of broad-band photometric observations by the Monitor of Ultraviolet Solar E n e r g y experiment ( M U S E ) from a rocket flight in A u g u s t 1966 and from satellites Nimbus 3 and 4 in April 1969 and April 1970, respectively. Another data were obtained with the double spectrometer on board of Nimbus 4 and Explorer 55 satellites, the latter launched in November 1975. The results are shown b y figure 13 based on N A S A (1977) and g i v i n g the ratio of solar irradiance measured at solar minimum to that at solar maximum. The solid c u r v e represents the proposed solar variability d u r i n g the cycle 20 for wave- lengths between 170 and 340 nm. The accuracies of the data obtained by the broad-band sensors are, respectively, ± 15% and ± 30% for the shorter and the longer wavelength detector and between ± 8% and ± 3%

for the double monochromator. Only two different ratios are available around 180 nm and the other values obtained around 290 nm with the double monochromator are higher by a factor of 15-20% than those obtained from the broad-band detectors at the same wavelength. A

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OCTOBER 1966

OCTOBER 1968

OCTOBER 1970

OCTOBER 1972

OCTOBER 1974 SMOOTHED SUNSPOT NUMBERS, CYCLE 20

SAMAIN & SIMON (1976) ACKERMAN & SIMON (1973) •

ROTTMAN (1974) ® HEROUX & SWIRBALUS (1976)

a. uj m 2 D z

o Q.

t/) 2 1/1 ZD O liJ t- X

o o

2

Fig. 11.- Comparison of solar irradiation flux measurements at 171 nm during the solar cycle 20. The smoothed sunspot numbers are also shown.

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OCTOBER

1966 OCTOBER OCTOBER

1968 1970 OCTOBER OCTOBER 1972 1974 SMOOTHED SUNSPOT NUMBERS, CYCLE 20

x BROADFOOT (1972) o SIMON (1974) • SIMON et at (1980)

Fig. 12.- Comparison of solar irradiation flux measurements at 230 nm during the solar cycle 20. The smoothed sunspot numbers are also shown.

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WAVELENGTH (nm)

Fig. 13.- Ratio of solar flux measured near solar-cycle minimum to that measured near solar-cycle maximum versus wavelength (see text for explanations).

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linear regression calculation is illustrated by the dashed lines for each type of measurements. Extrapolation to 320 nm of the solar irradiance ratio, obtained from the broad-band detectors data, give a variability ratio of 25% which disagrees with the variability ratio of 5% at the same wavelength based on the double monochromator measurements. In addition, extrapolation of these latter observation to 200 nm leads to a variability ratio of only 0.68 but there is no experimental evidence for either a linear dependance or for other functions of the solar variability dependance with the wavelength. Taking into account the possible experimental errors on the irradiance observations and the lack of data related to the solar activity, the validity of this curve needs to be confirmed by new measurements during solar cycle 21, with accurate cross-calibration making possible the intercomparison between the new data. The factor of 2 of variability claimed by Heath and Thekaekara (1977) seems also too high because there is no astrophysical reasons leading to variations with the 11-year cycle at 200 nm comparable to those observed for the Lyman a chromospheric line. This represents the maximum possible long-term variability in the extreme-ultraviolet wave- length range.

These comments seem to be confirmed by the new observations

made by means of a balloon-borne spectrometer in 1976 and 1977 already

mentioned (Simon et al, 1980) and corresponding to a period of solar

minimum activity. Comparing these data with those of Broad foot (1972),

obtained for solar maximum activity, variations between 210 and 240 nm

could be included between 23% and 12%, the ratio decreasing with in-

creasing wavelength, if the differences between these authors are

interpretated as only variability of the solar output and not as

experimental uncertainties which are of the order of ± 15%. Such a

variation range shows the need of new measurements with both better

accuracy and precision between 175 and 240 nm in order to deduce

correct quantitative variabilities in this wavelength interval. Around

300 nm, there is no more conclusive evidence of variabilities of the order

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of 15%, due also to the uncertainties associated with each available measurement. A r o u n d 400 nm and in the visible range new observations of solar irradiance performed by Shaw and Fröhlich (1979) by means of a filter radiometer located at the Mauna Loa observatory in Hawaii do not cover a sufficiently large period of time: only 6 months of results were published and do not allow quantitative long-term variation determina- tions in this part of the solar spectrum.

I I I . C O N C L U S I O N

The ultraviolet solar irradiation flux observations related to the middle atmosphere have been extensively described and the current position of the accuracies and of the discrepancies between the different measurements is summarized in table 3. Uncertainties of laboratory radio- metric standard sources and specific requirements for the future o b s e r - vations are also given.

The discrepancies are rather large and cannot be unam- bigeously interpreted as being caused by solar variabilities, due to the experimental e r r o r s associated with each measurement, to the inadequate time coverage d u r i n g solar cycle 20, and to the lack of intercomparison of the calibration results of the instruments. The laboratory radiometric standards have a smaller uncertainty than the present measurement, and future observation accuracy requirements could be f u I If i I led :

i) if a carefully calibration in the laboratory is made, allowing the transfer of the standard source accuracy to the instrument with little degradation, and

ii) if the e r r o r s introduced by the measurements on the s u n itself, in space environment, are reduced by appropriate techniques eliminating the degradation of the calibration accuracy.

The cleanliness of the experiment is certainly an important factor to

improve the validity of the future observations (Madden, 1978). A

different method consists to measure directly the aging of the instrument

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TABLE 3.- Uncertainties on solar ultraviolet irradiance measurements and future needs

A M n m ) Ly a 175 - 210

Quoted accuracy ± 30% ± 30% - ± 2 0 % Discrepancies

between relevant 100% 5 0 % observations

Uncertainties on

available stan- 5 % 5 % dard sources

Required accuracy 10% 5 % Required precision 5 % 1%

i M O

t

210 - 240 240 - 330 330 - 400

± 2 0 % - ± 10% ± 10% - ± 4 % ± 10% - ± 4 %

30% 2 5 % 15%

5 % 3 % 2 %

5 % 5 to 2 %

1% 1 to 0.3%

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sensitivity d u r i n g the preflight storage, the measuring mode in space and after the flight. T h i s implies that the instrument includes an in- flight calibration source and will be recoverable after the solar o b s e r - vation. The Space Shuttle, having a relatively short mission in space, without any severe limitation in weight and in power s u p p l y , seems a promising method since it allows the control of the g r o u n d calibration validity d u r i n g and after the space observations. In addition, it could be now conceivable to recover a free-flyer instrument after a long s u r v e y of the sun and to measure in the laboratory its degradation in order to see which components are really aging in space environment, improving our knowledge on the proper choice of the materials.

Moreover, observations with different experimental techniques are required to eliminate the systematic e r r o r s . However intercurnparison of calibration results for the various instruments should be performed before the flights to ensure a proper intercomparison of irradiance results after the flights.

The question of variability of the solar irradiance could be solved if new observations are performed with a correct time sampling, by means of repeated measurements with a v e r y high precision.

Variability measurements made from satellites will be only useful if' the aging of the instrument sensitivity is checked by means of c r o s s - calibrated observations with balloon, rocket or shuttle-borne in- struments.

S u c h a measurement strategy for the near future could be

applied to the "Solar ultraviolet Spectral Irradiance Measurement" and

the "Solar Spectrum" experiments both on board the Space Shuttle, to

the ultraviolet spectrometer on board the satellite "Solar Mesospheric

E x p l o r e r " , and to the rocket and the balloon observations performed

d u r i n g the same period by various laboratories.

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Figure

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