AERONOMICA ACTA

• 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° 244 - 1982

Current solar-spectral irradiance observations : a review

by

Paul 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 N O M I E 3 • Ringlaan

B - 1180 BRÜSSEL

FOREWORD

This paper has been presented at the Symposium on the Solar Constant and the Spectral Distribution of Solar Irradiance which was held in Hamburg (FRG) on August 17-18, 1981. It will be published in the proceedings of the conference.

AVANT-PROPOS

Cet article résume une communication présentée au Symposium sur "The Solar Constant and the Spectral Distribution of Solar

Irradiance" qui s'est tenu à Hambourg ( R F A ) le 17 et 18 août 1981. il sera publié dans les compte-rendus de la conférence.

VOORWOORD

Dit artikel werd voorgedragen op het "Symposium on The Solar Constant and the Spectral Distribution of Solar Irradiance" dat werd gehouden te Hamburg (BRD) van 17 tot 18 agustus 1981. Het zal gepubliceerd worden in de verslagen van deze conferentie.

VORWORT

Diese Arbeit wurde in Hamburg (DBR) zum Symposium "The Solar Constant and the Spectral Distribution of Solar Irradiance" (17 zum 18. August 1981) aufgetragen. Sie wird in den Vorträgen der Konferenz herausgegeben werden.

C U R R E N T S O L A R S P E C T R A L I R R A D I A N C E O B S E R V A T I O N S : A REVIEW

by

Paul C . SIMON

Abstract

Solar spectral irradiance measurements in the ultraviolet part of the solar spectrum will be reviewed and their divergences will be discussed. Deduced variations over the rising phase of solar cycie 2'i will be presented, showing that they are still masked by the un- certainties in available observations beyond 170 nm. Future observa- tional needs for climatology and aeronomy will be outlined.

Résumé

Les mesures dans l'ultraviolet de l'éclairement énergétique du soleil sont revues et leurs divergences discutées. Les variations déduites pendant la première phase du cycle solaire n°21 sont présentées. Elles montrent qu'elles sont encore masquées par les incertitudes sur les observations au-delà de 170 nm. Les observations futures nécessaires en aéronomie et en climatalogie sont soulignées.

Samenvatting

Er wordt een overzicht gemaakt van de zonneflux metingen in het ultraviolet gedeelte van het zonnespectrum. De afwijkingen tussen de verschillende metingen worden besproken. De variaties afgeleid voor de stijgende fase van de zonnecyclus n r . 21 worden voorgesteld. De bepaling van deze veranderingen zijn beperkt door onzekerheden in de voorhanden zijnde waarnemingen in het gebied beneden 170 nm. Waar- nemingen noodzakelijk in de toekomst voor klimatologie en aeronomie worden uiteengezet.

Zusammenfassung

Messugen der ultravioletten Sonnenausstrahlung und ihre Divergenzen sind diskutiert. Die Variationen während der steigenden Phase der 21. Sonnenzyklus zeigen dass sie noch durch die Beobachtungsfehler oberhalb 170 nm verdeckt sind. Zukünftige Beobachtungsbedürfnisse f ü r Aeronomie und Klimatologie sind vorgestellt.

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

An a c c u r a t e knowledge of u l t r a v i o l e t solar i r r a d i a n c e and its variations is fundamental in aeronomy and in climatology of t h e u p p e r atmosphere. I n d e e d , u l t r a v i o l e t solar i r r a d i a t i o n in wavelengths r a n g i n g from 120 to t h e visible p a r t of t h e spectrum initiates the photochemical processes of t h e n e u t r a l constituants in t h e lower thermosphere and in t h e middle atmosphere.

T h e purpose of this w o r k is to p r e s e n t the recent u l t r a v i o l e t solar i r r a d i a n c e measurements obtained d u r i n g the r i s i n g phase of solar cycle 21 and to discuss t h e i r d i v e r g e n c e s in terms of possible solar v a r i a b i l i t y .

T H E H I LYMAN a EMISSION LINE

I r r a d i a n c e measurements of H I Lyman a line d u r i n g the r i s i n g phase of solar cycle 21 have been performed by satellite and by r o c k e t s . T h e EUVS experiment on board the A E - E satellite p r o v i d e d observations from June 1977 leading to an increase of a factor of 3 from minimum to maximum conditions of solar a c t i v i t y with an estimated

11

minimum value of 3 x 10 h v/ s . c m ( H i n t e r e g g e r et a l . , 1981). T h e s e observations a r e not in agreement with those obtained by rocket by Mount et al. ( 1 9 8 0 ) and Mount and Rottman ( 1 9 8 1 ) . Measurements made since 1971 are r e p o r t e d on f i g u r e 1 for comparison purposes showing important discrepancies mainly in 1979 and in 1980. T h e dispersion in t h e rocket data d u r i n g the previous cycle is also well i l l u s t r a t e d . A c o r r e c t i n t e r p r e t a t i o n of these observations should obviously t a k e into account t h e i r r a d i a n c e v a r i a b i l i t y with t h e rotational period of the sun because rocket measurements correspond to snapshots. On the o t h e r h a n d , it clearly appears t h a t the l o n g - t e r m v a r i a b i l i t y deduced from the OSO 5 satellite d u r i n g solar cycle 20 implies smaller changes with solar a c t i v i t y t h a n the one deduced from t h e A E - E satellite d u r i n g solar cycle 21.

- 3 -

10

o

X 8

CM I 0

E

r— 1

u a> .

> w 6

LU U

z

< 4 o <

tr ac

t — i — i — r

Ly a

OBSERVATIONS

Cycle 20 —

• V

• A F G L Rockets + L A S P Rockets A J" A E - E Satellite 0 I OSO-5 Satellite

J

L

EUV m i n i m u m

I 1 1 1 1

71 72 73 74 75 76 DATE

77 78 79 80

Fig- 1- Comparison of HI Lyman u irradiance measured since 1971. References : IASB : Ackerman and Simon (1973); NRL : Prinz (1974); AFGL : Heroux and Higgins (1977); LASP : Mount et al. (1980), Mount and Rottman (1981); AE-E : Hinteregger et al.

(1981); 0S0-5 : Vidal-Madjar (1975), Vidal-Madjar and Phissamay (1980).

B o s s y and Nicolet (1981) have analysed all the available satellite data and have adjusted the measurements of Hinteregger et al.

(1981) to fit a linear relationship between H I Lyman a irradiance and the 10.7 cm flux. According to their formula, the variability over one solar cycle is only a factor of 2 with a minimum value of 2.5 x 10 p 11 hv/s.cm .

Consequently, the H I Lyman a irradiance value is still contro- versial as well as its long-term variability.

T H E W A V E L E N G T H I N T E R V A L 135-175 NM

D u r i n g solar cycle 20, several rocket measurements have been made in this wavelength interval. Heroux and Swirbalus (1976) and Samain and Simon (1976) have published the lowest full disk values which refer to a quiet solar d i s k . Rottman (1981) has recently proposed

revised values for its observations performed in 1972 and 1973. With 3 other flights made in 1975, 1976 and 1977, he has defined a so-called reference spectrum corresponding to minimum condition between solar cycle 20 and 21, by taking an average value of the 5 flights.

The ratios of irradiance values integrated over 5 nm from 135 to 175 nm taking as a reference the mean spectrum of Rottman (1981) are given on figure 2 which clearly shows that the previous values of Heroux and Swirbalus (1976) and Samain and Simon (1976) are 40 percent lower than the reference spectrum. On the other hand, the divergences between the 5 observations of Rottman (1981) give a good idea of the reproductibility reached in this wavelength range by means of snapshot measurements which include the variability related to the 27-day solar rotational period.

T h e data obtained for high solar activity level by Mount et al. (1980) and Mount and Rottman (1981) are systematically higher

2.4

2.2

2.0 ao

£ 1.8

0

1 1.6

(X o O 1.4

•O <b t 1.2 a>

a) 5 1 0 1 0.8

0.6 0.4

1 1 " 1 — 1 "1 r

-

-

MRT 79

- ~|_MR80 J

R5

-

... . r\M

R1 R3

_TmT.._TTTTT

R2

1 1

H78 -

1 1 HS73

1 1 1 I 1

—i 1

SS73 "

1 1 1 I 1

—i

1 1

135 140 145 150 155 160 WAVELENGTH (nm)

165 170 175

Fig. 2.- Observed solar irradiance in the spectral range 135-175 nm expressed as ratios relative to the solar reference spectrum of Rottman (1981). Values are integrated over 5 nm intervals. The labeling is as follows : HS73 : Heroux and Swirbalus (1976); SS73 : Samain and Simon (1976); Rl, R2, R3, R4, R5 .- Rottman (1981); H78 : Heath (1980);

MRT79 : Mount et al. (1980); MR80 : Mount and Rottman (1981).

than the reference spectrum. The differences represent true long-term variability which is confirmed by the A E - E satellite observations reported by Hinteregger et al. (1981). Nevertheless, data of Heath (1980) obtained in 1978 for intermediate level of solar activity are in better agreement with the minimum values proposed by Rottman (1981 i than with the values of Mount et al. (1980). They become even lower beyond 170 nm. Consequently, these differences cannot be interpretated in term of solar variability. Figure 3 gives the ratio of irradiances for maximum to minimum activity obtained from the L A S P rockets and by the A E - E satellite up to 190 nm.

It is clear from the available data that solar irradiance values refering to a quiet sun exhibit very large discrepancies, reaching a factor of 2 in the wavelength interval 135-175 nm. In addition, variability over the solar cycle is qualitatively evident but further observations are still required in order to give accurate value of variability especially around 170 nm.

T H E WAVELENGTH I N T E R V A L 175-210 NM

Solar irradiance and its variability are very poorly known in this wavelength range. Figure 4 gives the ratios of several observed irradiance values integrated over 5 nm to those of Samain and Simon (1976) corresponding to a quiet sun. Differences at shorter wavelengths could probably be due to long-term solar variability. There is no systematic divergences with wavelength and in addition measurements corresponding to similar level of solar activity are not consistent, it should be mentioned that according to figure 3 and to the plage model of Cook et al. (1980) the variability during the rising phase of solar cycle 21 beyond 180 nm does not exceed 30 percent if ratios from 180 to 190 nm obtained on the basis of Rottman's measurement are excluded because of larger errors. In addition the data of Mount et al. (1980) are not reliable beyond 190 nm because of experimental problems (Mount and Rottman, 1981). Consequently, the quantitative variation over the

- 7 -

WAVELENGTH ( n m )

Fig. 3.- Solar irradiance variability related to the rising phase of solar cycle 21 from 120 to 190 nm.

C D

CT>

%

o

J) E

T>

C

o o E

t/)

a

TJ <u t_ a>

<D a:

<

cr 185 190 195

WAVELENGTH (nm)

205

Fig. 4.- Observed solar irradiance in the spectral range 175-205 nm expressed as ratios relative to Samain and Simon (1976). Values are integrated over 5 nm intervals. The labeling is as follows : HS73 : Heroux and Swirbalus (1976); B73 : Brueckner et al. (1976); Rmin : Rottman (1981); H78 : Heath

(1980); MRT79 : Mount et al.. (1980); MR80 : Mount and Rottman (1981).

- 9 -

solar cycle is still masked by the uncertainties of all measurements in this wavelength range.

T H E S P E C T R A L R E G I O N B E Y O N S 210 NM

The wavelength interval 210-330 nm is extensively discussed by Simon et al. (1981). Beyond 330 nm, new irradiance values were recently published by Neckel and Labs (1981). These latter values which supercede those previously published by Labs and Neckel (1970) are probably the most reliable in the near ultraviolet and in the visible.

According to Table 3 in Simon (1981), they are in good agreement with the observation reported by Heath (1980). The measurements of A r v e s e n et al. have to be carefully adjusted because of e r r o r s in their wavelength scale below 400 nm and of uncertainties due to changes in the spectral irradiance scale of the National Bureau of S t a n d a r d s in 1973.

Long-term variability in this spectral range is generally considered as being negligible from the aeronomic point of view and is also masked by the uncertainties in available measurements. Variations related with the 27-day rotational period of the sun have been recently reported by Heath (1980) from Nimbus 7 observations.

C O N C L U S I O N S

The current position of irradiance measurement accuracies and of discrepancies between relevant observations is summarized in Table 1. Possible long-term variations are given on the basis of recent measurements performed d u r i n g solar cycle 21. The requirement related to aeronomy and climatology are specified for future observations for which important improvements are badly needed in order to reduce the uncertainties (see for example Simon, 1981 and N O A A , 1980).

TABLE 1 : Uncertainties on solar ultraviolet irradiance measurements and future needs.

WAVELENGTH INTERVALS (NM)

Ly a 135-175 175-210 210-240 240-330 330-400

QUOTED ACCURACY 30% 30-20% 30-20% 20-10% 10-4% 4-2%

DISCREPANCIES 200% 200% 50% 20% 20% 8%

ACHIEVED ACCURACY ^; + 30% i + 30% + 20% + 15% + 10% + 5%

UNCERTAINTIES ON LAMPS : 10% 6% 5% 3% 2%

IRRADIANCE STDS SYNCHROTRON : 2-5% ^ 27-D VARIABILITY 30% 30-4% 4%' 4-2% 2-1% < 1%

11-Y VARIABILITY 200% ? 100-20% 20-0% NEGLIGIBLE ^ ACCURACY 10% 5% 5% 5% 5-2% 2%

GOALS

PRECISION 5% 5% 1% 1% < 1 % < 0 . 1 %

R E F E R E N C E S

A C K E R M A N , M. and S I M O N , P . C . : 1973, S o l a r P h y s . 30, 345.

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 . 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.

B R U E C K N E R , G . E . , B A R T O E , J . D . F . , K J E D S E T H M O E , O. a n d V A N H O O S I E R , M . E . : 1976, A s t r o p h y s . J. 209, 935.

B O S S Y , L. a n d N I C O L E T , M. : 1981, Planet. S p a c e S e i . , 29, 1981.

C O O K , J . W . , B R U E C K N E R , G . E . and V A N H O O S I E R , M . E . : 1980, J.

G e o p h y s . R e s . 85, 2257.

H E A T H , D . F . : 1980, in P r o c e e d i n g s of the International C o n f e r e n c e on S u n and Climate, C e n t r e National d ' E t u d e s Spatiales, p. 445.

H E R O U X , L. and H I G G I N S , J . E . : 1977, J. G e o p h y s . R e s . 82, 3307.

H E R O U X , L. and S W I R B A L U S , R . A . : 1976, J. G e o p h y s . R e s . 81, 436.

H I N T E R E G G E R , H . E . , F U K U I , K . and G I L S O N , B . R . : 1981, G e o p h y s . R e s . Lett. 8, 1147.

L A B S , D . a n d N E C K E L , H. : 1970, Solar P h y s . , 15, 79.

M O U N T , G . N . a n d 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 . N . , R O T T M A N , G . J . and T I M O T H Y , J . G . : 1980, J.

G e o p h y s . R e s . 85, 4271.

N E C K E L , H. and L A B S , D . : 1981, Solar P h y s . 74, 231.

N O A A : 1980, P r o c e e d i n g s of the W o r k s h o p on S o l a r U V I r r a d i a n c e M o n i t o r s , Space E n v i r o n m e n t L a b o r a t o r y , National Oceanic and Atmosphieric A d m i n i s t r a t i o n , B o u l d e r , Colorado.

P R I N Z , D . K . : 1974, A s t r o p h y s . J. 187, 369.

R O T T M A N , G . L . : 1981, J. G e o p h y s . R e s . 86, 6697.

S A M A I N , D . a n d S I M O N , P . C . : 1976, Solar P h y s . 49, 33.

S I M O N , P . C . : 1981, Solar P h y s . , 74, 273.

S I M O N , P . C . , P A S T I E L S , R . , N E V E J A N S , D . and G I L L O T A Y , D. : 1981, P r o c e e d i n g s of the S y m p o s i u m on the Solar C o n s t a n t and the Spectral D i s t r i b u t i o n of Solar I r r a d i a n c e , I A M A P , H a m b u r g ( F R G ) .

V I D A L - M A D J A R , A . : 1975, Solar P h y s . 40/ 69.

V I D A L - M A D J A R , A . and PHISSAMAY, B . : 1980, Solar P h y s . 66, 259.

-13-