IV ~iii~/

1 5 0 - 2 1 0 n m

DENYS SAMAIN

Laboratoire de Physique Stellaire et Plan~taire, B.P. 10, F-91370 Verri~res le Buisson, France and

PAUL C. SIMON

Institut d'A~ronomie Spatiale de Belgique B-1180, Brussels, Belgium

(Received 7 April, 1976)

Abstract. Solar irradiation fluxes are determined between 150 and 210 nm from stigmatic spectra of the Sun obtained by means of a rocket-borne spectrograph. Absolute intensities at the disk center with a spectral resolution of 0.04 nm and a spatial resolution of 7 arc sec are presented. From center-to- limb intensity variations determined from the same spectra, mean full disk intensities of the quiet Sun can be deduced. In order to compare them with other measurements, the new solar fluxes have been averaged over a bandpass of 1 nm.

1. Introduction

The photodissociation of molecular oxygen by the ultraviolet solar irradiation flux ranging from 175 to 242 nm is the initial source of odd oxygen in the mesosphere and the stratosphere. This wavelength interval corresponds to the S c h u m a n n - R u n g e band system (204-175 nm) and to the H e r z b e r g continuum ( 2 4 2 - 2 0 4 nm) of molecular oxygen. In this spectral region, solar fluxes are not sufficiently well known, especially between 180 and 200 nm. The first complete measurements r e p o r t e d by Detwiler et al. (1961) are generally considered as being too high by an important factor. The other flux values covering also this whole spectral region which were published by Widing et al. (1970) correspond to values measured at the center of the solar disk and cannot be used in a e r o n o m y which requires total disk solar flux values. The most recent measurements have been obtained by means of rocket-borne spectrometers by R o t t m a n (1974) between 116 and 1 8 5 n m and by H e r o u x and Swirbalus (1976) between 125 and 1 9 4 n m . In addition, Brueckner et al. (1976) have deduced average disk intensities from a quiet Sun spectrum between 175 and 210 nm. O t h e r measurements have also been obtained by Simon (1974) by means of a balloon-borne spectrometer between 196 and 230 nm. R o c k e t measurements agree very well together below 180 nm leading to an equivalent blackbody solar temperature of the order of 4550 K in this spectral region. Simon's data lead to a brightness temperature of the Sun of the order of 4700 K at 196 nm. There is therefore an important increase in the solar flux between 180 and 196 nm which must be determined to calculate accurate photodissociation rate coefficients of minor stratospheric con- Solar Physics 49 (1976) 33-41. All Rights Reserved

Copyright 9 1976 by D. Reidel Publishing Company, Dordrecht-Holland

34 DENYS SAMAIN AND PAUL C. SIMON

stituents (Kockarts, 1976). Data published by Heroux and Swirbalus (1976) and by Brueckner et al. (1976) do not solve completely this problem, since systematic differences of roughly 40% can be seen between these observations. Furthermore, high spectral resolution fluxes are needed, to calculate, for example, the photo- dissociation rate coefficient of nitric oxide which has absorption bands with rotational structure in the Schuman-Runge region (Cieslik and Nicolet, 1973).

The purpose of this work is to determine the solar irradiation fluxes between 150 and 210 nm from stigmatic spectra of the Sun obtained by Samain et al.

(1975), by computing the mean flux over the whole disk from the center-to-limb variations determined from the same spectra.

2. Observational Data and Calibration

The stigmatic spectra of the Sun between 120 and 210 nm were obtained during a rocket flight, April 17, 1973, by means of a double Wadsworth mounting spectrograph. This experiment leads to the determination of solar disk intensities with a spectral resolution of 0.04 nm and to center- to-limb distributions with spatial resolution of 7 arc sec. Absolute intensities at the center of the disk have

1 0 t G

I I I I I I

'E

i

10 t5

~ >

,....

t.U U 2 "

Q

SOLAR D I S K CENTER INTENSITY

C

IV ~iii~/

Si II ~ Si I

, , Fe I I , k I~ ~ .,,~,

13 -

5x10 150 155 ^I 160 ^I 165 ^I 170 ^t 175 ^I 180 ^I

WAVELENGTH Into)

Fig. 1. Absolute intensities for the solar disk center between 150 and 185 n m with a spectral resolution of 0.04 n m and a spatial resolution of 7 arc sec.

! 85

E

" 3 "

E u

u,J Z

<~

I Z

10 17

1016

I I I I I

SOLAR D I S K CENTER INTENSITY

AI I

Sil

Mg i I

I i

i

1015 _--

5 xt0 175 180 185 190 195 200 205

WAVELENGTH (nm)

Fig. 2. A b s o l u t e intensities for t h e solar disk center b e t w e e n 175 and 2 1 0 n m with a spectral resolution of 0.04 n m a n d a spatial resolution of 7 arc sec.

210

been published by Samain et al. (1975). For wavelengths below 168 nm, values reported in their Figure 9 correspond to minima of the spectrum while b~yond this wavelength they correspond only to continuum peak intensities, excluding emission lines. Comparison with other measurements cannot be made directly since the spectral resolution of the instrument plays an important role in the measured intensities. In the spectral range 150-210nm, differences of +20%

from the highly resolved intensities can occur when the measurements are smeared by a triangular function of I nm half width. This fact could explain the equivalent blackbody temperature of the Sun of nearly 4900 K at 200 nm.

The absolute calibration of the instrument which is described by Samain et al.

(1975) was carried out at the Culham Laboratory (Abingdon, England) using standard detectors calibrated against ionization chambers as absolute standards (Burton et al., 1973). The film absolute sensitivity was measured at 120, 130, 149, 164, 174, 202 and 206 nm. The instrumental transmission was determined at the same wavelength, except at 164 and 202 nm. This determination is based on measurements made at the instrument aperture of a calibrated light which is related to film densities by means of the film characteristic curve. Moreover, by another experimental method, the instrument sensitivity was determined every

36 DENYS SAMAIN AND PAUL C. SIMON

5 nm between 120 and 190 nm. T h e instrumental response was also measured as a function of wavelength along the spectrograph slit. The average instrumental sensitivity curve is shown in Figure 3 by Samain et al. (1975). Intensity calcula- tions from the film optical densities are based o n t h e relative characteristic curves of the film obtained from inflight calibration spectra and on absolute sensitivity curves obtained by interpolating between values measured at the aforementioned wavelengths. Except below 155 nm, only densities between 0.3 and 1.3 have been considered in our work. Figures 1 and 2 show the solar intensities measured at the disk center.

3. Center-to-Limb Variations

T h e center-to-limb intensity variations which were initially determined in a bandpass of 0.05 nm of the solar continuum from the stigmatic spectra at 25 wavelengths (Samain et al., 1975) have been extended between 146 and 210 nm in o r d e r to determine accurately mean full disk intensities of the quiet Sun versus wavelength each 1 or 2 nm w h e n e v e r possible. T h e m e a n intensity f(~t) inte- grated over the whole disk can be deduced from the variation of the solar intensity 1(/~, )t) from the center (/a, = 1) to the limb (/~ = 0) through the relation:

1

=

2J

0

1.&

1.2

1.0

0.8

0.6

0.~

I~0

I I I I I I I

N INTE'NSITY-CENTER INTENSITY RATIO

Fig. 3.

t 1 ~ 1 I l I

150 160 1"/0 180 190 200 210

WAVELENGTH (nm)

Ratios b e t w e e n m e a n full disk intensities a n d specific intensities at the center of t h e Sun versus wavelength. O n l y d a t a points corresponding to the c o n t i n u u m have b e e n plotted.

Ratios b e t w e e n m e a n solar intensities I(3,) and specific intensities at the center of the disk I(0, ),) have been calculated for many wavelengths and are shown in Figure 3, where each point corresponding to the continuum is an average over 6 o b s e r v e d values in m o s t cases. T h e error on f(A)/I(O,)t) is deduced from the errors on the c e n t e r - t o - l i m b intensity variations and has b e e n estimated to be of the order of 6 to 8% b e t w e e n 150 and 210 nm and larger than 10% below 150 nm.

A n analysis has also b e e n u n d e r t a k e n for several absorption and emission lines of the solar s p e c t r u m in order to d e t e r m i n e their influence on the m e a n intensity integrated over 1 nm. Since w e a k lines are generally narrow in comparison with the spectrograph resolution, the c e n t e r - t o - l i m b m e a s u r e m e n t s lead to the same values as those deduced for t h e n e a r b y continuum. For b r o a d e r or stronger lines, the c e n t e r - t o - l i m b variations are different f r o m continuum 'values but their differences are m u c h less i m p o r t a n t than the possible error in the absolute intensity m e a s u r e m e n t s . L i m b darkening is w e a k e r in absorption lines than in the continuum, while l i m b brightening is m o r e intense in emission lines. F o r b o t h cases, the effect of smearing is to increase the average level of the m e a n intensity and t h e r e b y the level of the solar flux. If the detailed structure of the spectral intensity m a y be slightly modified at a few wavelengths, this effect is generally less than 3% and neglfgeable w h e n irradiation flux values are averaged over a spectral range of 1 rim. H o w e v e r , special care has b e e n t a k e n for the very b r o a d autoionization lines of aluminum at 193.2 and 193.6 nm, and for lines with intense emission or very large limb-brightening such as the lines of C iv at 154.82 and 155.08 nm, SiI/Fe n at 157.49 nm, F e n / H e ii at 164.02 nm, C I at 165.7 nm and Si Ii at 181.69 n m for which differences vary b e t w e e n 4 and 12%. T h e solar fluxes for intervals of 1 n m which include these lines have b e e n corrected consequently by introducing the correct c e n t e r - t o - l i m b variation in the calculations.

4. Results and Discussion

Full disk solar irradiation fluxes deduced b e t w e e n 150 and 210 nm f r o m the intensities at the center of the disk and f r o m the c e n t e r - t o - l i m b intensity varia- tions h a v e b e e n integrated over a spectral range of i n m (Figures 4 and 5). T a b l e I which gives also the flux values for 1 nm intervals in p h o t o n s s -1 cm -2 nm -1 and in m W m -2 nm -1, allows a direct comparison with other published values. A good a g r e e m e n t with data r e p o r t e d by R o t t m a n (1974) and H e r o u x and Swirbalus (1976) is obtained around 160 n m and at 175 nm, and with values deduced by B r u e c k n e r et al. (1976) over their whole wavelength range ( 1 7 5 - 2 1 0 nm). F o r wavelengths greater than 180 n m our m e a n values are roughly 5 0 % higher than those published by H e r o u x and Swirbalus (1976). Nevertheless, they are not different f r o m the recent m e a s u r e m e n t s of Simon (1974) a r o u n d 200 n m which were o b t a i n e d in the stratosphere n e a r 40 k m of altitude by m e a n s of a balloon- borne s p e c t r o m e t e r and a photoelectric detector with a spectral bandpass of 0.6 nm.

38 D E N Y S S A M A I N A N D P A U L C. S I M O N

1011

E c

m

z '~ 101(

<

7,

i1:

Fig. 4.

9 10140

SOLAR FLUX ~ -'

0 0 _

~800 K ~

/ / 1 4 6 ~ I DETWILER et o.L11961) -

L_.E'. ~ ,,r ---j HEROUX and SWlRBALUS(1976)-

~ 1 I I I I I

THIS WORK

145 150 155 160 ~ 165 170 175

WAVELENGTH (rim)

Comparison of the present solar irradiation flux determination integrated over i n m with various flux measurements from 140 to 175 nm and with different curves of

the blackbody temperatures.

-T E c ,-,,.

E

-'i

8 z

<

i012

,o"

2xlO 171

i i I I I I I

"-~--~ I

~ KERMAN Qnd SIMONl1973T

~ ~ " ~ . . . SIMON 1197z1

~ " ~ b~r--~O- | ~ r ~ ROTTMAN (197&]

O ~ ~ O NISHI {1975)

O / ~ . THIS WORK r----" HEROUX and SWIRBALUS{1975)

~ :. 9 = 9 BRUECKNER et o.l (1978)

1~5 1~3 , 851 1/0 1 1 5 2 0 0 1 2;5 210

WAVELENG?H Into)

Fig. 5. Comparison of the present solar irradiation flux determination integrated over 1 nm with various flux measurements from 170 to 210 nm and with different curves of

the blackbody temperatures.

D i s c r e p a n c i e s o f t h e o r d e r of 5 0 % in t h e s p e c t r a l r e g i o n b e t w e e n 180 a n d 194 n m a r e difficult to e x p l a i n . T h e r e s u l t s of H e r o u x a n d S w i r b a l u s ( 1 9 7 6 ) w e r e o b t a i n e d b y m e a n s 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 a n d a p h o t o n c o u n t e r a n d h a v e a n a c c u r a c y of + 2 0 % . T h e Cs I p h o t o c a t h o d e w h i c h w a s u s e d in t h e i r i n s t r u m e n t s h o w s a r a p i d d e c a y in efficiency f o r w a v e l e n g t h s g r e a t e r t h a n 180 n m (see F i g u r e 1 in H e r o u x a n d S w i r b a l u s , i 9 7 6 ) w h i c h sets a n u p p e r l i m i t at 194 n m t o t h e i r s p e c t r a . O n t h e o t h e r h a n d , R o t t m a n (1974), w h o also u s e d a p h o t o m u l - t i p l i e r w i t h t h e s a m e p h o t o c a t h o d e gives r e s u l t s o n l y b e l o w 185 n m w i t h a n a c c u r a c y o f + 1 5 % . T h e r e f o r e , it s e e m s t h a t s u c h m e a s u r e m e n t s l e a d t o less p r e c i s e v a l u e s o f t h e s o l a r flux in t h e n e i g h b o u r h o o d o f 190 n m b e c a u s e of t h e l o w e r d e t e c t o r efficiencies.

O u r r e s u l t s h a v e a n a c c u r a c y of + 3 0 % , t a k i n g i n t o a c c o u n t all p o s s i b l e s o u r c e s o f e r r o r o n t h e p h o t o g r a p h i c i n t e n s i t y c a l i b r a t i o n , i n s t r u m e n t a l s e n s i t i v i t y a n d c e n t e r - t o - l i m b v a r i a t i o n . C o n s e q u e n t l y , w e c o n s i d e r t h a t t h e r e is a s t r o n g n e e d , f o r a e r o n o m i c p u r p o s e s , f o r f u r t h e r a n d m o r e p r e c i s e m e a s u r e m e n t s , e s p e c i a l l y b e t w e e n 180 a n d 2 0 0 n m , since o u r r e s u l t s s h o u l d give an u p p e r l i m i t w h i l e t h o s e of H e r o u x a n d S w i r b a l u s (1976), s h o u l d give a l o w e r l i m i t f o r t h e s o l a r i r r a d i a t i o n flux b e t w e e n 180 a n d 194 nm.

TABLE I

Solar irradiation fluxes integrated over 1 nm between 151 and 209 nm at one astronomical unit.

Wavelength interval Irradiance

(nm) hv s -1 cm ~-2 nm -a mW m -z nm -1

151-152 5.29x 109 6.94x 10 -z

152-153 6.04 7.86

153-154 6.28 8.13

154-155 9.62 1.24x 10 -1

155-156 8.44 1.08

156-157 8.42 1.07

157-158 8.44 1.06

158-159 8.47 1.06

159-160 8.69 1.08

160-161 9.62 1.19

161-162 1.11 x 10 l~ 1.37

162-163 1.23 1.50

163-164 1.38 1.67

164-165 1.59 1.92

165-166 2.32 2.78

166-167 1.69 2.01

167-168 1.91 2.26

168-169 2.21 2.60

169-170 2.98 3.49

170-171 3.53 4.11

171-172 3.70 4.28

172-173 4.30 4.95

173-174 4.39 5.03

174-175 5.49 6.25

40 D E N Y S S A M A I N A N D P A U L C. S I M O N

Table I (Continued)

Wavelength interval Irradiance

(rim) hv s -~ cm -~ nm -1 mW m -2 nm -1

175-176 7.37 8.53

176-177 8.51 9.58

177-178 1.15 • 1011 1.29x 10 ~

178-179 1.32 1.47

179-180 1.29 1.43

180-181 1.54 1.69 .

181-182 2.00 2.19

182-183 1.86 2.02

183-184 1.97 2.13

184-185 1.66 1.79

185-186 1.91 2.04

186-187 2.37 2.53

187-188 2.66 2.81

188-189 2.80 2.95

189-190 2.93 3.08

190-191 2.94 3.06

191-192 3.33 3.45

192-193 - 3.48 3.59

193-194 2.54 2.61

194-195 4.46 4.56

195-196 4~27 4.34

196-197 4.86 4.91

197-198 4.87 4.90

198-199 4.92 4.93

199-200 5.53 5.50

200-201 6.25 6 . 1 9

201-202 6.30 6.21

202-203 6.42 6.30

203-204 7.63 7.45

204-205 8.96 .... 8.70

205-206 9.20 8.89

206-207 9.65 9.28

207-208 1.13• 1012 1.08• 101

208-209 1.27 1.21

References

Ackerman, M. and Simon, P. C.: 1973, Solar Phys. 30, 345.

Ackerman, M., Frimout, D., and Pastiels, R.: 1971,~ in Labuhn and Liist (eds.), New Techniques in Space Astronomy, D. Reidel Publishing Company, Dordrecht-Holland, p. 251.

Brueckner, G. E., Bartoe, J.-D. F., Moe, O. K., and Van Hosier, M. E.: 1976, Astrophys. J., to be published.

Burton, W. M., Hatter, A. T., and Ridgeley, A.: 1973, J. Phys. E. Sci. Instrum. 6, 302.

Carver, J. H., Horton, B. H., Lockey, G. W. A., and Role, B.: 1972, Solar Phys. 27, 347.

Cieslik, S. and Nicolet, M.: 1973, Planet. Space ScL 21, 925.

Detwiler, C. R., Garrett, D. L., Purcell, J. D., and Tousey, R.: 1961, Ann. Geophys. 17, 9.

Heroux, L. and Swirbalus, R. A.: 1976, J. Geophys. Res., to be published.

Kockarts, G.: 1976, Planet. Space Sci., to be published.

Nishi, K.: 1975, Solar Phys. 42, 37.

Rottman, G. L.: 1974, Trans. Am. Geophys. Union 56, 1157.

Samain, D., Bonnet, R. M., Gayet, R., and Lizambert, C.: 1975, Astron. Astrophys. 33, 71.

Simon, P. C.: 1974, in Broderick and Hard (eds.), Proceedings of the Third Conference on the Climatic Impact Assessment Program, DOT-TSC-OST-74-15, p. 137.

Widing, K. G., Purcell, J. D., and Sandlin, G. D.: 1970, Solar Phys. 12, 52.