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Values of the solar UV flux at 1216A, 1450A and 1710A obtained by means of rocket borne spectrometers are presented. They are compared with other results.

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1 2 1 6 ~ , 1 4 5 0 ~ A N D 1 7 1 0 ~

M. A C K E R M A N and P. S I M O N

Institut d'Adronomie Spatiale de Belgique, B-1180 Brussels, Belgium

(Received~22 January, 1973)

Abstract.

Values of the solar UV flux at 1216A, 1450A and 1710A obtained by means of rocket borne spectrometers are presented. They are compared with other results.

1. Introduction

The knowledge of the solar ultraviolet flux reaching the Earth is of fundamental importance in aeronomy.

In

the wavelength range between 1950 A_ and 1400 A, this radiation is mostly absorbed in the mesosphere and in the thermosphere. It initiates the photochemistry of neutral molecules up to the highest levels where physical processes such as diffusion take over to control the vertical distribution o f atmospheric consti- tuents. The photodissociation rate has to be determined as accurately as possible to define the lower boundary o f the heterosphere. Several determinations have been per- formed that belong to two categories. Those fully pertinent to aeronomy (Detwiler

et aL,

1961) and dealing with the global radiation of the Sun and those mostly interest- ing the solar physicists where the main objective is to characterize the solar atmosphere (Parkinson and Reeves, 1969; Widing

et al.,

1970; Brueckner and Kjeldseth Moe, 1972).

In this latter case fluxes of photons issued from relatively small portions of the Sun at suitable wavelength are mostly important. The total intensities is then obtained by spatial and spectral integration and the comparison with the global measurements is not straightforward. These determinations have however the advantage of giving the relative spectral distribution of the intensity with some uncertainty on the absolute value of the total UV flux.

This explains the need for accurate determinations of this absolute value at medium or low resolution and at some wavelengths to adjust the absolute scale of calibration for the high resolution data extending over the whole spectrum.

This is a report Of an absolute rocket measurement of the total solar UV flux at one astronomical unit, at 1450 A~ and 1710 A_ with respectively band passes of the order of 30 A~ and 10 A, This measurement has been undertaken with a double purpose. To help making a choise between the available values and to determine the vertical distri- bution of molecular oxygen in the thermosphere by absorptiometry of the solar UV.

These latter results will be reported elsewhere. To extend the range o f the oxygen measurement the Lc~ radiation was also determined.

Solar Physics

30 (1973) 345-350.

All Rights Reserved

Copyright 9

1973

by D. Reidel Publishing Company, Dordrecht-Holland

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346 M. ACKERMAN AND P. SIMON

2. Instrumentation

A 25 cm focal length Ebert-Fastie spectrometer shown in Figure 1 was used to select in sequence the three wavelength intervals shown in Figure 2. A diffusion lithium fluoride window was place din front o f the entrance slit to integrate the radiation over the whole solar disk. After diffraction by a fixed Lausch and L o m b 2.6 x 2.6 cm grating ruled at 1200 grooves per m m , the light was falling on an E M R 542 G-08-18 CsI

s R s

CONNECTOR

M,~

SPHERICAL MIRROR

Fig. 1. View of the spectrometer.

photomultiplier after passage through one o f the three exit slits. The slits were un- covered in sequence by means of a sliding shutter. The three wavelengths scan was performed every six seconds. The reflecting surfaces were coated with aluminium protected by a magnesium fluoride coating. The photomultiplier output was measured by means of a solid state electrometer followed by a three channel amplifier yielding a dynamic range of 104. Analog signals were transmitted to the ground. Two instruments were integrated by E S R O as part o f Skylark payloads S 84 and S 90 on Sun pointers.

During flight an aperture opened at the rear of the instruments allowed their outgassing.

3. The Photometric Calibration

The absolute calibration of the spectrometers was based on various standards. The

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transfer between these was made by means of a sodium salicylate coated photomulti- plier. This secondary standard was calibrated on the International Pyrheliometric Scale 1956 as for previous measurements (Ackerman et al., 1971) by means of a mercury lamp at 2537 ~ . At 1216 &, the instrumental sensitivity was measured by comparison with a CS2 ionization chamber. In addition, the sensitivity of the sodium

Fig. 2.

I i I

10 -18

~ d

% o 10 -19

i 9

E

10-17

>

p,-

E

Z

10.11~

lO-I~

I 0 - 2 c

5 90

^ %

X =1/.50A 10 -16

10-17 1

~k = 1707A

S 8l,

180 1200 1220 12/.0

~k = 1710A

I , I I , I I I ~ I/,20 IM, O I/,60 II,80

WAVELENGTH (,~)

I

1680 1700 1720 17~0

Instrumental sensitivity vs wavelength for the three channels of both experiments:

S 84 and S 90.

salicylate coated photomultiplier was measured by comparison at the Laboratory for Atmospheric and Space Physics, Boulder, Colorado, with a photodiode calibrated at the National Bureau o f Standards. All calibrations agreed within 25%. At Lc~ a decrease of the sodium salicylate efficiency was found in agreement with the results o f K n a p p and Smith (1964).

T h e instrumental functions are shown in absolute value in Figure 2.

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348 M. ACKERMAN AND P. SIMON

4. Observation

The t w o E S R O Skylark rockets were fired on February 28, 1972, at the Salto di Quirra launching range in Sardinia. The G M T launching t i m e s were 5h 5 4 m for S 84 and 9h 4 7 m 3s for S 90 and apogees o f 242 k m and 251 k m were respectively reached.

The Elliot Sun pointing altitude control unit performed very well in b o t h cases. The constant p h o t o m u l t i p l i e r outputs observed at altitudes where no a t m o s p h e r i c absorp- t i o n takes place have been used for the present determinations.

5. Results and Discussion

The results are s u m m a r i z e d in Table I and represented in Figure 3 where other obser- TABLE I

Measured solar fluxes

Let 1450 A 1710

photons cm -2 s -1 photons cm -~ s -1 (50/~)-1 photons cm -2 s -1 (50 A) -1

S 84 (2.1 • • 1011 (2.68 4-0.4) x 101~ (2.29 4-0.4) • 1011 S 90 (2.1 4-0.3) • 1011 (2.01 4-0.3) • 101~ (2.244-0.4) • 1011

Fig. 3.

11 10

"T

* ~ 10 I ~ to L

E o

10 9

I o Ly X et

;O0*K

I _ _ g

co o

o o o

8

I I

V

S O L A R F L U X ~ v ,,

10 14

DETWILER, GARRETT,

~ . - PURCELL ~, TOUSEY (1961) V BONNET (1968)

o PARKINSON ~, REEVES(1969)"

9 W l D I N G , PURCELL, 9

~ SANDLIN (1970) ACKERMAN, FRIMOUT

' o ~, PASTIELS (1971)

~ X ACKERMAN & SIMON(1973) 9 ARVESEN & et[. (1969)

I

2000 WAVELENGTH (/~)

13 10

12 t 0

108

I I I ~ I I I

01~

1000 1500 2500 3000 1

Comparison of the results obtaned by means of S 84 and S 90 with other observations.

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vations are also shown. The value obtained at L~ is lower than the usually accepted average value of 3 x 10 ~1 photons x cm -2 s -~. As shown in Figure 4, the S 84 and S 90 flights took place when the Ztirich sunspot number as well as the 10.7 cm solar flux were presenting low values. This can be correlated with low La fluxes also observ- ed from the OSO 5 satellite (Blamont and Vidal-Madjar, 1971, 1972). At 1450 ~ and

X ~ "

m b

_J

t) Z ">

3

c -

>''~ 2

._1

20C

100

I n D::

i i i

m 150

~E Z

5 ~00-

Q.

Z

5 0 - -I- 0

N 0

I ' I ' I ' I ' I ' I ' I

x $84 - $90

BLAMONT AND VIDAL-MAD3AR {1972}

I I I I I i I I I I I 1

1 10 20 I 10 20 30 DAY

Febr. March

Fig. 4. Daily variation of Ziirich sunspots numbers, radioelectric solar flux at 10.7 cm and Lc~ solar flux observed from the OSO-5 satellite. S 84 and S 90 experiments appear to have been performed

at low solar activity.

1700 A the flux values presented here are lower than the values published by Detwiler

et al.

(1961). They lead to solar blackbody temperatures intermediate between those

obtained by Widing

et al.

(1970) and by Parkinson and Reeves (1969). In these latter

cases the comparison is however not straightforward. The values presented here are in

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350

M. ACKERMAN AND P. SIMON

agreement with those obtained f r o m the satellite W R E S A T 1 (Carver et al., 1972).

It is n o w certain t h a t the a p p a r e n t b l a c k b o d y temperature o f the Sun can decrease by some 8 0 0 K f r o m 2100 to 1600 it. The steps are possibly as follows: 5 2 0 0 K at 2100 A, 4 8 0 0 K at 2000 A, 4 6 5 0 K at 1900 A, 4 5 5 0 K at 1700 A and 4 6 0 0 K at 1450 A with a m i n i m u m at 1600 A o f the order o f 4400K.

Acknowledgements

W e express o u r thanks to Professor M. Nicolet for his advice a n d f o r his interest in this work. D r F r i m o u t w h o p e r f o r m e d the calibration o f a transfer detector against the N a t i o n a l B u r e a u o f Standards p h o t o d i o d e at the L a b o r a t o r y f o r A t m o s p h e r i c and Space Physics Boulder, C o l o r a d o , is gratefully acknowledged. We t h a n k the Etudes Techniques et Constructions Aerospatiales ( E T C A ) in Charleroi for advices during the c o n s t r u c t i o n o f the p r o t o t y p e and flight instruments.

References

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

Aversen, J. C., Griffin, R. N., and Pearson, B. D.: 1969, Appl. Opt. 8, 2215.

Blamont, J. E. and Vidal-Madjar, A. : 1971, J. Geophys. Res. 76, 4311.

Blamont, J. E. and Vidal-Madjar, A. : 1972, Private communication.

Bonnet, R. M. : 1968, Space Res. 7, 458.

Brueckner, G. E. and Kjeldseth Moe, O. : 1972, Space Research 12, 1595.

Carver, J. H., Horton, B. H., Lockey, G. W. A., and Rofe, B.: 1972, Report ADP 111 of the Uni- versity of Adelaide, Department of Physics.

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

Knapp, R. A. and Smith, A. M.: 1964, Appl. Opt. 3, 637.

Parkinson, W. H. and Reeves, E. M. : 1969, Solar Phys. 10, 342.

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

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