relation variability.

Texte intégral

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OBSERVATION OF THE ULTRAVIOLET SOLAR IRRADIANCE IN THE STRATOSPHERE

by

P.C. SIMON

Abstract

Ultraviolet solar irradiance measurements for wavelengths greater than 175 urn are reviewed. Discrepancies between recent observations are pointed out and discussed in relation to the solar flux variability. Future needs

related to sbatospheric chemistry are emphasized.

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I. INTRODUCTION

Ultraviolet solar irradiation in wavelengths ranging from 175 to

400 nm initiates the photochemistry of the neutral constituants of stratosphere.

For instance, photodissociation of molecular oxygen by radiation of wavelengths shorter than 242 nm is the initial source of odd oxygen in the upper strato- sphere, leading in particular to the formation of ozone by reaction with the molecular oxygen in the presence of a third body (N

2, O

2). On the other hand ozone is destroyed by photodissociation in the Hartley continuum and the Huggins bands by radiation at wavelengths shorter than 360 nm, producing the

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electronically excited atomic oxygen O( D). These atoms are in fact the most important oxidizing agent in the stratosphere (Nicolet, 1975). An accurate knowledge of the solar irradiation flux at ultraviolet wavelengths and of its variations with the solar rotational period and the ll~ar cycle is thereby fundamental in stratospheric aeronomy.

The purpose of this work is to review the recent solar irradiance measurements between 175 and 400 nm. The discrepancies between the different

observations will be pointed out and discussed in relation to the solar irradiation flux variability.

II. OBSERVATIONAL DATA

Solar irradiation flux.es in the wavelength interval 175-242 nm are very important for upper stratospheric aeronomy. They are mainly responsible for the photodissociation of molecular oxygen which absorbs radiation of wavelength shorter than 242 nm. Some minor constituents such as nitrogen

oxides, water vapor and halocarbons also undergo photodissociation in this wavelength range. Unfortunatt1y, the solar spectrum is not sufficiently well known especially between 175 and 204 nm corresponding to the Schumann-Runge band system. With the exception of the data of Detwiler =E_~!. (1961) which are definitely to high, the only measurements covering this wavelength range

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165 -

are those deduced very recently by Samain and Simon

(1976)

and those of Brueckner et al.

(1976).

Both determined the solar irradiance fluxes from photographic radiance measurements using the limb-darkening values measured by Samain and Simon

(1976).

Solar spectra were obtained with resolutions of

0.04

and

0.007

nm respectively. The data, extending up to

210

nm were integrated over 1 nm interval for comparison and are reported on fig. 1 with the other measurements covering partially the same wavelength range. Fig. 2 illustrates the ratio between all the recent observations taking as reference the values of Samain and Simon

(1976).

The first conclusion to be drawn is that the data of Brueckner et al

(1976)

are quasi systematically

25%

lower than those of Samain and Simon

(1976).

A good agreement occurs at

175

nm with the results of Rottman

(1964)

and of Heroux and Swirbalus

(1975),

but discrepancies of the order of

50%

appears around

190

nm. Such a difference has been discussed by Samain and Simon

(1976)

who come to the conclusion that, from

180

to

194

nm, their own data should be considered as an upper limit for the solar irradiance while those of Heroux and Swirbalus should give a lower limit. Beyond

200

nm,

the most recent irradiance measurements are those obtained by Simon

(1974)

with a balloon borne spectrometer with a spectral bandpass of

0.6

nm. They are

4.0%

lower than 'the previous balloon measurements reported by Ackerman et al.

(1971)

and are also lower than Broadfoot's results

(1972),

except between

226

and

230

nm where the agreement is within

15%.

The accuracies quoted by these experimenters fall in the range

± 10

to

+ 30%

for all the measurements mentioned above.

The observations of Broadfoot

(1972)

obtained by means of a rocket borne spectrometer with a spectral resolution of

0.3

nm extend up to

320

nm.

They are the most reliable contribution covering continuously the most important wavelength range for the photodissociation of ozone (fig. 3). On the other hand, Arvesen et a1

(1969)

carried out, from an aircraft platform, measurements

between

300

and

2500

nrn by means of a double spectroradiometer having a resolution of

0.1

nrn in the ultraviolet. Unfortunately, they have some discrepancies with Broadfoot's data between

300

and

320

nm in which occurs the limit of photo- dissociation of 0

3 producing the excited atomic oxygen O(lD) which is particularly important for the stratospheric aeronomy (Nicolet;

1975).

The values of Simon

(1975)

exte.nding from

284

to

354

nm obtained by balloon observations are in

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- 166 -

very good agreement with those of Broadfoot (1972) up to 300 nm and with those of Arvesen ~~_~! (1969) beyond 330 nm (fig. 4). In fact, Broadfoot (1972) claimed an accuracy of

±

10% for his measurements except beyond 300 nm where the error is larger. On the other hand Arvesen ~~_~! (19~9) estimate their error to be 25% at 300 nm, 6% at 320 nm and 3.2% at 400 nm. The data of Simon (1975) could therefore provide useful irradiance values between 300 and 330 nm.

Other measurements obtained from aircraft are those of Thekaekara et al (1969) but the tables of irradiances integrated over 5 nrn are based on an average of data coming from four different instruments. More recently, irradiances at 0.1 nm intervals were pUblished by Thekaekara (1974) in the wavelength range 300-610 nm. Unfortunately, his result$ show many errors in the wavelength scale

(Simon, 1975) making difficult a correct comparison with the other measurements. Observations of De Luisi (1975) from 298.1 to 400 nm were obtained from ground- based measurement but only relative calibration of the spectrometer was performed.

Absolute irradiation fluxes were obtained adjusting the total energy in the wavelength range 300-400 tun on the standard solar spectral irradiance curve published by Thekaekara (1970). On the other hand, Labs and Neckel (1970) measured, from a high mountain observatory, the solar radiance near the disk

centre and computed very reliable irradiation fluxes from 330to 1250 nm integrated over 10 am, using the limb-darkening data of David and Elste (962). They stated a possible error of 2% below 400 nm. The accuracy of all these observ.ations allows an estimation of the solar irradiance flux within 10% around 300 nm and within 5% around 400 nm.

A compilation of the relevant irradiation flux values discussed in this work have been recently published by Delaboudiniere et al (1978).

III. DISCUSSION

A correct interpretation of the available data must take into account the flux variabilities arrising from changes in solar activity. These belong to three categories determined by thl;~ir time scale : firstly,· variations related to flares having a very short life time; secondly, those related to the solar rotation period (27- days) and finally, those related to the ll-year

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- 167-

cycle. The solar irradiation flux enhancement due to the flares could be considered as negligeable for wavelengths greater than 140 nm, according to Heath (1969).

Observations of the 27- day variability have only been reported by Heath (1973) and by Hinteregger ~~_~~ (1977) 0 Between 175 and 200 nm this variability appears to be less than 10% and decreases exponentially with decreasing wavenumbers, being of the order of 1% around 300 nm. It should be pointed out that a solar flux variability of 10% is le-ssli:!r than the quoted measurement accuracies and is comparable to that due to the sun-earth annual distance change involving periodic solar variation of 6.6% from Jal,1uary (max.) to July (min.).

The solar irradiation flux variability due to the 11-' year cycle is very poorly known. The inadequate time coverage of reliable data during

the solar cycle 20 and the errors associated with each measurement is illustrated in fig. 5 for 200 nm. Nevertheless, ,Heath and Thekaekara (1977) claimed a

11- year 'variability of a factor of 2 at the same wavelength on the basis of their 0\1711 measurements performed by satellites and by rockets si.nce 1966. This is illustrated in fig. 6 taken from Hudson (1977) and on which the full squares and triangles solar flux ratio were obtained from broadband photometric

observations and the full circles by means of double mon,oc-hromator experiment.

The solid curve represents the solar variability deduced by the author. It should be pointed out that there are only two points around 180 nm and that the solar flux ratio obtained around 290 nm with the double monochromators are higher by 15 - 2.0% than those obtai.ned from the hoadba.nd detectors at

the same, wavelength. In additi.on, the accuracy of such ra.tios is not indicated.

A linear regression calculation is shown by the dashed lines for each set of solar flux ra.tio. Extrapolation of solar flux ratio at 340 from the broadband detector measurements gives a variability of 25% between solar-cycle maximum and solar-cycle minimum irradiances. Such value(seems

ve~y

high

an~in

contradiction with the variability bc.3ed on the double monochromator data.

On the other hand, extrapolation of t.le latter at short,er wavelength gives a

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ratio of 0.68 at 200 nm.

In conclusion, taking into account all the irradiance measurements with their associated accuracies, there is no conclusive evidence for a

variability of a factor of 2 around 200 nm ,\lith the 11- year cycle. Lower value of the order of 30% seems more r:easonable. but should be experimentally p.roved.

New spectral irradiance measurements with ,an adequae time coverage during the solar cycle 21 are urgently needed i.n this ,.]avelength range 'with an improved accuracy in order to answer the question of the solar flux variability related with the stratospheric aeronomy. The required accuracy and resolution are summarized in table 1. In addition, 1.n situ me.asurements of solar flux at specific stratospheric altitudes would be very useful, especially in the wavelength range corresponding to the Schumann-Runge absorption bands of molecular oxygen,with a resolution of 0.003 rum in order to compare the experimental data with the calculated solar irrad:lance at the corresponding altitude.

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TABLE 1

Wavelength Interval (run) Quoted accuracy

Discrepancy

Required accuracy

Required spectral resolution

Variability (27-day) Variability (II-year)

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Mesosphere Higher stratosphere

175--240

+

10 to

+

30%

40 to 25%

5%

0.1 to 1 run (0.003 run from 175 to 204 nm) 10 to 1%

100% ?

Stratosphere Troposphere

240-400

±

5 to

±

10%

15 to 9% (1 measurement ' from 230 to 285 nm)

5% absolute

1 to 0.1% relative 0.1 to 1 run

(0.01 run for

occultation measurement)

<

1%

50

a

20% ?

.

.

i

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REFERENCES

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52, 1505.

ACKERMAN, M. and SIMON, P.C. (19?3). Rocket measurements of solar fluxes at 1216

A,

1450

A

and 1710

A.

Solar Phys. 30, 345.

--- --

ACKERMAN, M., FRIMOUT, D. and PASTIELS, R.: (1971). New ultraviolet solar flux measurements at 2000

A

using a balloon borne instrument, in New

!=~~~~g~=~_~~_~£~~=_~~~:~~~l' (Eds. Labuhn and LUst), p. 251, D. Reidel, Dordrecht, Netherlands.

ARVESEN, J.C., GRIFFIN Jr., R.N. and PEARSON Jr., B.D. (1969). Determination of extraterrestrial solar spectral irradiance from a research aircraft.

~£E~~_2£~~~~. ~, 2215.

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BROADFOOT, A.L. (1972). The solar spectrum 2100-3200

A.

Astrophys. J. 173, 681.

.

--- ---

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Absolute solar ultraviolet intensities and their variation with solar activity part I : th'e wavelength region 1750

A -

2100

A.

Astrophys. J. 209, 935.

--- ---

/

DE LUISI, J.J. (1975). Measurements of the extraterrestrial solar radiand flux from '298l to 4000

A

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intensity distribution in the ultraviolet solar spectrum. ~~~_2=~E~l~'

.!I,

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HEATH, D.F. (1969). Observatiof,s of the intensity and variability of the near ultraviolet solar flux from the Nimbus I I I satellite, J. Atm. Sc i.

l.£,

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HEATH, D.F. and THEKAEKAJ~A, M.P. (1977). The solar spectrum between 1200 and 3000

A,

in !~~_~~!~~_~~~~~E_!~~_!~~_Y~E!~!!~~, (ed, White), P. 193, Colorado Univ. Ass, Press, USA.

HINTEREGGER, H.E., BEDO, D.E, and MANSON, J.E. (1977). EUV flux variations 'vHh solar rotation observed during 1971j.-1976 from AE-C satellite.

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LABS, D. and NECKEr., H. (1970). Transformation of the absolute solar radiation data into the I!Internati.onal Practical Temperature Scale of 196811,

~~~~:_~~y~.

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M.

(1975). Stratospheric ozone: an introduction to its study.

~=Y:._9:.~£~Y~!._.~r:::::~_~~Z~:..

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ROTTMAl'i, G.L. (l97lt). Disc: values of the solar ultraviolet flux, 1150

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A. !:::~~::._~~~::._2=~E~I~:._~~~~~.

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A

and

o /

2300 A, in ~::~::~~~~~~_~~_~~:_~~.~::~_::?~~=:::~:=~_9~_~~=_:~~~::E~:

impact assessment program (Eds. Broderick and Hard), P. 137, DOT -TSC -OST -7 4-15.

SIMON, P.C. (1975). Nouvelles' mesures de l'ultravi.olet solai.re dans la s trat osphere, ~~!~;,._:::~::~:.._~:?r::,_~~~2~g~:1._~~!._~:~:' §l) 399.

THEKAEKARA, M.P. (1974), Extraterrestrial solar spectrum 3000-6100

A

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~pr~!._~E~~:~'

1J.>

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THEKAEKARA, M.P. , KRUGER, p. and D1J~iCAN, C.H. (1969). Solar irradiance measurements from a rese8:'::"ch. aircraft. Appl. Optics, 8, 1713.

--- -

THEKAEKARA, M.P, (1970). The solar constant and the solar spectrum measured from a research aircraft. ~ASA-TR-R-351.

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.

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.,

WI

->

.c. w 0 z « 0 « 0: 0:

SOLAR FLUX 1012 III II .. - DETWILER .1 al 119611 _._. __ ACKERMAN el al 119711 fIT" rrrr rr"!l I-++++i ,-J o r---j

...

ACKERMAN & SIMON (1973) SIMON (1974) ACKER~AN (1974) ° ROTTMAN (1974) NISHI (1975) HEROUX & SWIRBALUS (1976) BRUECKNER ., 01 (1976) SAMAIN & SIMON (1976) 10 t::"" I I I I I I 2)(10170 175 180 185 190 195 200 205 WAVElENGTH (nm)

210 Fig. 1.-Comparison of ultraviolet solar flux reported by various experimenters from 170 to 210 nm. Fluxes for different blackbody temperatures are also shown.

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<

c

o

f. If} "0 C o t::

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-1 l

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Aug30, 73

Brueckner et al

(1976) I ...

Heroux & Swirba!us(19?6)

0 .2 I !

j 180 190 200 210 WAVELENGT H (nm) Fig. 2.-Ratio of solar irradiation flux measurements from 175 to 210 nrn in comparison with the data of Samain and Simon (1976).

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10.4 1= Ii SOLAR FLUX ~ 1E 10

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UJ U z <t i5 <t ~ ~ 10'2 210 220 230 240 250 WAVELENGTH (nm)

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..

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-; l' E 10 c "t. E u \" ~ :lJ C,.) z <:

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~ 1013 0:::

SOLAR FLUX

---~-__1l-:-~iOFYrsz{~~ -:~ - - ·~- . ~. ~ __ ._~y~_~~~~~~l~d!IMf~ · ~, ;

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Fig. 4.-Comparison of u~travio1et solar flux reported by various experimenters from 280 to 350 nm. Fluxes for different blackbody temperatures are also shown.

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E

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I OCTOBER OCTOBER OCTOBER OCTOBER OCTOBER 1966 1968 1970 1972 1974 SMOOTHED SUNSPOT NUMBER, CYCLE 20 ~ ACKERMAN et al 1971

m

SIMON 1974 ® SAMAIN and SIMON 1976 0 BRUECKNER et al 1976 Fig. 5.-Comparison of solar irradiation flux measurements at 200 nm during the solar cycle 20. The smoothed sunspot numbers are also shown (after De1aboudiniere ~~_~~, 1978).

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(nm) Fig.

13.-

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

--,J --..J I

Figure

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