improved data of solar spectral irradiance from 0.33 to 1.25μ

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  • IMPROVED DATA OF SOLAR SPECTRAL IRRADIANCE

    FROM 0.33 TO 1.25tt*

    HEINZ NECKEL

    Hamburger Sternwarte, Hamburg-Bergedorf, F,R.G.

    and

    DIETRICH LABS

    Landessternwarte K6nigstuhl, Heidelberg, F.R.G.

    Abstract. The conversion of our centre of disk intensities published in 1968/70 into mean disk intensities has been repeated, using more accurate data for the centre-to-limb variation of both continuous radiation and strong absorption lines.

    The random observational mean error of the new irradiance data very likely is not larger than 1.5% in the UV and not larger than 1% in the visible and infrared. Comparison with the fluxes of Sun-like stars observed by Hardorp (1980) confirms these errors and seems to exclude the possibility of a systematic, wavelength-dependent scale error which would correspond to a temperature difference larger than 50 K.

    The resulting integral value of the irradiance between 0.33 and 1.25 Ix is 1.060, the corresponding value of the solar constant lies between 1.368 and 1.377 kW m -2.

    1. Introduction

    With respect to the divers plans to search for variations in the solar irradiance, it may be profitable to have a base from which to start and which is as solid as

    possible. Therefore we do not hesitate to represent here improved data of the

    irradiance, even if hereby observations are involved, which were made nearly 20 years ago.

    In the early 60's we measured absolute intensities at the centre of the solar disk

    in spectral bands 20 .0 /20 .5 /~ wide, using an almost rectangular apparatus profile.

    The corresponding intensity integrals we called X (Labs and Neckel, 1962, 1963, 1967):

    Ai+AA/2

    f AA=20.5 /~ for A i - -4001.05 /~, Zi J I , dA ' AA=20.0 /~ for A i - ->4020.0~. (1) Ai-AA/2

    Later on (Labs and Neckel, 1968, 1970) we used these X-integrals to derive the

    corresponding integrals of the mean intensity (4,) and of the irradiance (0), using the relations

    . . . . . . )

    ffOi = 2'i~i i = ~i \ I (0)/co,t [ 1 -- T'lcentreJ I (2)

    Oi = 6.800 X 10 -s &i, (3)

    (F/I(O))cont is the ratio of mean to central intensity as derived from centre-to- l imb

    * Proceedings of the 14th ESLAB Symposium on Physics of Solar Variations, 16-19 September 1980, Scheveningen, The Netherlands.

    Solar Physics "/4 (1981) 231-249. 0038-0938/81/0741-0231 $02.85. Copyright (~ 1981 by D. ReideI Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

  • 232 HEINZ NECKEL AND DIETR ICH LABS

    observations of the continuous radiation (in UV of 'window'-intensities!), and the r/'s are the line blocking coefficients for the 20 ~k bands for disk-averaged radiation (irradiance) and central intensity, respectively.

    Since Equation (2) has often been incorrectly interpreted, it should be empha- sized, that it does not mean a simple derivation of the irradiance from a continuum- curve and line blocking data. The basic observed quantity ~ includes a priori all lines, and the third factor takes into account just the centre to limb variation of their strengths.

    Most recently our irradiance data were used by Hardorp (1980) to compare them with the flux distribution of Sun-like stars. Thereby Hardorp found some differences between stellar and solar radiation, which demanded a clarification whether being real or not.

    Therefore we checked the observed central intensities ~ as well as the data used to derive the corresponding irradiance values. With respect to the three factors forming the right-hand side of Equation (2), the result may be summarized as follows:

    (1) Our observed ~ 's are even more accurate than we have quoted so far. (2) For the ratio F/I(O) of mean to central intensity more reliable values are now

    derivable from the recent centre-to-l imb observations of Pierce and Slaughter (1977a, b).

    (3) The former correcting factors taking into account the centre-to-l imb variation of the r/'s were erroneous for spectral bands with strong lines.

    2. Random Errors of Central Intensity Integrals

    While tests for possible systematic errors of our observations will be given in Sections 6 and 7, here we deal only with the random observational error, which can be infered from the agreement of repeated observations, but also - and more conclusively - from the scatter around an appropriate smoothing curve.

    Since the Z 's themselves don't follow any smoothed curve if plotted against wavelength, the most objective way to exhibit their accuracy is to plot instead the resulting continuum intensities or the related radiation temperatures. This was done in Figure 1.

    For A < 0.66tx, T was derived from the window-intensities in Z-cal ibrated atlases (Labs and Neckel, 1968, 1970, Table 5A), for A >0.66p~ from the quotients .~ / (1 - - r lcentre), where "]']centre was either obtained from summation of the equivalent widths given by Moore et al. (1966) or - for A > 0.877jx - deduced from the atlas of Delbouille and Roland (1963). The occasional inclusion of terrestrial lines, which so far had been neglected, has now been taken into account. The resulting tem- peratures can be smoothed rather precisely by two r.m.s, parabolas:

    A--3999.9A: T=6046+ 2021(A-0 .8699) 2 . (5)

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