Results of the Solar Disk-center
Spectral Intensity Measurements in the Range 310--1070 nm

K. A. Burlov-Vasiljev, Yu. B. Matvejev, & I. E. Vasiljeva

Main Astronomical Observatory of the National Academy of the Sciences
Goloseevo, 252650 Kiev-22, Ukraine

At the high-altitude station on the Peak Terskol (Central Caucasus, 3100 m a.l.s.) by the Main astronomical observatory of the National Academy of the Sciences of the Ukraine the spectral measurements of the solar disk-center intensity have been realized. Data published earlier (1995, Solar Phys. 157, 51) for near UV and visible range are now expanded on the longwave spectral region up to 1070 nm. The measurements are fulfilled with the specialized solar telescope. The method of of the solar disk-center brightness comparison with the brightness of the calibrated reference ribbon tungsten lamp was used. The atmosphere extinction was taken into account by Bouger's method with the parallel independent control of the atmosphere stability. The 1-nm integrals of the disk-center intensity in range 310---1070 nm now is available. The uncertainty of these values is estimated by 2%. In the regions with strong telluric absorption by oxygen and water vapour the supplementary reductions was interred, which was obtained from the synthetic atmosphere absorption spectra computed on the basis of molecular parameter atlas HITRAN and standard model of the atmosphere. With the help of solar disk darkening coefficients the values of the solar flux at 1 AU are derived. The comparison of the obtained data with the data by Neckel and Labs (1984, Solar Phys. 90, 205) is fulfilled as well as with the some other data. From the high-resolution FTS solar spectrum calibrated with our data we derived the solar continuum absolute location, which has proved more smooth than the continuum given by Neckel and Labs. Taking into account our previous measurements in the near UV and visible, we have compared the results of the solar and stellar spectro- and multicolor photometry in order to derive the most exactly adjusted data for solar spectral energy distribution, spectral energy distribution of Vega, and solar color-indices.

1. Objectives

2. Summary

FIGURE 1:Spectral intensity of the solar disk-center radiation in 1012 W m-3 ster-1 versus wavelength in nm. Thewavelength range 645---685 is the common for both series of the measurements (UV and visible, 1989: Burlov-Vasiljev et al., 1995; IR, 1992: Burlov-Vasiljev et al., 1997). The dotted curve corresponds to the data given by Neckel and Labs (1984).

FIGURE 2:Disk-center radiation temperatures for local maxima at 1-nm bands of the calibrated FTS Kitt Peak spectrum, continuum of Neckel and Labs (1984) (dashed line), our results (diamonds). See explanations in Summary section.

3. Apparatus

The instrumentation adapted for quasi-simultaneous registration of the solar and standard ribbon tungsten lamp spectra was described by Burlov-Vasiljev et al.} (1995). It has been installed at Peak Terscol (Central Caucasus, 3100 m a.l.s.) and consists of:

4. Observations

Atmospheric extinction was taken into account by a Bouger's method. Therefore the procedure of measurements represents the repeated scanning of an elected area of a spectrum during a day at various air mass in a direction on the Sun. Usually the air masses were limited by a range 1.2---5.0. The measurements of a solar spectrum were accompanied by parallel measurements of the solar aureole brightness for a control of optical stability of the atmosphere. Before fulfillment the day program of the solar radiation observations the spectrum of the standard lamp in the same area was measured. Thus the image of the ribbon of the standard lamp was formed on the entrance slit of the spectrometer with the help of the spherical collimator mirror.

The two main series of measurements were carried out: for the wavelength range 310---685 nm (1989) and for the wavelength range 645---1070 nm (1992). The result of 1989 was derived from measurements during 7 days in UV region (310---400 nm) and 9 days in visible (400---685 nm) during the period July---August. Totally during 1992 were conducted 3 series of observations (March 24---April 7, August 4---8, September 29---October 1). All together there are 19 days of observations. For final result in IR were used the observations on April 2, 3, 6, 7 and October 1. All the days in March, on April 1 and on September 29, 30 were extracted because of unsatisfactory stability of the atmospheric transparency.

The investigated spectral area was divided into few areas, each of which was measured at a certain position of the diffraction grating. The adjacent areas were overlapped among themselves on 1/2 length.

5. Calibration

The absolute scale was supported with the help of two tungsten lamps (type TRU 1100-2350 #359 and #92) with 2x1 mm calibrated area on the ribbon. They were calibrated as radiance standards at the State Institute of Optical and Physical Measurements (Moscow, Russia). The accuracy of calibration was stated as 2.5% at lambda = 300 nm and 1% at lambda = 1100 nm.

During 1989---1990 years we had conducted numerous comparisons of our lamps with the standards of some other laboratories of former Soviet Union and with the standard of the PMOD/WRC (Davos, Switzerland). These measurements showed that during this period of several years (namely from the moment of the first calibration in 1988) the lamps saved the brightness within the limits of a parts of a percent.

The lamps were calibrated in 1988, 1989 and 1990. The differences between calibrations were within the accuracy bar stated above. During measurements of a the solar spectrum the lamps were used in different modes. The lamp #359 was used for calibration daily. The lamp #92 was used not so often -- only for mutual comparisons with the lamp #359 in the very beginning and at the end of a series of observations. In result the time of burning of a lamp #92 is less in factor 10 than for a lamp #359. We believe that in this case the constancy of the ratio of two lamps brightness can be a criterion of preservation of an internal laboratory scale of measurements. It is very difficult to assume that both lamps will in coordination change the brightness at so different modes of use.

The measurements 1992 and result of calibration 1990 differ not more, than on 1%. We believe, that the scale of our measurements was constant within that limit.

6. Sources of Uncertainty

We have paid attention to the following possible sources of the measurements' uncertainty:

7. Extrapolation to Zero Air Mass

Reduction to zero air mass was fulfilled by the Bouger's method with the independent monitoring of the atmosphere optical stability (with the help of the halo photometer and pressure control). For the ``best'' day the formal uncertainty of the extrapolated to zero air mass solar disk-center intensity Ilambda,0 is about 0.3% (outside the molecular absorption bands). For the ``worst'' day adopted for the data evaluation this value is 1.5%. We have considered the following restrictions of the Bouger's method:

8. Results

FIGURE 3:Disk-center radiation temperatures for local maxima at 1-nm bands of the calibrated FTS Kitt Peak spectrum, continuum of Neckel and Labs (1984) (dashed line), our results (diamonds). See explanations in Summary section.

9. Synthetic UBV

As the solar spectral irradiance data are available for the near UV, visible and IR range, we can compute the solar color-indices. Here we presents the results for UBV system.

Different determinations give the value of 0.14---0.20 for (U - B)sun, and 0.63---0.69 for (B - V)sun (Makarova et al. (1991)). The values (U - B)sun = 0.18 and (B - V)sun = 0.67 are adopted. These values differ from the standard meanings for G2V star (0.10 and 0.62 respectively) and are more close G5V (0.20 and 0.68). The absolute magnitude of the Sun Vsun = -26.70 now is commonly used. To compute the solar color-indices and absolute magnitude we used the relative reaction curves for UBV system given by Buser (1978) calibrated us by spectral energy distribution for Vega. We consider here the following data set for Vega: by Hayes (1985), Archarov et al. (1989), Terez (1983), Knyazeva and Kharitonov (1988). In the case of necessary the absent data at 300 nm was added.

Vega calibration Sun (Neckel & Labs) Sun (Marakarova et al.) Sun (Present Work) Sun (Thekaekara)
Knyazeva & Kharitonov 0.1608 0.1770 0.1454 0.0944
Terez 0.0798 0.0961 0.0645 0.0134
Archarov et al. 0.1204 0.1367 0.1051 0.0540
Hayes 0.1024 0.1187 0.0871 0.0360
Vega calibration Sun (Neckel & Labs) Sun (Marakarova et al.) Sun (Present Work) Sun (Thekaekara)
Knyazeva & Kharitonov 0.6318 0.5893 0.6008 0.5645
Terez 0.6583 0.6158 0.6273 0.5911
Archarov et al. 0.6461 0.6036 0.6151 0.5789
Hayes 0.6767 0.6342 0.6457 0.6095
Vega calibration Sun (Neckel & Labs) Sun (Marakarova et al.) Sun (Present Work) Sun (Thekaekara)
Knyazeva & Kharitonov -26.7453 -26.7291 -26.7483 -26.6787
Terez -26.7469 -26.7307 -26.7499 -26.6803
Archarov et al. -26.6710 -26.6548 -26.6740 -26.6044
Hayes -26.7462 -26.7300 -26.7491 -26.6795

One can see that the values of the color-indices more close to G5V star are in the contradiction with the solar and star absolute spectrophotometry. Moreover, as the both of the color-indices are not independent, they must lie on the normal stars Main Sequence in the two colors UBV diagram We could find two close pairs of the spectrophotometer curves for the Sun and Vega, respectively (present work --- Archarov et al.), and (Makarova et al. --- Terez), which satisfy this requirement. In both cases the solar color-indices are typical for G2V star. The solar absolute magnitude Vsun obtained from spectrophotometry is close to the commonly-used value.


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