Comparison of the WRC85 Solar Spectral Irradiance with...

03.10.2006 14:41:56 SORCE Science Meeting, September 19-22, 2006, Orcas Island 1

Comparison of the WRC85 Solar Spectral Irradiance with RSSV1 and the SPM of VIRGO/SOHOClaus Fröhlich and Christoph WehrliPhysikalisch-Meteorlogisches Observatorium Davos, World Radiation Center, CH 7260 Davos Dorf, SwitzerlandeMail: [email protected]; http://www.pmodwrc.ch

Outline:Description of the WRC85 SpectrumComparison with the new Reference Spectra (RSSV1)SPM/VIRGO dataAbsolute values determined with SPM/VIRGO and during SIMBA-98Conclusions


The WRC85 Spectrum

The WRC85 was established to provide a reliable spectrum for use in atmospheric applicationsIt is composed of four parts: the ranges 199 to 310 nm, 310 to 330 nm, 330 to 869 nm, and 869 to 20000 nm are form Brasseurand Simon (1981), Arvesen et al. (1969), Neckel and Labs (1984), Smith and Gottlieb (1974), respectively. The composite is then scaled in order to yield a total solar irradiance of 1367.0 Wm-2.The values from 310 to 330 nm are re-binned from 0.4nm to 1nm intervals. The WRC85 spectrum used in the present comparison has the following resolution, which is then also usedfor the reference spectra• 1 nm from 199 to 630nm• 2 nm from 630 to 1000nm• 5 nm from 1000 to 2500nm


The Reference Spectra

The reference spectra are derived from measurements during the ATLAS 1 (March 1992) and 3 (November 1994) shuttle missions and are described in detail in Thuillier et al. (2004). We use here Version 1 values (RSSV1) which integrate to a total solar irradiance of 1367.7 and 1366.7 for ATLAS 1 and 3 respectively. For the comparison these higher resolution spectra are re-binned in the range from 199 to 2500nm to the resolution of the WRC85 spectrum.The reference spectra are normalized to the TSI values from the composite (41 61 0608) of 1366.65 and 1365.77 Wm-2, which correspond to the average TSI for March 1992 and November 1994, respectively. The WRC85 is scaled to the average of both, namely 1366.21 Wm-2.


Comparison of WRC85 with the Reference Spectra


VIRGO/SOHO SPM Results

The top panel shows the original data of the backup SPM-B. It is quite clear that these values do not reflect the solar irradiance and need some corrections.There are several effects: an increase at the beginning and the degradation. Moreover there are important temperature effects, and there should be solar cycle effect, which may be represented by a low-pass filtered TSI:

• Increase: a•(1−exp(−bt2))• Decrease: c•exp(−dt)• Temperature: e•(1+f•temp(t))• TSI: g•tsi(t),

with t = exposure time. The six coefficients are determined by least-square regression.The green channel looks different: the coefficient for TSI is with 1.179 much smaller than expected; it should be between the red and blue with 1.750 and 3.617. Also the temperature coefficient is with −8.55•10−5 different from the red and blue ones with +6.91•10−6 and +4.41•10−5, respectively.The corrected SPM-B is shown in the lower panel.


Comparison with VIRGO/SOHO

With the normalized filter transmissions for each SPM channel one can calculate the response of each channel with the ATLAS-3 spectrum (Table 1)These values can now be compared to the first light measurements of SPM-B (Table 2)The final comparison yields values of -0.25 to -3.6%, which are within the uncertainties of the lamp calibrations and their transfer to space (Table 3).We can further compare the measure-ments during the SIMBA-98 balloon flight with the results of VIRGO. The ratios SOHO to SIMBA amount 1.052, 0.988 and 1.013 for the red, green and blue channels. As all differences are positive they are most probably due to some under-estimation of the transmission corrections for the atmosphere above 40km.


Final Time series of SPM-A of VIRGO/SOHO

With the corrected SPM-B we can now also correct the SPM-A.It does not look perfect, but it is a first attempt. Moreover, it is no too bad for the period after the start of the SORCE mission.Comparison with SORCE/SIM will be important to determine whether the corrections are reasonable…….


Conclusions

The WRC85 spectrum is in the visible close to the reference spectra of Thuillier et al. (2004) which is due to the fact, that the Neckel and Labs (1984) spectrum is quite accurate.The differences in UV and IR regions are larger than what at thetime of the publication stated uncertainties would have suggested. The comparison with the SPM-B values on the other hand are indicating that these filterradiometers are capable to provide quite accurate data.As these results are still only a confirmation of the capability of the transfer of the calibration to space, comparison with SORCE will show whether the corrections to the SPM-B readings are correct.


That’s it, thanks.

You can get the poster – which is very similar to this presentation –from ftp://ftp.pmodwrc.ch/pub/Claus/SORCE Sep2006/SSI Poster.pdf.

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