Comparison of the WRC85 Solar
Spectral Irradiance with RSSV1 and the
SPM of VIRGO/SOHO
Claus Fröhlich and Christoph Wehrli
Physikalisch-Meteorlogisches Observatorium Davos, World Radiation
Center, CH 7260 Davos Dorf, Switzerland
eMail: cfrohlich@pmodwrc.ch; http://www.pmodwrc.ch
Outline:
ƒ Description of the WRC85 Spectrum
ƒ Comparison with the new Reference Spectra (RSSV1)
ƒ SPM/VIRGO data
ƒ Absolute values determined with SPM/VIRGO and during
SIMBA-98
ƒ Conclusions
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SORCE Science Meeting, September 19-22, 2006, Orcas Island
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The WRC85 Spectrum
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The WRC85 was established to provide a reliable spectrum for
use in atmospheric applications
It 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 Brasseur
and 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 used
for the reference spectra
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•
•
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1 nm from 199 to 630nm
2 nm from 630 to 1000nm
5 nm from 1000 to 2500nm
SORCE Science Meeting, September 19-22, 2006, Orcas Island
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The Reference Spectra
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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 rebinned 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.
SORCE Science Meeting, September 19-22, 2006, Orcas Island
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Comparison of WRC85 with the Reference
Spectra
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Comparison of WRC85 with the Reference
Spectra
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SORCE Science Meeting, September 19-22, 2006, Orcas Island
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Comparison of WRC85 with the Reference
Spectra
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Comparison of WRC85 with the Reference
Spectra
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VIRGO/SOHO SPM Results
ƒ 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.
ƒ 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 leastsquare regression.
ƒ 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.
ƒ The corrected SPM-B is shown in the
lower panel.
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Comparison with VIRGO/SOHO
ƒ With the normalized filter
transmissions for each SPM channel
one can calculate the response of
each channel with the ATLAS-3
spectrum (Table 1)
ƒ These values can now be compared to
the first light measurements of SPM-B
(Table 2)
ƒ 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).
ƒ We can further compare the measurements 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.
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SORCE Science Meeting, September 19-22, 2006, Orcas Island
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Final Time series of SPM-A of VIRGO/SOHO
ƒ With the corrected SPM-B we
can now also correct the
SPM-A.
ƒ 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.
ƒ Comparison with
SORCE/SIM will be important
to determine whether the
corrections are
reasonable…….
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Conclusions
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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 the
time 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.
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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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