Sun Reference Spectra From Solar Cycle 22 Measurements
1
2
3
5
Gérard Thuillier , Thomas N. Woods , Linton Floyd , Ernest Hilsenrath4, Richard Cebula , Michel
1
6
Hersé , Dietrich Labs
1
Service d'Aéronomie du CNRS, Verrières-le-Buisson, France
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO
3 Interferometrics Inc., Chantilly, VA, 4 NASA Goddard Space Flight Center, Greenbelt, MD.
5 Science Systems and Applications, Lanham, MD, 6 Landessternwarte, Heidelberg, Germany
2
The solar spectrum is a key input for the study of the planetary atmospheres.
It allows the understanding through theoretical modeling of the atmospheric
properties (e.g., composition and variability). Furthermore, a reference model is
useful for the preparation of instruments and platforms to be operated in space.
We present a composite solar spectral irradiance reference model for
wavelengths from EUV to 2400 nm, based on available data and modeling at
two distinct levels of solar activity, which are moderately high and low levels
encountered during the ATLAS 1 (March 1992) and ATLAS 3 (November
1994) Space Shuttle missions. Spectral irradiance accuracy varies from 40 % in
the EUV to a mean 3% in the UV, visible and near IR ranges. After integration,
a comparison with the Total Solar Irradiance measured at the same time shows
an agreement of the order of 1%.
1. RATIONALE OF THE SOLAR REFERENCE
SPECTRUM CONSTRUCTION
i) calibration source accuracy and procedure of
calibration, and
ii) aging of the instruments exposed to Sun radiation.
1.1 Requirements
At the 2001 International Solar Cycle Study ISCS
meeting (Longmont, Colorado, June 2001), the need for a
solar reference model was identified having the following
characteristics:
- absolute spectral irradiance with the best achievable
accuracy,
- spectral range from EUV to IR,
- two distinct levels of solar activity (close to
minimum and maximum),
- spectral sampling / resolution of 1 nm or better, and
- data corresponding to 1 AU.
With such requirements, no single instrument is able to
cover such a wide spectral range for which, in addition, the
solar spectral irradiance varies by more than a factor 105.
Therefore, a composite spectrum needs to be considered.
Obtaining high accuracy of a few percents is difficult to
achieve. In the 1970s, UV observations were carried out
from balloons and rockets. Comparisons of the results
showed discrepancies up to 20% above 200 nm and greater
below. The analysis of the reported discrepancies led to
consider their potential sources:
1
Taking into account the above difficulties, the following
strategy was implemented:
i) use of short term missions with instrument
retrieval allowing to make a post-flight
calibration,
ii) instruments intercomparison before flight and
after if possible, and
iii) two or more instruments in orbit at the same
time.
For most of the published spectra, authors made
comparisons with other available spectra and discuss the
differences, with an emphasis on simultaneous
measurements. These comparisons revealed the conditions
where the best agreement was obtained. In this case, data
obtained from different instruments having their own
design, their own calibration, if in agreement, insure that
the measurements are made in the absolute scale within
their quoted accuracy. Given a positive result, composite
spectra can be made. Such spectra should improve overall
accuracy over that of any of its individual component
spectra. The idea is that errors, be they random or
systematic, are usually reduced when more data are
included.
The different types of errors affecting the irradiance
spectra have distinct characteristics. Random measurement
errors are present, but these are typically much smaller than
the systematic uncertainties affecting the instrument
responsivity and scattered light effects. This is why
combining spectra from different instruments should reduce
the systematic uncertainties. The composite spectrum to be
built here will use data from different instruments due to the
large wavelength range (EUV to IR) and in each range
several spectra will be used if possible.
irradiance
presents
significant
difficulties
and
uncertainties in terms of absolute photometric scale.
However, between 120 and 156 nm the comparison with
simultaneous observations by SOLSTICE on board UARS
reveals an agreement of 15% [Wilhelm et al., 1999].
Nevertheless, given the limited SUMER irradiance data
and the available data from UARS and ATLAS, the
SUMER irradiance results are not used here in the
reference spectra.
The Solar Backscatter Ultraviolet model 2 (SBUV/2)
instruments flying on NOAA spacecrafts monitors the
long term global ozone column distribution in the
stratosphere. On a daily basis, the solar spectral irradiance
from 160 to 405 nm, and the Mg II core-to-wing ratio are
measured. DeLand and Cebula [1998] have shown that the
NOAA-11 SBUV/2 data have a long-term precision of
0.9 to 2.3 % (2) over 5.5 years. However, a comparison
of the absolute irradiances from the NOAA-11 SBUV/2
instrument with SSBUV [Cebula et al., 1998a] indicates
that the SBUV/2 data do not have the accuracy of the data
sets used herein to construct the reference spectra for solar
cycle 22.
The Global Ozone Monitoring Experiment (GOME) is
dedicated to the Earth observations. The instrument has
the capability to perform also direct solar observations
from 240 to 790 nm. A comparison with SSBUV 8 data
[Weber et al., 1998] reveals some photometric difficulties
by the presence of fringes pattern which will be certainly
corrected in a next future. As a result, the GOME data
were not included in the present analysis.
1.2 Available Data
As the reference spectrum will be mostly used in the field
of atmospheric physics, full solar disk, i.e. irradiance data
are required. As a consequence of the relationship between
resolution and instrument field of view, the data are
recorded at moderate resolution, typically a fraction of a
nanometer.
Due to the great discrepancies existing in UV
observations, a strong effort has been made in the 1980's to
cover with accuracy this range. As this effort is still pursued,
many data are existing. Curiously, the visible and IR
domains received little attention and few measurements
exist in these regions.
As two distinct levels of solar activity were required, we
take advantage of the Upper Atmosphere Research Satellite
(UARS) and three ATmospheric Laboratory for
Applications and Science (ATLAS) missions, allowing to
gather measurements at moderately high and close to
minimum solar activity.
The most recent solar UV irradiance measurements
shortward of 400 nm were obtained from orbital platforms
as reviewed by Rottmann [2002]. UARS measurements
include those from the Solar Ultraviolet Spectral Irradiance
Monitor (SUSIM) [Brueckner et al., 1993] and the SOLar
STellar Irradiance Comparison Experiment (SOLSTICE)
[Rottman et al., 1993]. Both of these UARS instruments
measure the solar UV irradiance from about 120 to 410 nm.
On board the three ATLAS missions were placed three
grating spectrometers operated at the same time, namely
SUSIM [Woods et al., 1996; VanHoosier, 1996; Floyd et
al., 2001] the twin instrument of the one run on board
UARS, the SOLar SPECtrum (SOLSPEC) spectrometer
[Thuillier et al., 1997, 1998a, b, 2002a] and the Shuttle
Solar Backscatter UltraViolet (SSBUV) instrument [Woods
et al., 1996; Cebula et al., 1996, 1998b].
The Solar Ultraviolet Measurements of the Emitted
Radiations (SUMER) [Wilhelm et al., 1995] high resolution
spectrometer carried out solar radiance observations from
46 to 160 nm on board the SOlar and Heliospheric
Observatory (SOHO). Transformation of radiance to
Table 1. Visible and infrared models, and available data.
 and obs. stand for spectral resolution and observations,
respectively.
Authors
Labs and Neckel
[1968]
Arvesen et al.
(1969]
Neckel and Labs
[1984]
Burlov-Vasiljev
[1995]
Colina et al.
[1996]
Kurucz [1995]
SOLSPEC
SOSP
Range (nm) (nm) Origin
205 - 100000 10
obs. and
model
300 - 2495 0.1
obs.
to 0.3
330 - 1247 2
obs.
332 - 1062
1
120 - 2500
1
obs.
obs. and
model
200 - 200000 ≈ 0.001@ opacities
500 nm and data
200 - 870
1
obs.
850 - 2500 20
obs.
From 400 nm to short IR, data were collected mainly
from ground and airborne experiments. Observations in
the visible and IR ranges were made by Labs and Neckel
2
[1962] from the Jungfraujoch (3600 m altitude) at the center
of the solar disk and corrected for center-to-limb variation
and atmospheric transmission. Absolute solar spectral
irradiances were revised in 1984 using their former
measurements and high resolution Fourier spectrometer data
which were afterward integrated in one nanometer bandpass.
Burlov-Vasiljev [1995] measured from ground the visible
and the near IR range up to 1062 nm. Arvesen et al. [1969]
and Thekaekara [1974] carried out observations from
airplanes at an altitude of about 12 km where, in principle,
most of the water vapor is left below.
Observations in the visible range were made by the
SOLSPEC instrument on board the three ATLAS missions
Detailed comparisons between these data show an
agreement better than 2% [Thuillier et al., 1998a and
1998b].
The Arvesen et al. [1969] and Thekaekara [1974] spectra
also cover the IR range. However, they contain spectral
features difficult to interpret which are likely due to
calibration and possible under-corrected water vapor
absorptions.
Labs and Neckel [1968] generated a spectral irradiance
model up to 100 m using measurementss and a semiempirical continuum model. Later, it was incorporated in
Smith and Gottlieb [1974] which was afterward itself
incorporated in the World Radiation Center (WRC)
spectrum [Wehrli, 1985] and in the recent ASTM [2000]
model.
The twin instrument of SOLSPEC, the SOlar SPectrum
experiment (SOSP) on board the EUropean REtrivial
CArrier (EURECA) space platform has covered this range
and comparisons will be shown in section 3.2.5. The
SOLSPEC and SOSP measurements are the primary data set
used for the visible and IR ranges of the reference solar
spectra. Finally, few data are existing in the IR range, and
this is why, synthetic spectra are also needed for this study.
Colina et al. [1996] have generated a solar spectrum from
120 to 2500 nm by assembling data from different origins,
and a solar model including Fraunhofer lines from Kurucz
[1993].
Kurucz [1995] has generated a solar spectral irradiance
model from the solar atmosphere opacity, and a continuum
model which appears to be about 2% and 1 % smaller than
the Labs and Neckel [1968] and Colina et al. [1996]
continua, respectively. These solar continua are close, but
no validation has been possible from the use of the Arvesen
et al. [1969] and Thekaekara [1974] spectra.
The above data and models characteristics are
summarized in Table 1 and detailed in Thuillier et al.,
2002b.
A validation effort has been undertaken concerning the
UARS and ATLAS spectrometers by comparison of their
results obtained in similar conditions. The instruments
having different design and different method and means of
calibration provide an important opportunity of
comparisons for the UV from 120 to 400 nm. The
validation of the UARS solar irradiances, through error
analysis of the pre-flight calibrations, comparison between
SUSIM and SOLSTICE, and comparisons to other
ATLAS solar measurements, indicate that the UARS solar
irradiances have an absolute uncertainty of 2 to 4% (all
instruments uncertainties are given at 2 ) [Woods et al.,
1996]. The ATLAS 1 and 2 solar irradiances were
included in this original validation effort for UARS.
Similarly, Cebula et al. [1996] have compared the solar
spectral irradiance measured by three independent
spectrometers placed on board the Space Shuttle for the
ATLAS 1 mission (March, 1992) namely SOLSPEC,
SSBUV, and the ATLAS version of SUSIM. They were
found consistent within 5%. Furthermore, the three spectra
were averaged and compared with the mean SOLSTICE
and SUSIM measurements carried out at the same time on
board UARS [Woods et al., 1996]. For the spectral range
200-350 nm, the mean deviation between these two mean
spectra was found to be 0.14 % ± 0.20 % with a RMS of
1% [Cebula et al., 1996].
The comparison with UARS data is continued here by
using the last available versions of the SUSIM and
SOLSTICE data. The dates chosen for these reference
spectra are March 29, 1992, April 15, 1993, and
November 11, 1994 for the ATLAS 1, 2, and 3 missions,
respectively. For these UARS mean spectra, the SUSIM
and SOLSTICE data are smoothed to a spectral resolution
of 0.25 nm before combining the different measurements.
The resulting spectra are obtained as an average of the
SUSIM and SOLSTICE measurements between 136 and
390 nm, as both instruments have very similar precision
across that wavelength range. Only the SOLSTICE
measurements are used from 390 to 420 nm due to SUSIM
having a shorter wavelength coverage and a possible
instrument artifact above 390 nm. For below 136 nm, the
SUSIM continuum levels have lower precision than the
SOLSTICE measurements but have similar precision as
SOLSTICE for the brighter emission lines. Therefore, the
SUSIM and SOLSTICE measurements at the brighter
emission lines below 136 nm are averaged to derive an
appropriate scaling factor that is used on the SOLSTICE
irradiances over the 119 to 136 nm range. These short
wavelength scaling factors, which are only a few percents
correction, are derived using a linear fit to the scaling
factors from the emission lines at H I 121.6 nm, O I 130.4
nm, C II 133.5 nm, and Si IV 139.4 and 140.3 nm. The
differences between the UARS reference spectra and the
individual SOLSTICE and SUSIM measurements are
typically less than 5% and even smaller at the longer
1.3 Results of Merging Spectra
3
wavelengths. These new UARS reference spectra are very
similar to the Woods et al. [1996] spectra for the ATLAS 1
and 2 mission periods; however, they are based on improved
algorithms, version 20 for SUSIM and version 17 for
SOLSTICE. The absolute accuracy for these UARS
reference spectra is approximately 3.5% and is essentially
constant across the 119 to 420 nm range.
These new spectra are compared with the mean ATLAS 1
spectra. Figure 1 a and b show respectively the comparison
of the two spectra and their ratio. Their mean is unity while
the RMS of their ratio at 5 nm resolution is 2.2%.
This ratio is presented in Figure 2 for the ATLAS 2 (for
ATLAS 3, the ratio behaves similarly). Their means depart
from unity of one percent while their RMS remain 2.5 and
2.2% respectively as for ATLAS 1. The RMS value
originates mostly from the strong Fraunhofer lines (Mg II,
Ca II) which have different measured depth as they depend
on the resolution and sampling of the compared spectra.
These comparisons and uncertainties analyses confirm that
the absolute uncertainty is between 2 to 4%. Furthermore,
the mean UARS and ATLAS spectra present less
differences than the contributing spectra.
2.2 Data Sets Composing the Reference Solar
Spectrum
Because of the wide spectral coverage for this reference
solar spectrum, the data for different wavelength ranges
are necessarily derived. The different data sets are
discussed next, broken down into the EUV range
shortward of 120 nm, the FUV range between 120 and
200 nm, the UV to near visible range from 200 to 400 nm,
the visible between 400 and 870 nm, and finally the IR
between 870 and 2400 nm.
2.2.1 EUV (0 to120 nm).
Woods and Rottman [2002] have produced an EUV to
200 nm spectrum using UARS SOLSTICE data and the
measurements by a rocket observation made in 1994
[Woods et al., 1998]. Variability was derived from proxy
models based primarily on the Atmospheric Explorer-E
data. The minimum solar spectral irradiance is provided as
well as the 11-year solar cycle variability applicable to
solar cycle 22.
Below Ly , the spectral solar irradiance was calculated
for the conditions of the ATLAS missions 1 and 3 (Figure
3) based on Table 2 and taking into account the maximum
of cycle 22 estimated when the sunspot number was equal
to 200 (August 1990).
2. CONSTRUCTION OF THE SOLAR SPECTRUM
2.1 Principles
Based on results shown in section 1.3, it appears that
merging spectra is certainly the best way to build a
composite spectrum, especially when differences are less
than the individual instrument uncertainties. As two levels
of solar activity are required, we have gathered data
corresponding to the ATLAS 1 and ATLAS 3 missions,
allowing the use of five instruments working at the same
time in the UV and near visible ranges. Obviously, such
number of measurements is not available for all time
periods.
The levels of solar activity as indicated by the sunspots
number, total solar irradiance provided by Fröhlich and
Lean [1998], and solar 10.7 cm radio flux (F10.7) are listed
in Table 2. We shall use these TSI data later to normalize
the composite ATLAS 1 and 3 reference spectra.
2.2.2 FUV (120 to 200 nm).
Three mean spectra from SOLSTICE and SUSIM
instruments observing together on board UARS were
generated for the three ATLAS periods (see section 1.3).
For the construction of the reference spectrum, the mean
UARS spectra during the ATLAS 1 and 3 missions are
used (Figure 4).
2.2.3 UV to near visible (200 to 400 nm).
In this region, many data are available from SOLSTICE
and SUSIM on board UARS, and SOLSPEC, SSBUV,
and SUSIM on board ATLAS 1 and 3. These data have
different spectral resolutions, slit functions and may differ
slightly in their wavelength scales. When doing the
averaging, this could result in an additional error in
presence of wide Fraunhofer lines. Furthermore, adjusting
each wavelength scale (within their quoted accuracy) of
the five instruments, is not manageable without ambiguity.
Consequently, the mean of the five spectra is calculated by
a linear interpolation to the wavelength scale of the
spectrum having the highest sampling. The results are
shown in Figure 5 for ATLAS 1 and 3 respectively. The
comparison with the UARS mean spectra in the 200-400
nm range at 5 nm resolution reveals an agreement better
Table 2. Sunspot number (monthly mean, Rz), TSI data, the
daily F10.7, and the 81-day smoothed <F10.7> for the
ATLAS 1 and 3 missions on March 29, 1992 and
November 11, 1994, respectively.
Mission
ATLAS 1
ATLAS 3
Rz
121
20
-2
TSI (Wm ) F10.7 <F10.7>
1367.7
192
171
1366.7
77.5
83.5
4
than 0.5% for the mean and a RMS difference of 2%. The 5
nm resolution for smoothing is chosen for consistency with
previous works using similar data sets. When two spectra
are compared, even though they agree on the photometric
scale, a RMS difference larger than the mean difference is
commonly found. These RMS differences are usually
associated with the Fraunhofer lines due to differences in
the wavelength scales and spectral resolutions of the
different instruments.
Burlov-Vasiljev [1995] spectra. Their ratio to the mean of
the three SOLSPEC spectra is shown at 5 nm resolution
on Figure 6b. Their means are centred on unity with a
RMS of 2 and 1.4%, respectively. However, the lower
spectral irradiance of the Neckel and Labs spectrum
[1984] with respect to the two others is shown below 450
nm. The difference of the ratios to unity is mostly due to
the strong Fraunhofer lines (Fe I at 427 nm and Ca II at
855 nm). The use of Burlov-Vasiljev et al. [1995]
spectrum was considered, but its sampling at 5 nm was too
low for our requirements.
The mean SOLSPEC visible spectrum has a resolution
of 1 nm while below 400 nm, the two composite spectra
have a resolution of 0.25 nm (see section 1.3). To insure
continuity, we operated with the following way using the
high resolution spectrum from Kurucz [1995]:
i) it has been degraded by a running mean in order to
keep the sampling, and integrated over 0.5 nm which
provides the best agreement with the Ca II lines as
obtained with the mean of the five UV spectra (see section
2.2.3). It has now to be scaled to SOLSPEC irradiance.
ii) this is why, it has been further degraded to 1 nm
(SOLSPEC resolution). The result compared to the
SOLSPEC original spectrum is used to derive correction
coefficients. This set of coefficients has been used to scale
the Kurucz [1995]' spectrum previously degraded at 0.5
nm resolution (see.i)
2.2.4 Visible (400 to 870 nm).
The visible spectrum measured by the SOLSPEC
spectrometer during the three ATLAS missions shows RMS
differences not greater than 1.7% while the mean difference
remains 2%. These three spectra are, therefore, considered
to be consistent. Comparison made with the BurlovVassiljev [1995] spectrum shows similar RMS differences
likely due to the Fraunhofer lines presence. As for the mean,
it is 1% from 350 to 870 nm, in particular below 420 nm.
Comparing with Neckel and Labs [1984] spectrum shows a
remarkable agreement below 2% (mean) from 350 to 870
nm. However, below 420 nm, the difference is increasing at
shortward wavelengths. This difference was originally
reported by Peytureaux [1968] quoting this difference to be
about 8%, in agreement with Shaw [1982] from the Mauna
Loa observations, reporting 4% at 416 and 460 nm. From
the SpaceLab 2 mission, SUSIM data [VanHoosier et al.,
1988] showed greater values than those given by Neckel and
Labs [1984] by a few percents, as well as the SSBUV
[Cebula et al., 1996] from ATLAS 1 and SOLSTICE data
[Woods et al., 1996]. From ground, similar conclusions
were obtained [Burlov-Vassiljev et al., 1995]. Detailed
comparisons between SOLSPEC results with available
visible spectra are made by Thuillier et al. [1998a, 1998b,
and 2002a].
Most of the solar irradiance variability is observed below
300 nm [Floyd et al., 1998; DeLand and Cebula, 1998]. It
also exists in the visible range particularly in the core of
certain Fraunhofer lines. Ca II (393 mn) and He II (1083
mn) show the most important variability. For the other lines,
the equivalent width weakly varies (Mn (539 nm) is quoted
to 2 %), and much lower for the others [Livingston, 1992].
Furthermore, for the 11-year cycle, a solar temperature
change of about 1.5 K was found by Gray and Livingston
[1997]. The very small solar activity effect on the visible
solar irradiance has been estimated by Fontenla et al. [1999]
who calculated a change of 0.1% on the solar continuum.
Then, at one nanometer resolution, the solar variability is
barely detectable.
Consequently, for reducing noise effects during solar
observations and uncertainties occurring in the photometric
calibration, we shall use the mean of the three SOLSPEC
visible spectra. The merging of the three spectra is shown in
Figure 6a together with the Neckel and Labs [1984] and
As the Kurucz sampling is higher than the SOLSPEC
one, the set of coefficients is linearly interpolated
between two consecutive SOLSPEC measurements. This
is why, we have afterward verified that the original
SOLSPEC spectrum was reproduced after integration. To
do that, their ratio was calculated as well as their RMS.
-3
The mean was found smaller than 10 with respect to
unity and the RMS was found to be 0.7%.
2.2.5 IR (870 to 2400 nm).
The SOSP instrument, the twin instrument of SOLSPEC
was operated on board the EURECA platform. The
thermal conditions were stable and rather cool, which
allowed the IR measurements to be of better quality than
IR SOLSPEC data for the ATLAS missions. For scanning
from 850 to 2400 nm, different second order filters, as
well as density filters, were used as a function of
wavelength. The density filters take into account the large
decrease of the solar irradiance and instrument
responsivity toward the long wavelengths. The filters
generally induced a few percents perturbation at the
wavelength where they are changed. Taking into account
that no Fraunhofer lines are clearly detectable at 20 nm
resolution by the SOSP IR spectrometer, the spectrum has
been smoothed by a polynomial fit. It is shown in Figure
7a.
5
Comparison with existing solar spectrum models, show
no feature related to water vapor absorption or instrument
characteristics as found in the Arvesen [1969] and
Theakekara [1974] spectra. The SOSP spectrum is very
close to Labs and Neckel [1968], Colina et al. [1996] and
Kurucz [1995] solar models continua, but provides an
higher irradiance of 4% between 1500 and 2000 nm and 3%
at 2400 nm (Figure 7b) [Thuillier et al., 2002a] before
normalization.
As no Fraunhofer lines are present in the smoothed IR
SOSP spectrum, we have used those of the Kurucz [1995]
spectrum and we have applied the same method as in the
visible (see section 3.2.4). For that, the Kurucz spectrum has
been smoothed at 50 nm resolution in order to generate a
spectrum as smooth as the IR SOSP results shown in Figure
7a. The same verification has been also made resulting in a
-3
mean smaller than 10 with respect to unity, and a RMS
equal to 0.2%. The result is illustrated in Figure 8a. This
being done, it is possible to extend the reference spectrum
above 2400 nm using section 3.3.2. results with the Kurucz
[1995] spectrum.
LN [1968]
RSSV0-ATLAS 1
RSSV0-ATLAS 3
1366.36 1.2
The TS2397 is slightly greater for ATLAS 1 than for
-2
ATLAS 3 as expected, by an amount of 0.16 Wm . This
change is due to the spectral irradiance below 400 nm
since above it is the same for both ATLAS 1 and 3
spectra. The TSI difference between the two ATLAS
-2
-2
periods being 1 Wm , a change of 0.16 Wm in UV is
consistent with the expected change due to solar activity
effect in this range.
2.3.2 Normalizing with the Total Solar Irradiance.
We have now calculated the TSI, the energy above
2397.5 nm (TS>2397) and the spectral solar irradiance at
2397.5 nm (I2397) for the spectra indicated in Table 4.
-2
Table 4. TSI, TSI above 2397.5 nm in Wm , and solar
-2
spectral irradiance at 2397.5 nm in mWm nm-1 for
Kurucz [1995] and Labs and Neckel [1968] spectra
(respectively indicated by K[1995] and LN[1968]) and
RSSV0. A unique value is given at 2397.5 nm by
construction of the reference spectrum.
2.3 Normalization to the Total Solar Irradiance
Spectra such as WRC [Wehrli, 1985], Labs and Neckel
[1968], Kurucz [1995] are normalized on a value of the TSI.
The rationale of doing that is based on the radiometric
absolute scale (0.1% accuracy) which is significantly better
than the photometric scale of the spectrometers which are of
the order of 2% at best.
The various contributions from XUV to 2400 nm have
been assembled in a unique spectrum named RSSV0 (for
Reference Solar Spectrum version 0, before normalization).
For normalization, two ways are considered as described
next.
K [1995]
TSI
1368.11
TSI>2397 51.46
I2397
59.97
LN [1968] RSSV0
1366.36
52.48
60.44
61.33
-
The RSSV0 (table 4) solar irradiance is 61.33 mWm
-1
nm at 2397.5 nm. Its ratio to the Kurucz [1995], and
Labs and Neckel [1968] spectral irradiances at that
wavelength, is used to estimate the solar irradiance above
2397.5 nm. This allows to calculate the TSI for the
RSSV0. For example, for ATLAS 1 and using the Kurucz
spectrum, we obtain 1330.28 + 51.46*61.33/59.97. We
proceed similarly for ATLAS 3 and using the Labs and
Neckel [1968] and Kurucz [1995]'spectra.The results is
given in Table 5.
2
2.3.1 Normalizing below 2400 nm.
We have calculated the total irradiance up to 2397.5 nm
(TS2397) for the spectra listed in Table 3. After comparing
the TSI (up to 2397.5 nm) with the corresponding number
obtained from the Kurucz [1995], Colina et al., [1996] and
Labs and Neckel [1968], we obtained the percentages of
difference given in column 3 of Table 3. The percentage of
adjustment is then between 1.0 and 1.4%.
Table 5. Calculated TSI for ATLAS 1 and 3 reference
spectra using the total solar irradiance up to 2397.5 nm
complemented by K [1995] and LN [1968]. Percentages
of difference is given for each ATLAS period using
K[1995] and LN[1968], respectively (last line).
Table 3. Total solar irradiance up to 2400 nm and
percentage of difference with our reference spectra in
-2
version V0. Unit is Wm . LN[1968] stands for Labs and
Neckel [1968].
Spectra
TS2397
Kurucz [1995]
1316.79
Colina et al. [1996] 1311.58
1314.10
1330.28
1330.12
TSI Source
measured TSI
calculated TSI
with K[1995]
TSI
%
1368.11 1.0
1.4
6
ATLAS 1
1367.7
1382.91
ATLAS 3
1366.7
1382.74
calculated TSI 1383.53
1383.37
with LN[1968]
difference in % 1.11 / 1.16 1.17 / 1.22
accuracy between 2 to 4%. A similar situation is
encountered between Neckel and Labs [1984], BurlovVasiljev et al. [1995], and SOLSPEC spectra, thus
providing an accuracy around 3%. Above 870 nm, the
accuracy is only based on error analysis [Thuillier et al.,
2002a] since comparisons with other data are not relevant
due in particular, to the presence of undercorrected water
vapor absorption. It is found to vary from 2 to 3% between
870 and 2400 nm.
The RSS version 0 spectra for ATLAS 1 and 3 have
been corrected to the measured TSI value by 1.11 and
1.17%, respectively. Figures 3 to 7 are displaying V0
results. Normalization is taken into account in Figure 8,
which illustrates the full spectrum for the ATLAS 1
period.
The results are very consistent and we could adopt 1.11
to 1.16 % and 1.17 to 1.22% normalization percentages for
ATLAS 1 and ATLAS 3 respectively as shown by Table 5.
Furthermore, these results are in agreement with the method
of normalization explained in the above section (2.3.1). We
also note that the normalization percentage should likely be
greater for ATLAS 3 than for ATLAS 1, given the identical
spectrum used for both above 400 nm.
A mean percentage of adjustment may be chosen based
either on Labs and Neckel [1968] or Kurucz [1995] spectra.
However, for planetary atmosphere studies, the use of a
spectrum with Fraunhofer lines may be required. This is
why we have chosen to operate the normalization with the
Kurucz [1995] spectrum.
The adjustment percentages are finally 1.11 and 1.17 %
for ATLAS 1 and ATLAS 3 period. As expected it is below
the uncertainties of the spectral measurements which are of
the order of 2 to 3 %, to be compared with the absolute
radiometers accuracy quoted to 0.1 %. This adjustment is
justified by noting that, within the TSI, most of the energy is
provided by the visible and IR ranges which are measured
by a single instrument.
Applying the 1.11 and 1.17 % to the ATLAS 1 and 3
spectra in version 0, we obtain the Reference Solar Spectra
in Version 1 which are available on request via e-mail to
<gerard.thuillier@aerov.jussieu.fr>. Wavelengths are in nm
and spectral irradiances at band center are given in mWm2nm-1. For consistency, a unique exponential notation (E
format) is used to six decimal places for spectral
irradiances. We acknowledge it is superfluous in certain
wavelength ranges.
An extension of these two spectra above 2400 nm is
achievable after taking the ratio of solar spectral irradiance
of each RSSV1 to the Kurucz [1995] spectrum at 2397.5
nm as we did to operate the normalization.
3.2 Comparison With ASTM [2000]
Figure 8c compares the ATLAS 3 reference spectrum in
version 1 (after normalization) with the recent ASTM
[2000] spectrum by taking their ratio at 10 nm resolution.
Below 400 nm, this ratio is smaller than unity because the
UARS spectral irradiance used in this range has been
reduced by 3.2 % to match with the Neckel and Labs
[1984] spectrum for constructing the ASTM spectrum.
Around 900 and 1200 nm, the ratio of 3% above unity
originates from the Smith and Gottlieb [1974] spectrum in
this range. Above 1300 nm, the ratio below unity by a few
percent originates from the SOSP-EURECA data which
are not in agreement by this amount with the solar
continuum model of Smith and Gottlieb [1974].
3.3 Solar Variability From ATLAS 1 to ATLAS 3
The solar activity has decreased from ATLAS 1 to
ATLAS 3 periods as shown by the solar indices given in
Table 2. Figure 9 shows the ratio ATLAS 1 to ATLAS 3
ratio at 1 nm resolution. This ratio decreases as expected
from EUV to IR. Below Ly , the variabity is obtained
from the variabilty given by Woods and Rottmann [2002].
At Ly  the variablity reaches a factor 1.5 and 3 for the
He II line. The variability in the Mg II and Ca II lines is
also detectable.
Above Ly  the variability in the two reference spectra
originates directly from the data sets gathered at the
ATLAS 1 and 3 periods. We also calculated the Mg II
indices from our two reference spectra and compared with
other indices from SUSIM and SBUV/2 spectrometers
(table 6).
3. PROPERTIES OF THE REFERENCE SPECTRA
3.1 The Reference Solar Spectra
The accuracy of the two reference spectra varies over the
wavelength range because the data are derived from
multiple sources. Following the estimate of Woods and
Rottmann [2002], below the He II 30.4 nm line, the
accuracy is quoted to be about 40% and in the EUV, to be
30%. Above the Ly  line at 121 nm and below 200 nm, the
UARS mean spectra have an accuracy better than 3.5%
[Woods et al., 1996]. Between 200 and 400 nm, the
availability of five instruments (Cebula et al.1996; Woods et
al., 1996] has allowed several comparisons which show an
Table 6. Mg II indices from the two reference spectra,
SUSIM and SBUV/2 spectrometers.
RSS
7
SUSIM
SBUV/2
ATLAS 1
ATLAS 3
0.2748
0.2600
0.2708
0.2596
0.2756
0.2625
4. FUTURE MISSIONS
Table 6 shows consistent results. The Mg II indices
derived from the composite spectra are within the two other
determinations given by the two other single instruments.
This also allows to verify that there is no significant
wavelength shift between the five instruments.
The TSI variability originates partly from the wavelength
range below 400 nm. Table 7 shows a variation of 0.2 Wm
2
. This represents 20 % of the TSI variation. This number is
consistent with the estimate of Lean et al. [1997] quoting
about 30 % for a maximum to minimun solar activity
variation.
The ENVISAT 1 platform, launched in March 2002
carries two spectrometers, MERIS (Medium Resolution
Imaging Spectrometer) and SCIAMACHY (SCanning
Imaging Absorption SpectroMeter for Atmospheric
CHartographY) [Bovensmann et al., 1999]. They study the
land surface and middle atmosphere by observing the
backscattered light. They will also be able to make solar
spectral observations as GOME. The NOAA-16 SBUV/2
instrument was launched in fall 2000 and additional
SBUV/2 instruments are scheduled for launch throughout
the first decade of the 21st century. The SBUV/2
instruments will also continue to provide solar spectral
observations in the near UV.
SOlar Radiation and Climate Experiment [SORCE,
Rottman et al., 1997; Woods et al., 2000] is an
investigation designed to operate from mid 2002 on board
a free-flying satellite for a 6-year duration. It is made of
four instruments:
-2
Table 7. Energy per spectral ranges in Wm .
ATLAS 1
ATLAS 3
0-200 nm
0.116
0.106
200-400 nm > 400 nm
109.1
1258.51
108.91
1257.74
i) TIM monitoring the total solar irradiance (TSI),
ii) SIM measuring the spectral irradiance from 200
to 2000 nm,
iii) SOLSTICE observing the spectral irradiance
from 120 to 320 nm, and
iv) XPS observing several XUV bands from 1 to
35 nm.
4.4 Characteristics of the two Reference Solar Spectra
The sampling and resolution as a function of wavelength
is not constant, as listed in Table 8, because they depend on
the origin of the data. The ATLAS 1 and 3 spectra were
built from the data sources listed in Table 9.
Table 8. Sampling (s) and resolution (r) of the reference
spectra. From 400 to 2400 nm the sampling is quasi linearly
increasing.
Ranges
XUV-EUV
Ly  to 400 nm
400 to 2400 nm
s (nm)
1
0.05
0.2 to 0.6
On board the International Space Station (ISS), a solar
pallet to be operated for a duration of three years, is now
scheduled for 2004. It consists of three instruments
described by Thuillier et al., [1999]:
r (nm)
1
0.25
0.5
i) SOVIM measuring the total solar irradiance
(TSI),
ii) SACES observing from 17 to 220 nm,
iii) SOLSPEC observing from 180 to 3000 nm.
Table 9. Summary of data and models used for the
reference solar spectrum.
Ranges
XUV and EUV
UV up to 200 nm
UV 200 to 400 nm
Vis to 2400 nm
2400 nm to 100 m
Atmospheric, climate and solar physics are the basic
objectives of these two missions by measuring the total
and spectral solar irradiance and studying how the TSI
variations are partitioned into different spectral ranges.
These two missions are closely related in terms of spectral
ranges, but differ in design and calibration principles.
Furthermore, the International Space Station (ISS) solar
pallet extents more in the IR range while SORCE extents
more towards the EUV. An advantage of the ISS is the
retrieval of the instruments for a post-mission laboratory
check and calibration
SORCE will overlap with both the Space Station and
ENVISAT 1 missions, but also with the two UARS
experiments as recently decided allowing continuity over a
period longer than a Schwabe cycle. Consequently,
Source
rocket measurements
SOLSTICESUSIM/UARS
SOLSTICE-SUSIM/UARS
SOLSPEC-SSBUVSUSIM/ATLAS
SOLSPEC/ATLAS
SOSP/EURECA
Kurucz [1995] model
8
continuity and useful comparisons for both spectral and total
solar irradiance data will be achieved. We finally note that
the strategy of overlapping missions will be still applied
insuring that TSI and absolute spectral irradiance and their
specific variability will be accurately measured as required
for atmospheric, climate and solar physics. These new data
sets will provide the opportunity to improve the reference
solar spectra.
Cebula, R.P., G. Thuillier, M. R. J. Vanhoosier, E.
Hilsenrath, M. Hersé, P. C. Simon, Observations of the
solar irradiance in the 200-350 nm interval during the
ATLAS 1 mission: A comparison among three sets of
measurements-SSBUV, SOLSPEC, and SUSIM,
Geophys. Res. Lett., 23, 2289, 1996.
Cebula, R. P., M. T. DeLand, and E. Hilsenrath, NOAA-11
SBUV/2 solar spectral irradiance measurements 19891994: I. Observations and long-term calibration, J.
Geophys. Res., 103, 16235, 1998a.
Cebula, R. P., L. K. Huang, and E. Hilsenrath, SSBUV
sensitivity drift determined using solar spectral
irradiance measurements, Metrologia, 35, 677, 1998b.
Colina, L., R. C. Bohlin, and F. Castelli, The 0.12-2.5 m
absolute flux distribution of the sun for comparison with
solar analog stars, Astrophys. J., 112, 307, 1996.
David, K. H. and G. Elste, Der Einfluss von Streulicht auf
die Photometrie der Sonnenoberflaeche, Z.. Astrophys.,
54, 12, 1962.
DeLand, M. T., and R. P. Cebula, Solar Backscatter
Ultraviolet, model 2 (SBUV/2) instrument solar spectral
irradiance measurements in 1989-1994, 2, Results,
validation, and comparisons, J. Geophys. Res., 103,
16251, 1998.
Fontenla, J., O. R. White, P. A. Fox, E. H. Avrett, and R. L.
Kurucz, Calculation of solar irradiances. I. Synthesis of
the solar spectrum, Astrophys. J., 518, 480, 1999.
Floyd, L. E., P. A. Reiser, P. C. Crane, L. C. Herring, D. K.
Prinz, and G. E. Brueckner, Solar Cycle 22 UV Spectral
Irradiance Variability: Current Measurements by SUSIM
UARS, Solar Phys., 177, 79, 1998..
Floyd, L. E., D. K. Prinz, P. C. Crane, L. C. Herring, Solar
UV Irradiance Variation during cycles 22 and 23, Adv.
Space Res., in press, 2001.
Fröhlich, C., and J. Lean, Total Solar Irradiance Variations,
in New Eyes to see inside the Sun and Stars, edited by
F.L.Deubner et al., pp. 89-102, Proceedings IAU
Symposium 185, Kyoto, August 1997, Kluwer
Academic Publ., Dordrecht, The Netherlands, 1998.
Gray, D. F. and W. C. Livingston, Monitoring the solar
temperature: spectroscopic temperature variations of the
sun, Astrophys. J,, 474, 802, 1997.
Kurucz, R. L., Smithsonian Astrophys. Obs. CD rom N°
19, 1993.
Kurucz, R. L., Smithsonian Astrophys. Obs. CD rom N°
23, 1995.
Labs, D. and H. Neckel, Die absolute Strahlungsintensitaet
der Sonnenmitte im Spektralbereich 4010<<6569, Z.
Astrophys. 55, 269, 1962.
Labs, D. and H. Neckel, The radiation of the solar
photosphere from 2000 Å to 100 m, Z. Astrophys. 69,
1, 1968.
Lean, J., G. J. Rottman, H. L. Kyle, T. N. Woods, J. R.
Hickey, and L. C. Puga, Detection and parametrization
of variations in solar mid-and near-ultraviolet radiation
(200-400 nm), J. Geophys. Res., 102, 29939, 1997.
Livingston, W. C., Observations of solar irradiance
variations at visible wavelengths, in Proceedings of the
Workshop on the Solar Electromagnetic Radiation Study
for Solar Cycle 22,, Ed. R. F. Donnelly,11, 1992.
5. CONCLUSION
With the most recent existing data, we have built two
reference solar spectra close to moderately high and low
solar activity conditions as encountered during the ATLAS
1 and 3 periods, made by assembling data from different
instruments, primarily from the UARS and ATLAS
missions. They extend from XUV to IR. Their accuracy
depends of the spectral range, typically 30% below Ly ,
3.5% up to 200 nm and about 2 to 4% above. The sampling
is also variable with wavelength. Solar variability is
included in these two spectra as described by their Mg II
indices. After extension in the IR the calculated TSI is
found at about 1 % close to the observed TSI during the
ATLAS missions. Future missions will surely improve these
results by using new data with better accuracy and
measurements taken at the same time (e.g., SORCE and ISS
solar pallet).
Acknowledgments. Each participants have provided the
necessary data to build this reference spectrum. Important
discussions took place in recent meetings to define the
principles of the spectrum construction. Data handling was
carried out by Georges Azria from Service d'Aéronomie.
R. P. Cebula was supported by NASA contract NAS198106.
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Figure 3 : The EUV spectrum for ATLAS 1 (solid line)
and ATLAS 3 (dash line) periods.
Figure 1 : Comparison between the mean UARS spectrum
(dash line) and the mean ATLAS 1 spectrum
(solid line). a: these two spectra displayed
together. b : ratio of these two spectra at 5 nm
resolution.
Figure 2 : Comparison between the mean UARS spectrum
and the mean ATLAS 2 spectrum by their ratio
at 5 nm resolution.
11
Figure 4 : The UV spectrum up to 200 nm
for ATLAS 1 (solid line) and ATLAS 3
(dash line) periods.
Figure 5 : The 200-400 nm spectrum for ATLAS 1 (solid
line) and ATLAS 3 (dash line) periods.
Figure 7 : The IR range up to 2400 nm. a : comparison
between spectra from Kurucz [2001] (dash
line) and SOSP without Fraunhofer lines
(solid line). b : ratio of Kurucz [2001] (short
dash), Labs and Neckel [1968] (long dash),
and Colina et al.[1996] (solid line) to SOSP
at 50 nm resolution.
Figure 6 : The visible spectrum. a: comparison of the
Burlov-Vasiljev [1995] (solid), Neckel and
Labs [1984] (short dash) spectra and the mean
ATLAS 1-2-3 SOLSPEC spectrum long dash).
b: ratio of Burlov-Vasiljev [1995] (dash) and
Neckel and Labs [1984] spectra (solid) to the
mean ATLAS 1-2-3 SOLSPEC spectrum at 5
nm resolution.
12
35
30 8 : The reference spectrum for ATLAS 1. a: in linear
Figure
coordinates. b: in logarithmic coordinates
25
showing data as short wavelenths, and the
quasi linear logarithmic irradiance above 500
20
nm.c: its ratio to ASTM [2001].
Janvier
Février
Mars
Avril
Mai
Juin
15
10
5
0
Repas
Essence
Hotel
Figure 9 : Ratio of the ATLAS 1 to ATLAS 3 spectra at 1
nanometer resolution.
13