\(\mathbf{\Upsilon}\) represents the \(\mathbf{\beta}\) datasets at the\(\mathbf{\alpha}\) spectrum irradiance scale. Υdatasets differ from the \(\mathbf{\beta}\) datasets in their broad
baseline features, but share the same native spectral and sampling
resolutions. The TSIS-1 HSRS is the concatenation of these\(\mathbf{\Upsilon}\) datasets. In transition regions, where one\(\mathbf{\Upsilon}\) dataset overlaps in wavelength with another, we
adopt an average of the irradiance values for the HSRS.
3 Data
3.1 High Accuracy (\(\mathbf{\alpha}\)) Spectrum
Our high accuracy \(\mathbf{\alpha}\) spectrum is space-based SSI
observations from the TSIS-1 Spectral Irradiance Monitor (SIM) and
Compact SIM (CSIM). TSIS-1 SIM has measured daily SSI between 200-2400
nm since March 2018. The CSIM dataset, spanning 210-2800 nm, began in
late-March 2019. The SIM instruments have variable spectral resolution
of approximately 0.25 to 40 nm (Richard et al., 2019; 2020). TSIS-1 SIM
and CSIM data are available from:
https://lasp.colorado.edu/home/tsis/data/ssi-data/ and
https://lasp.colorado.edu/home/csim/data-and-ham-radio/.
TSIS-1 SIM meets climate-record quality requirements (NOAA, 2010) and
has order-of-magnitude reductions in radiometric uncertainty relative to
the heritage SORCE SIM instrument (Harder et al., 2005) through an
extensive component level calibration program that characterized the
instrument as an absolute sensor and verified the instrument in
irradiance across the spectrum against an SI-traceable cryogenic
radiometer using stable tunable laser sources (Richard et al. 2020). The
instrument level validation and final end-to-end absolute calibration
placed relative pre-launch accuracy uncertainties at 0.24%
(>460 nm) to 0.41% (<460 nm). On-orbit
calibration stability is maintained by instrument degradation
corrections that utilize observations made by redundant and independent
instrument channels that are exposed to the Sun at varying duty cycles
(Mauceri et al., 2020). Precision is 0.01% to 0.05% (Richard et al.,
2020).
CSIM is a 6U CubeSat technology demonstration mission for the NASA Earth
Science Technology Office. CSIM radiometric accuracy is tied to the same
SI-traceable cryogenic radiometer with the same laser sources used in
the TSIS-1 SIM calibrations, but by calibration transfer as opposed to
absolute calibration verification. The CSIM measurement uncertainty is
<1% from 300-2000 nm and 1.26% above 2000 nm (Richard et
al., 2019).
Specifically, the \(\mathbf{\alpha}\) spectrum, from 200 to 2365 nm, is
an average of daily TSIS-1 SIM irradiance observations from 1-7 December
2019, which coincides with the solar activity minimum between solar
cycles 24 and 25
(https://www.swpc.noaa.gov/news/solar-prediction-scientists-announce-solar-cycle-25).
We extend this spectrum from 2365 to 2730 nm with averaged CSIM
observations from April to September, 2019. A wavelength-independent
offset factor of 0.9921 (i.e., 0.8%; within the measurement
uncertainty) ensures the CSIM irradiance portion of the\(\mathbf{\alpha}\) spectrum (i.e. \(\mathbf{\geq}\) 2365 nm) matches
TSIS-1 SIM irradiance at 2365 nm.
3.2 High Spectral Resolution (\(\mathbf{\beta}\)) Datasets
The \(\mathbf{\beta}\) datasets are the Air Force Geophysical Laboratory
(AFGL) solar irradiance observations, the Kitt Peak National Observatory
(KPNO) solar transmittance atlas, the Quality Assurance of Spectral
Ultraviolet Measurements In Europe (QASUME) Fourier transform
spectrometer (QASUMEFTS) solar irradiance observations, and the Solar
Pseudo-Transmittance Spectrum (SPTS) atlas.
Grating spectrometer observations of the Sun’s ultraviolet irradiance
from high-altitude balloons dating to the 1970’s and 1980’s by the Air
Force Geophysical Laboratory (AFGL) (Hall & Anderson, 1991) are the
only SSI dataset available to date between 200 nm and 310 nm with a
spectral resolution of 0.01 nm or better. Corrections for atmospheric
ozone absorption attenuation were applied to the data. The spectral and
sampling resolution of the AFGL irradiance dataset are 0.01 nm and the
radiometric uncertainty is typically 5-10%, but can reach 25% near 200
nm.
Additional high resolution data are solar transmittances between 300 and
1000 nm (Kurucz, 2005) derived from Kitt Peak National Observatory
(KPNO) ground-based Fourier transform spectroradiometer (FTS)
observations between 296 and 1300 nm at ~0.001 nm
resolution (Kurucz et al., 1984). Converting from FTS observation to
transmittance was achieved through a multi-step process (Kurucz, 2005).
First, continuum atmospheric absorption features based were removed
based on a model followed by the estimation and removal of the solar
continuum with subjective fits of the FTS observations to a simulated
solar spectrum. Sharp telluric spectral features, attributed to
molecules in Earth’s atmosphere, were identified with the HITRAN
database (Rothman et al., 2005) and removed. The KPNO residual
irradiance wavelength scale accuracy, reassessed for this study, is
found to be better than 3.2x10-4 nm above 305 nm and
better than 3.0x10-3 nm at shorter wavelengths,
unchanged from that reported in Chance and Kurucz (2010).
An additional source is the high-resolution extraterrestrial solar
irradiance spectrum measured by an FTS between 305 nm and 380 nm from a
high-altitude, ground location during the Quality Assurance of
Ultraviolet Measurements In Europe (QASUMEFTS) campaign. The measured
spectrum was extended down to 300 nm and up to 500 nm with the KPNO
atlas (Gröbner et al., 2017). The extraterrestrial solar spectrum was
derived from QASUME observations by the Langley plot technique. The FTS
observations were adjusted to the absolute irradiance scale of a
lower-resolution, reference spectroradiometer with accuracy traceable to
the primary spectral irradiance standard of the Physikalisch Technische
Bundesanstalt (PTB) laboratory in Germany. QASUMEFTS radiometric
uncertainty (k=2) reaches 4% at wavelengths lower than 310 nm and 2%
between 310 and 500 nm. The spectral resolution of QASUMEFTS is better
than 0.025 nm and uncertainty in the wavelength-scale is 0.01 nm or
better.
Version 2016 of the ‘disk-integrated’ Solar Pseudo-Transmittance
Spectrum (SPTS) (Toon, 2014) contains the transmittance from 40,000
solar absorption lines spanning 600-26316
cm-1 (380-16600 nm), sampled every 0.01
cm-1. It is an empirically-generated dataset, where
telluric line contributions to the observed spectra from multiple FTS
instruments are identified with the HITRAN database and iteratively
removed. Measured KPNO spectra are the predominant observation source in
the SPTS database, supplemented with observations from high-altitude
balloons and satellites (Toon, 2013).
We adopt a vacuum wavelength scale for the HSRS. The AFGL and QASUMEFTS
datasets were converted from air-to-vacuum scale using Edlén (1966).
4 Results
In this section, we present the TSIS-1 Hybrid Solar Reference Spectrum
(HSRS) and make comparisons to independent datasets.
4.1 Q factor
When the spectral ratio method is used to adjust an irradiancedataset, Q is unitless and represents a magnitude adjustment to
the radiometric calibration of the original dataset. However, when the
method is applied to adjust a solar transmittance dataset,Q has units of SSI (W/m2/nm) and approximates
the solar continuum when devoid of absorption and emission features. In
either case, Q adjusts broad features while leaving fine spectral
features undisturbed.
Figure 2 shows the Q factors used to produce the HSRS at the\(\mathbf{\alpha}\) spectrum irradiance scale. The adjustments are
smaller than 25% for AFGL and 2.5% for QASUMEFTS datasets, which falls
within their respective reported radiometric uncertainties. The
adjustments for the KPNO and SPTS solar transmittance datasets have the
expected spectral shape of the Sun’s continuum.
Applying Q to the \(\mathbf{\beta}\) datasets forms the TSIS-1
HSRS at 0.01 to ~0.001 nm spectral resolution and
spanning 202 to 2730 nm (Figure 3, top). We also produce four variants
of the HSRS that standardize the reference spectrum to fixed, lower
spectral resolutions using Gaussian convolution filters. The integrated
SSI of the HSRS and the HSRS variants is within 0.2% of the integrated\(\mathbf{\alpha}\) spectrum between 202 and 2730 nm (1324.94 W
m-2). We produce an additional variant of the HSRS
dataset (not shown) over the spectral range 202 to 500 nm with variable
Gaussian convolution kernels that approximate the spectral resolution,
but not the true spectral shape, of the SORCE Solar-Stellar Irradiance
Comparison Experiment (SOLSTICE) (McClintock et al., 2005) and the Aura
Ozone Monitoring Instrument (OMI) (Levelt et al., 2006). This final
variant has utility for developing new, higher resolution, solar
irradiance variability models (Lean et al., 2020). The HSRS and its
variants are reported on fixed wavelength grids of at least 4 points per
resolution element (Table 1).
4.2 Uncertainties
The total TSIS-1 HSRS uncertainty (Table 1) is the root-sum-square of
the following error sources: the uncertainties of the TSIS-1 SIM and
CSIM measurements that comprise the \(\mathbf{\alpha}\) spectrum,
including those incurred from instrument degradation corrections, and
the methodology accuracy. The methodology uncertainty is the
1- \(\mathbf{\sigma}\) standard deviation of the relative percent
difference of the HSRS and the \(\mathbf{\alpha}\) spectrum computed
separately for the UV (< 400 nm) and VIS-NIR (400-2365 nm) and
long NIR (> 2365 nm) portions of the spectrum (Figure 3;
bottom) and equal to 1.2%, 0.16% and 0.36%, respectively. The HSRS
uncertainty is equivalent to 0.3% over most of the spectrum, increasing
to 1.3% below 400 nm and above 2365 nm. It reflects the uncertainty of
the HSRS for the same spectral resolution as the TSIS-1 and the
CSIM instruments . At very high spectral resolution, the relative
differences in individual lines from different solar line databases can
reach several tens of percent (not shown).
4.3 Comparison to Other Datasets
Figures 1 and 3 establish the difference between the HSRS and the
ATLAS-3 and LASP WHI solar reference spectra that is several percent
between 500 nm and 1300 nm, increasing to 8-10% for wavelengths outside
of that range. The SOLAR-ISS differs at individual wavelengths by -3.3%
(~-0.06 W m-2) near the peak of the
solar spectrum at 520 nm and by +2 to +4% (-0.01 to -0.03 W
m-2) between 800 nm and 1400 nm. Above 1500 nm, the
agreement is generally within 2%. In the ultraviolet, differences
between the HSRS and the other reference spectra can approach 10%.
In Figure 4, we compare the HSRS to high-resolution TANSO Fourier
Transform Spectrometer (TANSO-FTS) observations obtained during solar
calibration scans of the Greenhouse Gases Observing Satellite (GOSAT)
mission (Kuze et al., 2009). For the comparison, the HSRS resolution has
been reduced to match that of the TANSO-FTS instrument. We also apply
adjustments to the TANSO-FTS data. First, we correct the wavelength
scale for the Doppler shift that occurs with changing spacecraft
velocity. Second, we convert the s- and p-polarized solar radiance to
solar irradiance under the assumption of a perfect solar diffuser plate.
Third, we average the Doppler-corrected, s- and p-polarized irradiance
to get the unpolarized solar irradiance spectrum. Finally, we adjust the
irradiance scale to match that of the HSRS using the spectral ratio
method described in Section 2. The resulting
1- \(\mathbf{\sigma}\) standard deviation of the HSRS and
TANSO-FTS relative percent difference is smaller than 0.4% in all bands
(not shown), demonstrating robust HSRS solar line positions and depths
in these wavelength ranges.
5 Conclusions
The TSIS-1 Hybrid Solar Reference Spectrum (HSRS) is a new solar minimum
irradiance reference spectrum developed by normalizing high spectral
resolution solar line data to the absolute irradiance scale of the
TSIS-1 SIM and CSIM. TSIS-1 SIM and CSIM observe SSI at higher accuracy
than attained by predecessor instruments and, notably, show the
near-infrared solar spectrum is 8-10% lower in magnitude than the
ATLAS-3 and LASP WHI reference spectra. The SOLAR-ISS (v2) reference
spectrum agrees with the TSIS-1 SIM over most wavelengths above 1600 nm
but disagreements persist from 500-1600 nm. Differences can reach 10%
below 300 nm. Therefore, the HSRS provides an important new constraint
for science analyses in a broad array of fields.
The HSRS spans 202 to 2730 nm, encompassing an integrated energy that
exceeds 97% of the total solar irradiance. The HSRS accuracy is 0.3%
to 1.3% and the spectral resolution is 0.01 nm to
~0.001 nm. Variants of the HSRS are also provided for
lower, fixed spectral resolutions.
Acknowledgments, Samples, and Data
The TSIS-1 HSRS is available from
https://lasp.colorado.edu/lisird/. The authors extend their
gratitude to the teams responsible for the development and maintenance
of the AFGL, QASUMEFTS, KPNO, and JPL SPTS databases. Additionally, the
authors extend their gratitude to F. Kataoka and A. Kuze for provision
and assistance with the GOSAT solar calibration radiance data, to M.
Snow for provision of the SORCE SOLSTICE instrument line shape
information, and to S. Marchenko for the Aura OMI instrument line shape
information. OC, ER, DH, PP, and TW are grateful for the support of the
NASA TSIS-1 project (80GSFC18C0056) and NASA’s Solar Irradiance Science
Team (80NSSC18K1304) in performing this analysis. Authors at the
Harvard-Smithsonian Center for Astrophysics thank NASA for ongoing
support for development of satellite measurements of the Earth’s
atmosphere.
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