\(\mathbf{\Upsilon}\) 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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