The Solar Radiation and Climate Experiment (SORCE) Mission: Final Calibrations and Data Products

The Solar Radiation and Climate Experiment (SORCE) was a NASA mission that operated from 2003 to 2020 to provide key climate-monitoring measurements of total solar irradiance (TSI) and solar spectral irradiance (SSI). This topical collection provides an overview of some of the key SORCE science results, an overview of mission operations and how anomalies impacted the science observations, a detailed description of the updated algorithms used in producing the final data products of TSI and SSI from the four SORCE instruments, and results from an underflight calibration-rocket experiment flown in June 2018. The 17-year-long SORCE mission has made many contributions to the climate records of TSI and SSI that date back to the 1970s, and, fortunately, similar observations from the Total and Spectral Solar Irradiance Sensor (TSIS-1) are able to continue these Sun-climate records after SORCE without a gap.

contributes to the TSI climate record and how SORCE's three SSI instruments added to and extended in wavelength the SSI climate record. The SORCE SSI instruments are the Spectral Irradiance Monitor (SIM: 240 -2413 nm), SOLar STellar Irradiance Comparison , and X-ray UV Photometer System (XPS: 0.1 -40 nm and H I 121.6 nm). Woods et al. (2021) also provide overviews of the spacecraft and instrument anomalies during the mission and how those anomalies impacted the solar observations. Despite these anomalies, it is remarkable that the four SORCE instruments all made excellent solar measurements until the last day of the SORCE mission and that SORCE was able to provide daily averages of the solar irradiance for more than 95% of the days over its 17-year mission. The most significant data gap of 210 days, being 71% of the 296 days with gaps, was caused by a significant battery-capacity issue between August 2013 and February 2014. The end of the SORCE mission was a planned passivation of the spacecraft following a successful two-year overlap with the TSIS-1 mission, which continues the TSI and SSI climate records.
A significant portion of this topical collection is the four articles about the final calibrations and algorithm revisions for the four SORCE instruments. While the concept of converting a detector signal into irradiance units seems straightforward, there is actually much complexity to understand the performance of every instrument component in order to provide the most accurate solar-irradiance results. The preflight calibrations and characterizations for the SORCE instruments laid a solid foundation for the data-processing algorithms, but much was learned about the instruments and their trends over the long 17-year mission. The main purpose of these four articles is to describe in detail the improvements to the algorithms and the trends of the instrument degradation over the mission, so that future reprocessing efforts could be made to further improve upon the SORCE data products. Kopp (2021) provides those results for the TIM Version 19 data products, as well as discussing six science highlights with the TIM observations. Harder et al. (2022) discuss the SIM-algorithm improvements and details of the instrument trends for the SIM Version 27 data products. Snow et al. (2022) present the improvements and trend analysis for the SOLSTICE Version 18 data products, the slow migration of the South Atlantic Anomaly (SAA) over the SORCE mission, and several comparisons of SOLSTICE data to other measurements and models of the SSI variability. With the reduced capability of the SORCE spacecraft to provide routine stellar calibrations for SOLSTICE in the later years, a special rocket experiment was flown in June 2018 to provide underflight validation for SOLSTICE; those results are presented by Thiemann et al. (2023). Woods and Elliott (2022) provide the inflight calibration results used for the XPS Version 12 data products and also the improved reference spectra in the 0 -40 nm range for the XPS Level 4 (model) product.
Another aspect of this topical collection is to provide some example science results from the SORCE mission. In addition to the science results presented in the aforementioned articles, there are two additional articles. Cahalan, Ajiquichi, and Yatáz (2022a, 2022b) present data analysis of the SIM spectra to derive the solar-brightness temperature as a function of wavelength in the 240 -2400 nm range. The spectral-brightness temperature effectively provides a probe of the emissions from different layers in the solar atmosphere, as well as identifying various ions affecting the solar spectrum. Furthermore, Cahalan, Ajiquichi, and Yatáz (2022a) estimate how much the solar-brightness temperature varies with changing solar activity. In the other article,  present SSI variability results from the ≈ 27-day solar rotations and the ≈ 11-year solar cycles during the SORCE mission and compare those results to variability results derived from three different composite SSI data sets and two different models of SSI variability. Those comparisons indicate that the best agreement between SORCE and these other variability estimates is for wavelengths shorter than 240 nm and that the SORCE variability results for Solar Cycle 24 (2008 -2019) appear to be more accurate than the results for Solar Cycle 23 (prior to 2008).  also compare the SORCE irradiance measurements from the 2008 -2009 cycle minimum and the 2019 -2020 cycle minimum and conclude that there is minimal, if any, long-term trend of the solar irradiance over the SORCE mission.
Thanks to the dedicated SORCE team members at the Laboratory for Atmospheric and Space Physics (LASP), the Goddard Space Flight Center (GSFC), the Naval Research Laboratory (NRL), and Orbital Sciences (now Northrop Grumman Space Systems), the SORCE mission operated long beyond its five-year prime mission. The SORCE TSI and SSI observations have been critical for the continuation of the 42-year-long TSI and SSI climate records, as well as improving upon those records with unprecedented accuracy and measurement precision. The next-generation TIM and SIM instruments are now flying on the TSIS-1 mission with further improvements in sensor technology that enable even better measurements of the TSI and SSI to extend these important Sun-climate records.

Competing Interests
The authors declare no competing interests.
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