Maintaining Continuity of the Solar Spectral Irradiance Data Record in the 21st Century: On-Orbit Validation of Future Observational Strategies

The continuous measurement of solar spectral irradiance (SSI) is essential to interpreting how the Earth system responds to solar spectral variability and understanding the physical mechanisms driving the interactions among the atmosphere, oceans, ice, and land. Fundamental objectives related to SSI monitoring extend to a broad range of studies that includes Earth’s energy budget, process-oriented remote sensing applications, climate and atmospheric modeling, and atmospheric composition, among many others. The current implementation for continuous SSI monitoring (from 200 – 2400 nm, approximately 96% of the total solar irradiance) is the Spectral Irradiance Monitor (SIM) on the Total and Spectral Solar Irradiance Sensor (TSIS-1). TSIS-1 was launched in December 2017, with SIM acquiring first light spectra in March 2018. Advances in both instrument design and spectral irradiance calibration techniques have resulted in the TSIS-1 SIM being the most accurate space-borne SSI radiometer to date, with irradiance uncertainties at 0.2% across the spectrum. Furthermore, both temporal and spectral observational overlap with the extant SORCE SIM measurements that began in 2004 will secure the SSI data record continuity for the near-term. There are many challenges to maintaining an accurate SSI measurement record over the longer term that must be balanced within the broader goals and future fiscal realities of NASA Earth Science Division. The recent emergence and rapid growth of CubeSat developments commensurate with accurate and highly capable compact instruments provide much needed flexibility for achieving and maintaining critical long-term solar data records with the constraints of NASA’s larger mission. With the launch of the Compact Solar Spectral Irradiance Monitor (CSIM) we advance a low-risk, cost efficient sustainable measurement strategy for maintaining an accurate SSI data record that meets the stringent requirements of TSIS-1. Moreover, the operational overlap of CSIM with the TSIS-1 SIM provides SSI measurement advantages that have not been exploited previously: their common, traceable calibration tie presents a unique opportunity to verify and validate on-orbit, the accuracy and temporal and spectral stability of the CSIM mission concept. This will provide substantial dividends toward securing the continuity of this important climate decade record well into the next decade and beyond.