Solar irradiance is one of the longest and most fundamental of all data records derived from space-based measurements. The continuous, 40-year total solar irradiance (TSI) observational data record is the result of independent instruments flown on many different missions where observational overlap is central to establishing a reliable composite solar radiation data record. The long-term, continuous measurements of the nearly full spectrum (UV through IR) solar spectral irradiance (SSI) began with the Solar Radiation and Climate Experiment (SORCE) mission launched in 2003 and has provided continuous SSI data for over 15 years. The long-term measurement of SSI is fundamental to interpreting how the atmosphere responds to solar variability, identifying the physical mechanisms of response, and validating climate model sensitivity to spectrally varying solar forcing driving the interactions among the atmosphere, oceans, ice, and land.
After nearly two decades of delays and programmatic changes, the first implementation of NASA’s Total and Spectral Solar Irradiance Sensor (TSIS-1) launched on December 15th, 2017 and was integrated on the International Space Station (ISS). The TSIS-1 mission continues SSI observations with an improved version of the LASP Spectral Irradiance Monitor (SIM). Extensive advances in both instrument design and new spectral irradiance calibration techniques have resulted in the TSIS-1 SIM being the most accurate space-borne SSI radiometer to date (continuous 200 – 2400 nm SI-traceable spectral absolute uncertainties < 0.5%). The TSIS-1 SIM measurements will secure the SSI data record continuity for the near-term by establishing the link to the SORCE end-of-mission data record and extend the observational SSI composite to validate and improve solar irradiance models.
Challenges remain to maintaining an accurate SSI measurement record into the future that must be balanced within the broader goals and future fiscal realities of NASA Earth Science budgets. Now is the time for developing and investing a framework that ensures solar irradiance data continuity for the coming decades. The recent emergence and rapid growth of more advanced small satellite and CubeSat developments – commensurate with accurate and highly capable compact instrument and advanced detector designs – offer the potential to demonstrate the viability of achieving and maintaining critical long-term solar data records. With the launch and demonstration of the Compact Spectral Irradiance Monitor (CSIM) we advance a new concept for a low-risk, cost efficient measurement strategy to continue and maintain an accurate SSI data record.