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The stability and longevity of the Ozone Monitoring Instrument (OMI), flying on board the (active) Aura Earth-observing satellite since July 2004, facilitates creation of accurate, long-term record of relative (normalized to a solar minimum) solar spectral irradiances (SSIs). Here we discuss technical details of the most recent version (V3) of the SSI product that provides approximately daily measurements for the period July 2006 to April 2018 in the 265-to 500-nm domain with average 0.5-nm resolution. We compare OMI SSIs with concurrent independent observations and model estimates. The short-term (solar rotational cycle) observations and model predictions mostly agree to ∼0.1-0.2% in the ultraviolet domain, with an excellent, down to ∼0.01% agreement in the visible range. The long-term (solar cycle) comparisons pose more challenges in the ultraviolet domain, where the differences between observations and models frequently exceed the rather conservative ∼0.1% (both point-to-point and long-term) OMI uncertainties. In the visible range, these differences gradually diminish to <0.05% yet again pointing to reliability and robustness of the amassed SSI data in this domain. Key Points: • We describe the new version (V3) of the OMI solar spectral irradiances that provides a record of Solar Cycle 24 variability in the 265-to 500-nm range • The short-term (solar rotation) SSI variations: Comparison with the concurrent independent observations and model predictions show good-to-excellent (predominantly, to ∼0.1% in UV, and down to ∼0.01% in the visible) agreement between the various data sets • The long-term (solar cycle) SSI variations in the UV domain pose serious challenges, with model-observation and observation-observation disagreements frequently exceeding 0.5%. In the visible the inter-comparisons show good agreement, down to ∼0.05%

We compare solar spectral irradiance (SSI) measurements obtained from the Spectral Irradiance Monitor (SIM) on board the Solar Radiation and Climate Experiment (SORCE), corrected for instrument degradation using three different methods; SIM Version 25, Multiple Same-Irradiance-Level and SIM constrained Version 2 (SIMc V2), to quantify their differences and gain understanding about the relative performance of each method. Furthermore, we compare the three data sets to independent SSI observations that include the SORCE SIM successor Total and Spectral Solar Irradiance Sensor SIM and solar irradiance models. While we find agreement between the SSI for the three correction methods for wavelengths in the visible, differences persist for shorter and longer wavelengths, on time scales on the order of the 11-year solar cycle. The comparisons to independent SSI observations reveal remaining instrument trends in some of the corrected SORCE SIM data.

An understanding of solar variability over a broad spectral range and broad range of timescales is needed by scientists studying Earth's climate. The Total and Spectral Solar Irradiance Sensor (TSIS) Spectral Irradiance Monitor (SIM), is designed to measure solar spectral irradiance (SSI) with unprecedented accuracy from 200 nm to 2400 nm. SIM started daily observations in March 2018. To maintain its accuracy over the course of its anticipated 5-year mission and beyond, TSIS SIM needs to be corrected for optical degradation, common for solar viewing instruments. The differing long-term trends of various independent solar-irradiance records attest to the challenge at hand.The correction of TSIS SIM for optical degradation is based on piecewise linear fits that bring the three instrument channels into agreement. It is fundamentally different to the correction applied to the TSIS SIM predecessor on SORCE. The correction facilitates reproducibility, uncertainty estimation and is measurement-based. Corrected, integrated TSIS SIM SSI agrees with independent observations of total solar irradiance to within 45 ppm as well as various solar-irradiance models. TSIS SIM SSI