Long-term Solar Spectral Irradiance Observations by the TSIS-1 Spectral Irradiance Monitor

The NASA’s Total and Spectral Solar Irradiance Sensor (TSIS-1) launched on December 15th, 2017 and was integrated on the International Space Station (ISS) to measure long-term total solar irradiance (TSI) and solar spectral irradiance (SSI). The direct measurement of the SSI is made by the LASP Spectral Irradiance Monitor (SIM) and provides data essential to interpreting how the Earth system responds to solar spectral variability. Extensive advances in TSIS-1 SIM instrument design and new SI-traceable spectral irradiance calibration techniques have resulted in improved absolute accuracy with uncertainties of less than 0.5% over the continuous 200 to 2400 nm spectral range. Furthermore, improvements in the long-term spectral stability corrections provide lower trend uncertainties in SSI variability from those of the previous SORCE SSI instruments. We present the early mission results of the TSIS-1 SIM SSI observations for the first 5 years of operations – a time-period that includes the descending phase of solar cycle 24, the last solar minimum, and the ascending phase of solar cycle 25. Comparisons are made to previous spectral measurements both in the absolute scale of the solar spectrum and the time dependence of the SSI variability. The TSIS-1 SIM SSI spectrum shows lower IR irradiance (by as much as 6% near 2400 nm) and small visible irradiance increases (~0.5%) from the previous ATLAS3 and WHI reference solar spectra, but more consistent agreement with recent SCIAMACHY and SOLAR2 reanalysis results. We also show initial comparisons to current NRLSSI2 and SATIRE-S SSI model results both for short-term (solar rotation) spectral variability and, for the first time, the longer-term (near half solar cycle) spectral variability across the solar spectrum from the UV to the IR.