SORCE and TSIS comparison: absolute scale reconciliation

Abstract:

The Solar Radiation and Climate Experiment (SORCE) and Total and Spectral Irradiance Sensor (TSIS) conducted an intercomparison for the two Spectral Irradiance Monitors (SIM) spanning 712 days from 2018/03/14 to 2020/02/25 and permitted 530 time-matched pairs of observations. This comparison was conducted during the extremely quiescent Solar Cycle 24 minimum, so all observed differences and drifts between the two sensors are instrumental in nature. The TSIS SIM benefited from advanced calibration capabilities base on SI standards that were not available during the preflight calibration time period of SORCE. For this reason, a revision of the SORCE SIM absolute scale is appropriate. As expected, wavelength dependent differences in absolute agreement are a function of detector sensitivity and local changes in spectral slope. At the time of the comparison SORCE SIM has been on-orbit for 17 years while TSIS observations commenced immediately after a 6 week out-gassing period. Peak-to-peak absolute scale differences are less ±9% with a mean fractional difference of 1.1%. The greatest scale differences occur at the change-over between the UV and visible photodiodes in the 310nm region, and a systematic disagreement is present in the 850-2400 nm range. A scale correction factor has been developed to reconcile the TSIS-SIM difference with a wavelength dependent error on the mean typically less than 0.01% derived from every matched pair of observations.