The Development and Maintenance of a Longterm Observational Solar Spectral Irradiance Data Record

The space-based measurements of the solar spectral irradiance (SSI) covering a continuous wavelength range >96% of the total solar output have been made by several missions for over two decades – a time-period covering solar cycles 23-25. The continuity of these observations by overlapping missions provides data essential to interpreting how the Earth system responds to solar spectral variability and improving operational solar irradiance models for the long-term climate data record. Extensive advances in instrument design and new SI-traceable spectral irradiance calibration techniques have resulted in improved SSI absolute accuracies. Furthermore, improvements in the long-term instrumental spectral stability corrections provide lower on-orbit trend uncertainties in SSI variability measurements.

These extant and future solar spectral irradiance instruments depend heavily on radiometric standards for their calibrations and characterizations in order to achieve uncertainties of less than 0.5% over a broad spectral range from the ultraviolet to the infrared. Recent advances in these radiometric standards permit room temperature, rather than cryogenic operation. This greatly reduces the size and complexity of the entire ground calibration system, which in turn reduces the cost and complexity of the instrument calibration.

We present an overview of the recent evolution of these radiometric calibration techniques and traceable absolute detector advancements required for the maintenance of a low uncertainty solar irradiance scale. We also discuss how these advances are critical for the next-generation operational SSI missions that are being proposed for the continuity of the observations into the next decade.