A Comparative Assessment of Solar Irradiance Observations and Models at
the Dawn of TSIS
Abstract
A wide variety of research applications require knowledge of total solar
irradiance (TSI) and solar spectral irradiance (SSI) on time scales from
minutes to centuries. The current satellite data record of TSI and
ultraviolet SSI is 40 years long while observations of solar irradiance
at visible wavelengths through the near-infrared span 15 years. In late
2017, the NASA Total and Spectral Solar Irradiance Sensor-1 (TSIS-1)
mission was deployed on the International Space Station (ISS); these new
TSI and SSI datasets are now extending the observational solar
irradiance record with a planned 5-year mission. Recognizing the need
for ongoing specification of solar irradiance, the National Centers for
Environmental Information established the Solar Irradiance Climate Data
Record (CDR) in 2014. The CDR includes a composite record of TSI
observations and estimates of solar total and spectral irradiance
variations during, and prior, to the space-based record based on the
Naval Research Laboratory (NRL) models. Utilizing as inputs proxies of
sunspot darkening and facular brightening, the models specify TSI and
SSI annually since 1610 and daily since 1882. Both the observational
composite and the model specifications are updated regularly and will
eventually utilize the new TSIS-1 observations, both to extend the
observational composite and to validate and improve the models. With the
goal of establishing the utility of the NRL models in specifying the
time and wavelength dependence of solar variability for the Solar
Irradiance CDR, we compare the latest NRLTSI2 and NRLSSI2 modeled
irradiances with observations, including composite records, and with
independent models of solar irradiance variability. Our assessments
quantify current understanding of solar irradiance variability on
multiple timescales and identify areas where TSIS-1 observations are
expected to provide improved understanding of solar irradiance
variability. We use the following datasets in our comparisons: TSIS-1,
Solar Radiation and Climate Experiment (SORCE), Ozone Monitoring
Instrument (OMI), Solar Irradiance Data Exploitation (SOLID), Spectral
and Total Irradiance Reconstructions for the Satellite Era (SATIRE-S), a
three-dimensional extension of the SATIRE-S model (SATIRE-3D), and
Empirical Irradiance Reconstruction (EMPIRE).