In a study published in Light:Science&Applications , ZHU Ping and YE Xin from Changchun Institute of Optics, Mechanics and Physics (CIOMP) of the Chinese Academy of Sciences (CAS), Gerard Thuillier from Physikalisch-Meteorologisches Observatorium Davos World Radiation Centre (PMOD/WRC), Martin Snow from University of Colorado Boulder, and ZHANG Peng from National Satellite Meteorological Center and Innovation Center, China Meteorological Administration, studied the characteristics of seven SSI models. 

The researchers conducted comprehensive comparisons between theoretical SSI models, semi-empirical SSI models, and SSI measurements to get a better understanding of SSI. Great attention was paid to the identification of the date when the Sun reached its minimum activity during the 23rd and 24th 11-years solar cycle. The Sun is currently picking up its activity towards the 25th cycle and the number of space weather related solar events (flares, strong solar wind, etc.) will increase in the next few years.

SSI covers the electromagnetic radiation from extreme ultraviolet (EUV) to near inferred (IR) wavelength range. After nearly forty years of development in SSI modeling and measurements, there are two popular theoretical SSI models: SATIRE and NESSY; four frequently used composite models ATLAS3, SOLID, SOLAR 2rev, and SOLAR3; and the longest SSI time series as measured in space by the Solar Radiation and Climate Experiment (SORCE) mission. A global systematic comparison of these seven SSI datasets remains absent.

The researchers pointed out that the seven studied SSI series agree within a few percent overall. Focusing on 3 to 4% differences among seven SSI spectra, they found that the seven spectra used here all produced a maximum brightness temperature near 1600 nm, however, the temperature values of the observed data are about 200K higher than those of the semi-empirical and radiative transfer models. Those discrepancies could be originated from absolute calibrations, pass-band profiles, and their centering, which the latter two effects are the most important.