Earth’s primary source of energy is incoming radiation from our Sun. This “income” side of our planet’s energy budget sets the baseline for determining how quickly Earth is warming. When combined with measurements of the total amount of energy that’s emitted and reflected from our planet back into space, this information allows scientists to calculate how Earth’s temperature is changing—and to hone their predictions of what’s most likely to happen to our planet in the future.
Accurately determining how much radiation Earth receives across the electromagnetic spectrum, particularly in ultraviolet, visible-light, and near-infrared wavelengths, is crucial for understanding the rate and degree to which Earth’s climate is changing. But previous measurements of the spectrum of solar radiation were less accurate, increasing uncertainty in global climate models and other Earth Science applications
To address this, the Committee on Earth Observation Satellites has recommended that a new spectrum of solar radiation created by researchers at the Laboratory for Atmospheric and Space Physics (LASP) be used as the world standard. This Hybrid Solar Reference Spectrum is a highly accurate, composite spectrum based on direct measurements by LASP’s Total and Spectral Solar Irradiance Sensor (TSIS-1) Spectral Irradiance Monitor (SIM) instrument, which has been operating aboard NASA’s International Space Station since 2018, and the Compact Spectral Irradiance Monitor (CSIM) technology demonstration mission that operated from 2019 to 2022.
The new composite spectrum includes high spectral-resolution solar lines from databases of observations independently acquired by the Air Force Geophysical Laboratory, the Kitt Peak National Observatory, the Quality Assurance of Ultraviolet Measurements In Europe measurement campaign, and the Jet Propulsion Laboratory’s Solar Pseudo-Transmittance Spectrum.
In March, the committee’s Working Group on Calibration and Validation voted to formally adopt LASP’s TSIS-1 HSRS spectrum as the new global reference. This means the new spectrum will be incorporated into global climate models, such as the National Center for Atmospheric Research (NCAR) Whole Atmosphere Community Climate Model (WACCM), and many more applications used by scientists around the globe.
LASP’s TSIS-1 HSRS spectrum was selected in part due to its unprecedented accuracy. The TSIS-1 SIM and the CSIM instruments underwent careful unit-level calibrations, validation, and final end-to-end absolute calibration in the lab’s state-of-the-art spectral radiometer facility, leading to order-of-magnitude reductions in the uncertainty of direct observations of the Sun’s energy spectrum. The integrated energy in the new spectrum is about 97% of the Sun’s total energy, or total solar irradiance.
LASP scientists are now busy at work extending the TSIS-1 HSRS to span the full electromagnetic spectrum from the far ultraviolet to the longwave infrared, beyond the observing range of the instruments, using independent observations and theoretical understanding. This will increase the integrated energy in the composite spectrum from ~97% to 99.99% of the total solar irradiance and greatly increase its utility for energy budget and climate studies.
Creation of the new spectrum involved many partners, including the support of NASA’s Earth Science Division and Earth Science Technology Office for the TSIS-1 and CSIM missions, respectively. A partnership with the National Institute of Standards and Technology was key to developing both advanced detector technologies and state-of-the-art calibration facilities for these solar instruments.
Written by Terri Cook – Head of LASP’s Office of Communication Management