Satellite Measures Sun's True Power

Scientists expect to gain important information about the sun's effect on the Earth's atmosphere and climate from sophisticated instruments—all calibrated by NIST or relying on components measured by NIST—launched on a satellite January 25.

The Solar Radiation and Climate Experiment (SORCE), a satellite mission of the National Aeronautics and Space Administration, has a five-year assignment to provide precise and accurate daily measurements of the sun's radiant power (or light intensity). The satellite will collect data on both total light output and amount of optical radiation at particular wavelengths in the ultraviolet (UV) to near-infrared parts of the electromagnetic spectrum. The mission will measure variations in solar irradiance much more accurately than ever before and observe some of its spectral properties—the relative intensity of the different wavelengths—for the first time.

Two NIST-calibrated instruments will take spectral readings of the UV intensity. Performing such measurements by satellite is essential because the solar UV radiation is mostly absorbed by the Earth's atmosphere and thus is not well characterized by ground-based measurements. Even small variations in the sun's radiation at these wavelengths lead to significant changes in atmospheric chemistry.

NIST also calibrated an assortment of small photodiodes for another instrument that will measure extreme ultraviolet radiation (EUV) and low-energy X-rays. EUV radiation exhibits dramatic fluctuations. These data will enhance understanding of the sun's corona (outer atmosphere), solar events that affect satellite communications, and other phenomena. For additional instruments, NIST performed accurate measurements of the areas of optical apertures, which are used to define the conditions for various solar irradiance measurements. These apertures provide a means for determining the total solar optical radiation output from sampling of a small fraction of the radiation.

The calibrations were performed at NIST's Synchrotron Ultraviolet Radiation Facility (SURF III), which offers specialized beamlines with unique calibration equipment and a highly accurate radiometric standard. NIST previously calibrated solar UV irradiance instruments that were launched on a satellite in 1991 and continue to make measurements today. That mission has focused on ozone chemistry in the Earth's atmosphere and has provided data on the solar UV irradiance, clarifying how and why the sun's output varies over time scales from minutes to years.

More information about the mission can be found at http://lasp.colorado.edu/sorce/index.html.