The Sun-Earth Connection: NCAR Scientist Quantifies Variations In Sun's Radiation

February 11, 1998

BOULDER--Until now, there's been a lot of uncertainty about the way the sun's radiation varies over time. Scientist Peter Fox and colleagues at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, are using precise observations, theoretical atomic physics, and computer modeling to get the best representation so far of the total radiative output of the sun (including the ultraviolet, visible light, and infrared wavelengths that reach the earth). "This is a step towards accurately quantifying what the entire sun is doing, which will give us a better understanding of the sun's influence on earth's climate," says Fox. NCAR's primary sponsor is the National Science Foundation.

Fox presented the findings of his research (with NCAR's Oran White, Juan Fontenla of GTE, and Eugene Avrett of the Smithsonian Astrophysical Observatory) at the annual meeting of the American Association for the Advancement of Science, in Philadelphia on February 14.

Previous methods relied on a single parameter estimate of the sun's activity (the flux in the 10.7-centimeter radio waveband) to represent radiative changes in the sun. Those methods are based on correlations between the sun's activity and radiative variations. "However," explains Fox, "the physical connections that the correlations imply have not been established; these relations are purely statistical. So the use of the parametric representation has no physical basis--it's an indirect relationship."

The NCAR team's breakthrough is the ability to represent more accurately the physical processes of the entire surface of the sun. They do this by taking a detailed look at how individual features, like sunspots, change the amounts of radiation in several different wavelengths. This allows them to estimate all of the radiation reaching the earth over several different wavelengths as the physical state of the sun continually changes.

The team uses observations from the precision solar photometric telescope (PSPT), combined with computer modeling that incorporates prior observations and solar physics theory. The PSPT is operated by NCAR's High Altitude Observatory at its Mauna Loa Solar Observatory in Hawaii. The PSPT project is part of the National Science Foundation's Radiative Input from the Sun to the Earth (SunRISE) program.

"Tree rings and ice cores tell us a little about variations in the sun over centuries. But we have only the 11-year sunspot record for the last 300 years to link directly to today's sun, and we don't have direct measurements of the radiation before the 1970s," Fox explains. New information from the team will help scientists understand the effects of solar variability on the chemistry of the upper atmosphere, including the ozone layer, the shield protecting the earth from too much ultraviolet radiation. The new techniques will also provide better information to scientists modeling the earth's climate by providing more accurate data about the sun's infrared radiation. In addition, the green and yellow bands in the visible light spectrum are now believed to affect ocean circulation. The team expects to make the first accurate estimates of solar radiation in those bands, adding important information on the link between the oceans and the atmosphere.

NCAR is managed by the University Corporation for Atmospheric Research (UCAR). UCAR is a consortium of more than 60 universities offering Ph.D.s in the atmospheric or related sciences.
Writer: Zhenya Gallon

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