A satellite designed, built and controlled at the University of Colorado at Boulder for NASA has returned data indicating both Venus and sunspots have something in common -- they block some of the sun's energy going to Earth.
Data from The Solar Radiation and Climate Experiment, or SORCE, satellite show when the shadow of Venus tracked across the face of the sun June 8 as the planet passed between Earth and the sun, the amount of sunlight reaching Earth decreased by 0.1 percent, said CU-Boulder's Gary Rottman.
The SORCE team saw similar reductions in October 2003 when three sunspots moving across the sun's face dimmed solar radiation coming toward Earth by 0.3 percent, he said. Rottman is the principal investigator for the SORCE mission and a senior research associate at CU-Boulder's Laboratory for Atmospheric and Space Physics.
"Because of its distance from Earth, Venus appears to be about the size of a sunspot," he said. Venus transits come around every 122 years, when two transits occur eight years apart like the one in 2004 that will occur again in 2112.
"This is an unprecedented large decrease in the amount of sunlight," said Rottman, who noted it is comparable to the solar decrease that scientists believe occurred in the 17th century during the so-called Little Ice Age. "That decrease lasted almost 50 years, and was likely associated with the exceptionally cold temperatures throughout Europe at that time."
Solar conditions during the Little Ice Age were quite different because there essentially were no sunspots, he said. Astronomers of the time, including Galileo, kept a good record of sunspot activity before and during the period, encountering only about 50 sunspots in 30 years.
"Something very different was happening during the 17th century, and it produced a much more permanent change in the sun's energy output at the time," said Rottman. Today, large sunspots are surrounded by bright areas known as faculae, which more than compensate for the decrease in sunlight from sunspots and provide a net increase in sunlight when averaged over a few weeks, he said.
A larger number of sunspots occurring in October and November 2003 indicated a very active sun accompanied by many very large solar flares, Rottman said. The massive, record-setting solar flares in the X-ray portion of the light spectrum were accompanied by large sunspots, which produced the 0.3 percent decrease in the sun's energy output.
SORCE can simultaneously collect solar energy from the soft X-ray, ultraviolet, visible and near-infrared wavelengths of the light spectrum, something that has never been done before, said SORCE Program Manager Tom Sparn of LASP.
Part of NASA's Earth Observing System, the $88 million satellite is collecting a long-term data set of natural solar variation as a tool to determine the role the sun plays in global change. SORCE was launched in January 2003 and is controlled by researchers and students from LASP's CU Research Park facility.
A performance evaluation of the SORCE mission last June by NASA rated it excellent in all categories in a performance evaluation of the mission, including quality, timeliness, cost and leadership -- the highest ratings given, Sparn said.
Such precise knowledge of variations in the sun's energy input to Earth is a necessary prerequisite to understanding Earth's changing climate, said Robert F. Calahan, a SORCE project scientist and head of the Climate and Radiation Branch at NASA's Goddard Space Flight Center in Greenbelt, Md.
The SORCE measurements not only provide atmospheric and climate scientists with essential information on the sun's current energy output to the Earth, they also will be valuable to future scientists, said Rottman. Likewise, Galileo's findings about the sun almost 400 years ago have increased in value as understanding of the sun and its importance for Earth has advanced.
For more SORCE information and images on the Web, visit the NASA site at: , and the LASP site at: .