Mar. 13, 2000 A week's warning of potential bad weather in space is now possible thanks to a new use of the Solar and Heliospheric Observatory (SOHO) spacecraft. Two astrophysicists supported by the National Science Foundation (NSF) have developed a technique to image explosive regions on the far side of the sun, using ripples on the sun's surface to probe its interior.
"This is a major breakthrough in our ability to forecast solar storms that can affect us on earth," said Morris Aizenman of NSF's Astronomical Sciences Division.
Explosive regions on the sun are hidden until they rotate to the side of the sun visible from earth, giving little advance warning. The new imaging technique uses computer modeling developed since the early 1990s with support from NSF and observations taken with the Michelson Doppler Imager (MDI) on NASA's SOHO satellite to detect and locate these hidden solar storms.
"We've known for 10 years that in theory we could make the sun transparent all the way to the far side," said Charles Lindsey of Solar Physics Research Corp. in Tucson, Ariz. "But we needed observations of exceptional quality. In the end we got them, from MDI on SOHO." Lindsey and his colleague Douglas Braun of NorthWest Research Associates, Boulder, Colo., describe the research in the March 10 edition of Science.
Active regions on the sun are often the sites of spectacular explosive events, called solar flares, which are associated with eruptions of plasma (hot, electrically charged gas). The radiation and plasma from these events sweep past the earth and can disrupt spacecraft, radio communications and power systems. Scientists watch closely for these eruptions because modern systems are increasingly sensitive to solar disturbances. But experts can still be taken by surprise as the sun rotates, bringing hidden active regions into view.
To locate these regions in advance, the scientists developed a technique of using ripples on the sun's surface to image the interior. The ripples are caused by sound waves reverberating through the sun. Analysis of these solar sound waves, a science known as helioseismology, has opened the sun's gaseous interior to investigation in much the same way as seismologists learned to explore the earth's rocky interior through the analysis of earthquake waves.
Lindsey and Braun's technique examines sound waves that emanate from the far side of the sun and reach the near side by rebounding internally from the solar surface. They used observations from MDI taken on March 28-29, 1998, to detect a group of sunspots on the far side of the sun that was not visible on the near side until 10 days later.
SOHO is a cooperative project between the European Space Agency and NASA. The far-side helioseismology research was funded by NSF and NASA.
Editors: Images are available at: http://umbra.nascom.nasa.gov/soho/ssu/rightthrough.html
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