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The Sciences

98. Twin Probes Watch Sun’s Fury in 3-D

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Astronomers got a new perspective on the sun in April, when NASA’s Solar Terrestrial Relations Observatory (STEREO) probes began sending back the first three-dimensional images of our nearest star. NASA built the twin spacecraft to learn more about coronal mass ejections, or CMEs—billion-ton spitballs of electrically charged particles that sporadically fire off from the sun. When CMEs slam into Earth, their electric fields can blow out the circuits of communications satellites or overload regional power grids. “Anything that’s electromagnetic can be affected by their charged particles,” says NASA astrophysicist Madhulika Guhathakurta, a program scientist for STEREO.

Despite their destructive power, CMEs are so wispy that they are hard to observe without blotting out the sun’s light, and seeing them from only one vantage point, such as the Earth, makes it difficult to determine their 3-D structure. “All you’re seeing is stuff that’s moving across the plane of the sky, like the shadow of a smoke ring,” Guhathakurta says. “If you want to model it—if you want to know its mass, if you want to know its velocity—you need a three-dimensional view.”

In addition to providing 3-D images of solar eruptions and the sun’s surface, STEREO will help space-weather forecasters figure out which CMEs are likely to make earthfall; this may extend the warning time for space squalls—now only hours—to several days.

The new spacecraft are also clarifying the broader relationship between the sun and the rest of the solar system. “We literally live in the outer atmosphere of the sun,” Guhathakurta says.

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