Massive solar flares captured in 3-D

Distance between orbiting STEREO craft allows better imaging of coronal mass ejections

WASHINGTON — For the first time, scientists can accurately assess the size, shape and speed of massive flares as they leave the sun, allowing better estimates of when the flares might strike Earth and cause widespread electronic disruptions.

FROM BOTH SIDES NOW These images of a coronal mass ejection, a massive solar flare (arrows), are part of a series captured December 12 and 13, 2008. The images allow scientists to assess the size, shape, speed and trajectory of the flare as it leapt into space. NASA

Since early 2007, NASA researchers have been gathering solar data using sensors onboard two craft known as STEREO, the Solar Terrestrial Relations Observatory (SN: 2/10/07, p. 93). One of those golf-cart-sized, 620-kilogram probes now orbits the sun about 50 million miles ahead of Earth, and the other orbits about 50 million miles behind the planet. The broad span between the two craft, like the separation between human eyeballs, only now provides scientists with two sidelong views of the most massive flares, or coronal mass ejections. The resulting three-dimensional depth perception helps to track the flare as it speeds through space.

Previously, data from STEREO enabled scientists to track a coronal mass ejection from the sun to Earth (SN: 3/3/07, p. 133). Now, the distance between the craft has broadened, allowing researchers to also accurately assess the 3-D structure of flares as they develop on their way to Earth, says Angelos Vourlidas, a project scientist at the Naval Research Laboratory in Washington, D.C who works on the STEREO mission. “We can actually see the shape of the material” as it speeds through space, he said at a news conference April 14 at NASA headquarters.

Coronal mass ejections spew billions of tons of charged particles into space. When those eruptions sweep past Earth, they can trigger geomagnetic storms that disrupt radio communications and knock out satellites (SN: 7/31/04, p. 74). Particularly bad episodes of such space weather can pummel electrical grids across large regions and threaten high-flying jets at far-north latitudes, says Michael Kaiser, a project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

Before STEREO, many observations of solar flares came from SOHO, the Solar and Heliospheric Observatory. It orbits the sun and perpetually sits on a direct line between the star and Earth. Because SOHO looks at oncoming solar flares from one position — a spot about 1.5 million kilometers sunward from Earth — it can’t judge those flares’ speeds accurately, Kaiser said at the news conference.

Scientists’ predictions of when a coronal mass ejection might slam Earth were good “only plus or minus 12 hours,” before STEREO data were available, says Kaiser. Now, he notes, using data from STEREO and other craft, impact predictions can be made with as little as two hours’ error and at least 24 hours in advance.

Not only do the STEREO craft have a good view of solar flares, they measure the speed and the composition of the solar wind as the material sweeps by, says Antoinette Galvin, an astrophysicist at the University of New Hampshire in Durham who works on the STEREO mission. That data, as well as the orientation of a flare’s magnetic lines, can’t be discerned from images alone and are vital for assessing how strongly Earth might be affected by a massive solar flare, she notes.

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