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John Roach

for National Geographic News

Published April 23, 2010

NASA's new Solar Dynamics Observatory may be getting all the press this week for its retina-searing first pictures of the sun. But two old sun-observing warhorses recently showed they're not quite ready for pasture yet.

The twin, golf cart-size spacecraft of NASA's Solar Terrestrial Relations Observatory (STEREO) mission filmed, in ultraviolet light, the largest solar "prominence" in 15 years, according to the space agency. (See the video above, which compresses about 19 hours of solar activity on April 12 and 13).

Stretching nearly halfway across the sun, the looping eruption of ionized helium "is definitely one of the largest" solar prominences ever witnessed, said Madhulika Guhathakurta, a STEREO program scientist at NASA headquarters in Washington, D.C.

(See more pictures of solar eruptions.)

Solar Eruptions Still Something of a Mystery

The looping, gaseous eruptions are linked to changes in the strength of the sun's magnetic field, though the details of how solar prominences form remain an active area of research, Guhathakurta said.

Often when a prominence is rising above the surface of the sun, the magnetic field becomes so stressed that the gaseous loop "actually erupts, opens up," creating what's known as a coronal mass ejection, expelling "a huge amount of material into space," she said.

Coronal mass ejections can send bursts of charged particles, called solar wind, streaming toward Earth, where they can overload our planet's magnetic shield, knocking out satellite communications and power grids.

(Read "Magnetic-Shield Cracks Found; Big Solar Storms Expected.")

The eruption in the video, though, was directed away from Earth, so the effects on our planet, if any, were negligible, Guhathakurta said.

The detection of the giant eruption, she added, is an indication that, after a few years of subdued solar activity, "we are entering a phase of the solar cycle where activity is gradually picking up."

More: See the first 3-D pictures of the sun >>

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