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Solar tsunamis move at astronomical speeds

Image: Solar tsunami
A solar tsunami racing across the sun at millions of kilometers per second.
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Images of a tsunami blasting its way through the sun's lower atmosphere have been taken for the first time.

NASA's twin STEREO spacecraft captured one of the massive solar waves in action May 19, 2007, as it moved through four layers of the solar atmosphere. These images and videos, released today, have helped astronomers to revise estimates of the waves' speeds.

Astronomers think that solar tsunamis, initially discovered by the SOHO spacecraft in the late 1990s, are something like the tsunamis in Earth's oceans. Like these monster ocean waves, solar tsunamis are the result of a release of energy that creates a pressure wave that propagates through some kind of medium. On Earth, that medium is ocean water, but on the sun, it is hot, roiling solar gases.

Tsunamis and CMEs
Early on and still today, there are many unknowns about solar tsunamis. The speed of the waves as calculated based on the first SOHO snapshots didn't match up with their estimated intensity. "They seemed to be going very slowly for the amount of energy we saw in the explosion," said study leader Peter Gallagher of Trinity College Dublin. The explosions release about two billion times the annual world's energy consumption in just a fraction of a second.

STEREO's cameras took more images per day than SOHO, so Gallagher and his colleagues were able to more accurately clock the speed of the solar tsunamis at more than 1 million kilometers per hour.

"They're actually traveling a lot faster than we previously thought," Gallagher told LiveScience. "The speeds are astronomical, literally. These things [take] blinks of an eye to traverse the Earth."

STEREO's Extreme Ultraviolet Imager (EUVI) instruments also allow astronomers to monitor the sun at four wavelengths which correspond to temperatures from 60,000 to 2 million degrees Celsius. Each wavelength corresponds to a different layer of the solar atmosphere. To the team's surprise, the tsunami seemed to move just as speedily through dense layers as it did through less dense layers, Gallagher said.

Unclear causes
What causes these giant solar waves isn't clear. Astronomers know they are associated with coronal mass ejections (CMEs) which are like "a rope of gas and magnetic fields that gets accelerated away from the sun," Gallagher explained.

Solar tsunamis could be the shockwave that results from the CME, or they could simply be related phenomena that have a common trigger. But whenever they see a solar tsunami, there's always an associated CME, Gallagher said. "When (a solar tsunami) goes off, it tells you that there's been an explosion on the sun."

This relationship could be important in predicting CMEs, which can launch damaging material at Earth and the other the planets. Gallagher thinks further STEREO observations will help astronomers decide what causes what.

Gallagher and his colleagues will present their findings on April 2 at the Royal Astronomical Society National Astronomy Meeting in Belfast, Northern Ireland.