Incredibly powerful waves of plasma rippling across the
surface of the sun and dubbed "solar tsunamis" were first observed
years ago, but were thought to be an optical illusion. Scientists have now
confirmed, though, that they are really real.
When scientists first saw the phenomenon, it was hard to
believe that a towering wave of hot plasma was actually racing along the sun's
surface. One of the waves rose up higher than the diameter of Earth and rippled
out from a central point in a circular pattern millions of miles wide, like a
gargantuan pattern of waves created by a pebble dropped in a pond.
Skeptical observers suggested it might be a shadow of some
kind – a trick of the eye. But new observations from NASA's STEREO (Solar
Terrestrial Relations Observatory) spacecraft are telling researchers that this
controversial phenomenon isn't an illusion.
This week, NASA released a remarkable video
of a solar tsunami.
"Now we know," said Joe Gurman of the Solar
Physics Lab at NASA's Goddard Space Flight Center in Greenbelt, Md. "Solar
tsunamis are real."
Reality confirmed
The twin STEREO spacecraft confirmed their reality in images
captured in February when sunspot 11012 unexpectedly erupted. The blast hurled
a billion-ton cloud of gas (a coronal mass
ejection, or "CME") into space and sent a tsunami racing along
the sun's surface.
STEREO recorded the wave from two positions separated by 90
degrees, giving researchers an unprecedented view of the event.
"It was definitely a wave," said Spiros
Patsourakos of George Mason University in Virginia and lead author of a paper
reporting the finding in the Astrophysical Journal Letters. "Not a wave of
water," he adds, "but a giant wave of hot plasma and magnetism."
The technical name is "fast-mode magnetohydrodynamical
wave" – or "MHD wave" for short. The one STEREO saw reared up
about 62,000 miles (100,000 km) high, and raced outward at 560,000 mph (250
km/s) packing as much energy as 2,400 megatons of TNT.
Solar tsunamis were discovered
back in 1997 by the Solar and Heliospheric Observatory (SOHO). In May of
that year, a CME blasted up from an active region on the sun's surface, and SOHO recorded a tsunami rippling away from the blast site.
"We wondered," Gurman recalled, "is that a
wave – or just a shadow of the CME overhead?"
Stereo view
SOHO's single point of view was not enough to answer the
question–neither for that first wave nor for many similar events recorded by SOHO in years that followed, until STEREO launched in 2006. The mission uses two spacecraft
— one orbiting the sun ahead of the Earth, the other behind it — to get,
literally, a stereo view of the sun.
"We've seen the waves reflected by coronal holes
(magnetic holes in the sun's atmosphere)," Vourlidas said. "And there
is a wonderful movie of a solar prominence oscillating after it gets hit by a wave.
We call it the 'dancing prominence.'"
Solar tsunamis pose no direct threat to Earth. Nevertheless,
they are important to study, scientists say.
"We can use them to diagnose conditions on the
sun," Gurman said. "By watching how the waves propagate and bounce
off things, we can gather information about the sun's lower atmosphere
available in no other way."
"Tsunami waves can also improve our forecasting of
space weather," Vourlidas added, "Like a bull-eye, they 'mark the
spot' where an eruption takes place. Pinpointing the blast site can help us
anticipate when a CME or radiation storm will reach Earth."
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