Monster Waves on the Sun are Real (STEREO)
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http://science.nasa.gov/headlines/y2009/24nov_solartsunami.htm
Monster Waves on the Sun are Real
NASA Science News
11.24.2009
November 24, 2009: Sometimes you really can believe your eyes.
That's what NASA's STEREO (Solar Terrestrial Relations Observatory)
spacecraft are telling researchers about a controversial phenomenon on
the sun known as the "solar tsunami."
Years ago, when solar physicists first witnessed a towering wave of hot
plasma racing along the sun's surface, they doubted their senses. The
scale of the thing was staggering. It rose up higher than Earth itself
and rippled out from a central point in a circular pattern millions of
kilometers in circumference. Skeptical observers suggested it might be a
shadow of some kind - a trick of the eye - but surely not a real wave.
"Now we know," says Joe Gurman of the Solar Physics Lab at the Goddard
Space Flight Center. "Solar tsunamis are real."
The twin STEREO spacecraft confirmed their reality in February 2009 when
sunspot 11012 unexpectedly erupted. The blast hurled a billion-ton cloud
of gas (a "CME") into space and sent a tsunami racing along the sun's
surface. STEREO recorded the wave from two positions separated by 90^o ,
giving researchers an unprecedented view of the event:
"It was definitely a wave," says Spiros Patsourakos of George Mason
University, 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 100,000 km high, and
raced outward at 250 km/s (560,000 mph) packing as much energy as 2400
megatons of TNT (10^29 ergs).
Solar tsunamis were discovered back in 1997 by the Solar and
Heliospheric Observatory (SOHO). In May of that year, a CME came
blasting up from an active region on the sun's surface, and SOHO
recorded a tsunami rippling away from the blast site.
"We wondered," recalls Gurman, "is that a wave - or just a shadow of the
CME overhead?"
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.
The question remained open until after the launch of STEREO in 2006. At
the time of the February 2009 eruption, STEREO-B was directly over the
blast site while STEREO-A was stationed at right angles - "perfect
geometry for cracking the mystery," says co-author Angelos Vourlidas of
the Naval Research Lab in Washington DC.
The physical reality of
the waves has been further confirmed by movies of the waves crashing
into things. "We've seen the waves reflected by coronal holes (magnetic
holes in the sun's atmosphere)," says Vourlidas. "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. "We can use them to diagnose conditions on the sun,"
notes Gurman. "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," adds
Vourlidas, "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."
And they're pretty entertaining, too. "The movies," he says, "are out of
this world."
Author: Dr. Tony Phillips <mailto:james.a....@earthlink.net> |
Credit: Science@NASA <http://science.nasa.gov>