pulse Report This Comment Date: November 26, 2009 05:21AM
Above: A solar tsunami seen by the STEREO spacecraft from orthogonal points of
view. The gray part of the animation has been contrast-enhanced by subtracting
successive pairs of images, resulting in a "difference movie."
Source: [
science.nasa.gov]
Monster Waves on the Sun are Real 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 90o,
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 (1029 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. (diagram)
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.'"
Right: The dancing prominence (circled). Watch it bounce up and down after
getting hit by a faint but powerful solar tsunami: 4 MB gif animation, 54 MB
Quicktime movie.
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."
Edited 1 time(s). Last edit at 26/11/2009 05:22AM by pulse.
