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Dec.
11, 2007: NASA's fleet of THEMIS spacecraft, launched
less than 8 months ago, has made three important discoveries
about spectacular eruptions of Northern Lights called "substorms"
and the source of their power. The discoveries include giant
magnetic ropes that connect Earth's upper atmosphere to the
Sun and explosions in the outskirts of Earth's magnetic field.
 "The
mission is only beginning but THEMIS is already surprising
us," says Vassilis Angelopoulos the mission's principal
investigator at the University of California, Los Angeles.
The
discoveries began in March less than a month after the five
THEMIS satellites had been activated. "On March 23, 2007,
a substorm erupted over Alaska and Canada
producing vivid auroras for more than two hours." A network
of ground cameras organized to support THEMIS photographed
the display from below while the satellites measured particles
and fields from above.
Right:
Auroras over Alaska on March 23-24, 2007. Photo credit: Daryl
Pederson. [More]
Right
away the substorm surprised investigators: "The auroras
surged westward twice as fast as anyone thought possible,
crossing 15 degrees of longitude in less than one minute,"
says Angelopoulos. The storm had traversed an entire polar
time zone in 60 seconds flat!
Also,
"the display was surprisingly bursty." Photographs
taken by ground cameras and NASA's Polar satellite (also supporting
the THEMIS mission) revealed a series of staccato outbursts
each lasting 10 minutes or so. "Some of the bursts died
out while others reinforced each other and went on to become
major events."
Scientists
have been tracking and studying substorms for more than a
century, yet these phenomena remained mostly unknown until
THEMIS went into action.
Even
more impressive was the substorm's power. Angelopoulos estimates
the total energy of the two-hour event at five hundred thousand
billion (5 x 1014) Joules. That's approximately
equivalent to the energy of a magnitude 5.5 earthquake.
Where
does all that energy come from? THEMIS may have found an answer:
"The
satellites have found evidence for magnetic ropes connecting
Earth's upper atmosphere directly to the Sun," says Dave
Sibeck, project scientist for the mission at the Goddard Space
Flight Center. "We believe that solar wind particles
flow in along these ropes, providing energy for geomagnetic
storms and auroras."
 A
"magnetic rope" is a twisted bundle of magnetic
fields organized much like the twisted hemp of a mariner's
rope. Spacecraft have detected hints of these ropes before,
but a single spacecraft is insufficient to map their 3D structure.
THEMIS's five satellites were able to perform the feat.
Right:
A magnetic map of a magnetospheric rope observed in cross-section
by the THEMIS satellites on May 20, 2007. [Larger
image]
"THEMIS
encountered its first magnetic rope on May 20, 2007,"
says Sibeck. "It was very large, about as wide as Earth,
and located approximately 40,000 miles above Earth's surface
in a region called the magnetopause."
The magnetopause is where the solar wind and Earth's magnetic
field meet and push against one another like sumo wrestlers
locked in combat. There, the rope formed and unraveled in
just a few minutes, providing a brief but significant conduit
for solar wind energy. Other ropes quickly followed: "They
seem to occur all the time," says Sibeck.
THEMIS
has also observed a number of relatively small explosions
in Earth's magnetic bow
shock. "The bow shock is like the bow wave in front
of a boat," explains Sibeck. "It is where the solar
wind first feels the effects of Earth's magnetic field."
When a knot of magnetism within the solar wind hits the bow
shock--"Bang!" he says. "We get an explosion."
The
technical term for these explosions is "hot flow anomalies"
or HFAs. HFAs boost the temperature of solar wind particles
ten-fold (as high as 10 million degrees) and they can stop
the solar wind dead its tracks. "This is no mean achievement
considering the fact that the solar wind moves at supersonic
speeds near a million miles per hour."
Above:
A cartoon of a hot flow anomaly observed by THEMIS on July
4, 2007, and a computer simulation of the explosion. Credit:
N. Omidi. [More]
"Hot
flow anomalies may not play a major role in energizing auroral
substorms--they happen too infrequently, less than once a
day," notes Jonathan Eastwood of the University of California,
Berkeley, who is studying them. "Nevertheless they are
of interest. This is a fundamental physical process that accelerates
particles to high energies and we are delighted to be able
to study it."
Powerful
substorms, giant magnetic ropes, explosions that stop the
solar wind in its tracks: "We have much more to learn
about all these things," says Angelopoulos. "I
can't wait to see what comes next."
For
more information about THEMIS, visit http://www.nasa.gov/themis/.
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Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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