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July
24, 2008: Duck! Plasma bullets are zinging past Earth.
That's
the conclusion of researchers studying data from NASA's five
THEMIS spacecraft. The gigantic bullets, they say, are launched
by explosions 1/3rd of the way to the Moon and when they hit
Earth—wow. The impacts spark colorful outbursts of Northern
Lights called "substorms."
Right:
A substorm of Northern Lights photographed from the window
of an airplane over Hudson Bay, Canada, on Feb 27, 2008. Credit:
Jeff Hapeman. [more]
"We
have discovered what makes the Northern Lights dance,"
declares UCLA physicist Vassilis Angelopoulos, principal investigator
of the THEMIS mission. The findings appear online in the July
24 issue of Science Express and in print August 14
in the journal Science.
The
THEMIS fleet was launched in February 2007 to unravel the
mystery of substorms, which have long puzzled observers with
their unpredictable eruptions of light and color. The spacecraft
wouldn't merely observe substorms from afar; they would actually
plunge into the tempest using onboard sensors to measure particles
and fields. Mission scientists hoped this in situ
approach would allow them to figure out what caused substorms--and
they were right.
The
discovery came on what began as a quiet day, Feb 26, 2008.
Arctic skies were dark and Earth's magnetic field was still.
High above the planet, the five THEMIS satellites had just
arranged themselves in a line down the middle of Earth’s magnetotail—a
million kilometer long tail of magnetism pulled into space
by the action of the solar wind.
That's
when the explosion occurred.
A
little more than midway up the THEMIS line, magnetic fields
erupted, "releasing about 1015 Joules of energy,"
says Angelopoulos. "For comparison, that's about as much energy
as a magnitude 5 earthquake."
Although
the explosion happened inside Earth's magnetic field, it was
actually a release of energy from the sun. When the solar
wind stretches Earth's magnetic field, it stores energy there,
in much the same way energy is stored in a rubber band when
you stretch it between thumb and forefinger. Bend your forefinger
and—crack!—the rubber band snaps back on your thumb. Something
similar happened inside the magnetotail on Feb. 26, 2008.
Over-stretched magnetic fields snapped back, producing a powerful
explosion. This process is called "magnetic reconnection"
and it is thought to be common in stellar and planetary magnetic
fields.
The
blast launched two "plasma bullets," gigantic clouds
of protons and electrons, one toward Earth and one away from
Earth. The Earth-directed cloud crashed into the planet below,
sparking vivid auroras observed by some 20 THEMIS ground stations
in Canada and Alaska. The opposite cloud shot harmlessly into
space, and may still be going for all researchers know.
Above:
An artist's concept of the THEMIS satellites lined
up inside Earth's magnetotail with an explosion between the
4th and 5th satellites. [Larger
image]
The
THEMIS satellites were perfectly positioned to catch the shot.
"We
had bulls-eyes on our solar panels," says THEMIS project
scientist David Sibeck of NASA's Goddard Space Flight Center.
"Four of the satellites were hit by the Earth-directed
cloud, while the opposite cloud hit the fifth satellite."
Simple geometry pinpointed the site of the blast between the
4th and 5th satellite or "about 1/3rd of the way to the
Moon."
No
damage was done to the satellites. Plasma bullets are vast,
gossamer structures less dense than the gentlest wisp of Earth's
upper atmosphere. They whoosh past, allowing THEMIS instruments
to sample the cloud’s internal particles and fields without
truly buffeting the satellite.
This
peaceful encounter on the small scale of a spacecraft, however,
belies the energy deposited on the large scale of a planet.
The bullet-shaped clouds are half as wide as Earth and 10
times as long, traveling hundreds of km/s. When such a bullet
strikes the planet, brilliant auroras and geomagnetic storms
ensue.
Right:
A collection of ground-based All-Sky Imagers (ASI) captures
the aurora brightening caused by a substorm. Credit: NASA/Goddard
Space Flight Center Scientific Visualization Studio. [animation]
"For
the first time, THEMIS has shown us the whole process in action—from
magnetic reconnection to aurora borealis," says Sibeck.
"We are finally solving the puzzle of substorms."
The
THEMIS mission is scheduled to continue for more than another
year, and during that time Angelopoulos
expects to catch lots more substorms--"dozens of them,"
he says. "This will give us a chance to study plasma
bullets in greater detail and learn how they can help us predict
space weather."
"THEMIS
is not finished making discoveries," believes Sibeck.
"The best may be yet to come."
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Author: Dr.
Tony Phillips | Credit: Science@NASA
more
information |
THEMIS
home page
THEMIS
is the fifth medium-class mission under NASA's Explorer
Program. The program, managed by The Explorers Program
Office at Goddard Space Flight Center, Greenbelt, Md.,
provides frequent flight opportunities for world-class
space investigations in Heliophysics and Astrophysics.
The University of California, Berkeley's Space Sciences
Laboratory managed the project development and is currently
operating the THEMIS mission. Swales Aerospace, Beltsville,
Md., built the THEMIS satellites.
NASA's
Future: US
Space Exploration Policy
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