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March
6, 2008: Imagine living on a planet where Northern
Lights fill the heavens at all hours of the day. Around the
clock, even in broad daylight, luminous curtains shimmer and
ripple across the sky, mesmerizing anyone who bothers to look.
 News
flash: Astronomers have discovered such a planet. Its name
is Earth.
"Our
own planet has auroras 24 hours a day," says Jim Spann
of the Marshall Space Flight Center, "and we can see
them even in broad daylight." The trick, he explains,
is picking the right wavelength. "If we look at Earth
from space using an ultraviolet (UV) filter, we see there
are auroras underway at all times. It is a beautiful sight."
Right:
An artist's concept of NASA's Polar satellite observing Earth.
[More]
The
extent of the phenomenon was revealed twelve years ago by
NASA's Polar satellite. As its name suggests, Polar orbits
over Earth's poles, slowly arcing as high as 57,000 km above
the arctic where it can take a good long look at the auroras
below. Polar is equipped with special UV filters that allow
it to photograph auroras through the glare of sunlight and
"we were amazed to see Northern and Southern Lights active
non-stop," says Spann, one of the scientists who led
development of the satellite's UV instrumentation.
As
an example he offers this
movie of a recent auroral display over Antarctica. Just
before the video, a solar wind gust buffeted Earth's magnetic
field causing a mild geomagnetic storm. Visible light auroras—the
kind we see with the human eye--were weak, but "Polar's
UV camera picked up a magnificent outburst," says Spann.
Above:
Click on the image to play a 2 MB movie of UV auroras over
Antarctica. [More]
Watch
the movie again;
it highlights an ancient mystery. Long ago, arctic sky watchers
noticed that mild auroras—the ho-hum kind they saw almost
every night--would sometimes erupt without warning in a stunning
riot of light and color. 20th-century scientists called these
events, with some understatement, "substorms." A
good substorm can unleash a hundred thousand billion (1014)
Joules of energy, as much as a magnitude 5 earthquake. Although
auroras, generally speaking, are understood (they are caused
by solar activity), the sudden power of substorms is one of
the biggest mysteries of space science.
The
Antarctic outburst is a classic substorm. "We see plenty
of them at UV wavelengths," says Spann. "Polar's
ability to monitor both night and day allows us to catch substorms
that other satellites might miss."
This
ability is being put to good use. Polar is now assisting THEMIS,
a fleet of five spacecraft launched by NASA in Feb. 2007 to
solve the mystery of the substorm. What triggers the events?
Where does their power come from? These are some of the questions
THEMIS has set out to answer.
The
five THEMIS satellites are equipped with sensors to map the
complex ebb and flow of particles and fields in Earth's magnetosphere.
(The magnetosphere is a vast magnetic bubble around Earth.
It is the "force field" that protects us from solar
wind, and which lights up with auroras when solar wind gusts
come crashing into it.) While a single spacecraft might be
confused by the magnetosphere's suddenly shifting electrodynamics,
the THEMIS quintet, working together, is able make sense out
of very complicated events. Researchers hope this will lead
to an understanding of the substorm phenomenon.
Polar
is valuable because no other spacecraft can match its global
view of the auroras. "We provide the big picture while
THEMIS sorts out the crucial details."
At
an age of 12 years, Polar is well past its design lifetime.
"It's amazing that we're still in business," notes
Spann. Moreover, by joining THEMIS, the veteran spacecraft
is on the verge of new discovery.
Even
for Polar, however, the auroras must stop sometime. During
the writing of this story, the spacecraft ran out of fuel,
limiting its ability to track Earth's poles. Nevertheless,
mission planners believe they can squeeze another one or two
month's worth of observing out of Polar in support of THEMIS.
Its final images may be key pieces to the auroral puzzle.
Farewell,
Polar--and thanks for all the substorms!
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Author: Dr.
Tony Phillips | Credit: Science@NASA
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