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March
29, 2007: So you thought Northern Lights were big
in Alaska? "That's nothing," says Randy Gladstone
of the Southwest Research Institute in San Antonio, Texas.
"Jupiter has auroras bigger than our entire planet."
Last
month, Gladstone and colleagues used NASA's Chandra X-ray
Observatory to capture this picture:
Above:
X-ray auroras observed by the Chandra X-ray Observatory overlaid
on a simultaneous optical image from the Hubble Space Telescope.
[More]
The
purple ring traces Jupiter's X-ray auroras. Gladstone calls
them "Northern Lights on steroids. They're hundreds of
times more energetic than auroras on Earth."
Chandra
has observed Jupiter's auroras many times before, but this
recent dataset is exceptional both in length and quality.
Gladstone hopes it will help him solve some mysteries lingering
for almost 30 years.
Jupiter's
auroras were discovered by the Voyager 1 spacecraft in 1979.
A thin ring of light on Jupiter's nightside looked like a stretched-out
version of our own auroras on Earth. But those early photos
merely hinted at the power involved. The real action, astronomers
soon learned, was taking place at high-energy wavelengths invisible
to the human eye. In the 1990s, ultraviolet cameras on the Hubble
Space Telescope photographed raging lights thousands of times
more intense than anything ever seen on Earth, while X-ray observatories
saw auroral bands and curtains bigger than Earth itself.
Jupiter's
hyper-auroras never stop. "We see them every time we
look," says Gladstone. You don't see auroras in Alaska
every time you look, yet on Jupiter the Northern Lights always
seem to be "on."
Gladstone
explains the difference: On Earth, the most intense auroras
are caused by solar storms. An explosion on the sun hurls
a billion-ton cloud of gas in our direction, and a few days
later, it hits. Charged particles rain down on the upper atmosphere,
causing the air to glow red, green and purple. On Jupiter,
however, the sun is not required. "Jupiter is able to
generate its own lights," says Gladstone.
The
process begins with Jupiter's spin: The giant planet turns
on it axis once every 10 hours and drags its planetary magnetic
field around with it. As any science hobbyist knows, spinning
a magnet is a great way to generate a few volts—it's the basic
principle of DC motors. Jupiter's spin produces 10 million
volts around its poles.
"Jupiter's
polar regions are crackling with electricity," says Gladstone,
"and this sets the stage for non-stop auroras."
 The
polar electric fields grab any charged particles they can
find and slam them into the atmosphere. Particles for slamming
can come from the sun, but Jupiter has another, more abundant
source nearby: the volcanic moon Io, which spews oxygen and
sulfur ions (O+ and S+) into Jupiter's
spinning magnetic field.
Right:
A volcano on Io, photographed by New Horizons in Feb. 2007.
[More]
Somehow,
these ions make their way to Jupiter's poles where electric
fields send them hurtling toward the planet below. Upon entering
the atmosphere, "their electrons are first stripped away
by molecules they run into, but as they slow down they start
grabbing electrons back. The 'charge exchange reaction' produces
intense X-ray auroras," he explains.
So
Jupiter's Northern Lights are, in a sense, volcano powered.
Mystery solved? Not quite.
No
one knows exactly how volcanic exhaust meanders from Io out
through Jupiter's magnetosphere and back to Jupiter's poles.
"We're still trying to figure it out," says Gladstone.
But
that is a minor detail compared to another, even bigger puzzle:
There is an X-ray "pulsar" inside Jupiter's northern
auroras. Sometimes Chandra sees it, sometimes not. When it's
on, the pulsar emits gigawatt bursts of X-rays with a regular
beat of 45 minutes.
 Right:
X-ray flashes from Jupiter's north pole. [More]
Gladstone
suspects the pulsar has nothing to do with Io's volcanoes,
but instead is caused by the sun. "Maybe Jupiter's magnetic
field, when it gets hit by a solar wind gust, rings like a
bell with a 45-minute period," he speculates. "There
are many other possibilities as well."
The
February 2007 dataset may hold important clues. "Chandra
observed the auroras for 15 hours, and we weren't the only
ones watching," he says. The Hubble Space Telescope,
the FUSE satellite, XMM-Newton (a European X-ray observatory),
the New Horizons spacecraft and many ground-based observatories
were all taking data at the same time. The campaign was timed
to coincide with New Horizons flyby of Jupiter—a slingshot
maneuver designed to increase its velocity en route to Pluto.
"Jupiter's
auroras have never been observed by so many telescopes at
once," says Gladstone. "I'm really excited by these
data, and the analysis is just beginning."
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NASA's
Marshall Space Flight Center, Huntsville, Ala., manages the
Chandra program for the agency's Science Mission Directorate.
The Smithsonian Astrophysical Observatory controls science
and flight operations from the Chandra X-ray Center, Cambridge,
Mass.
Author: Dr.
Tony Phillips | Production Editor:
Dr. Tony Phillips | Credit: Science@NASA
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