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- The force fields that
surround all respectable intergalactic
space ships are wimpy imposters
of what Nature created long ago, namely
the protective magnetic shield
around our planet, fueled by mysterious
processes deep within.
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- Until recently, we've
known nearly as much about the
source of the force fields around
fictional star ships as we've known about
the natural one that
envelopes Earth.
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- But advances in computer modeling, discussed in tomorrow's
issue of the journal Nature, are helping scientists grasp how the planet
continually regenerates its invisible, protective magnetic shield, and
the work confirms earlier suggestions that the Earth's mantle indeed plays
a role in fueling the magnetism.
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- Further, the new simulations
provide fresh insight into
how the magnetic north and south poles might
migrate and, over time, even
trade places.
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- Earth, the Giant Magnet
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- Magnetic energy
generated in the Earth's core results
in a geomagnetic field, the thing
that makes compass navigation possible
while at the same time
deflecting and absorbing harmful solar radiation.
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- The solid inner core, mostly
iron, is surrounded by a
more fluid outer core, which is mostly molten
iron. The interaction of
these two regions, in which material flows at
different rates, creates
what scientists call a "hydromagnetic
dynamo," something like
an electric motor that results in a
magnetic field akin to a giant bar
magnet.
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- Invisible geomagnetic lines
stretch from one pole, curving
far out into space and back to the
opposing pole. Left alone, the curved
lines might appear like a
wire-frame model of some giant pumpkin, but the
electrically charged
solar wind blows the pumpkin into a teardrop shape.
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- As best as researchers can
tell, this protective field
has existed in various forms for at least 3
billion years, periodically
growing stronger and weaker, shifting
around, and on a few occasions even
flipping its polarity entirely.
Scientists examine old rocks for indications
of previous reversals,
which leave directional clues in solidified magnetic
minerals.
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- At times, the field
is thought to weaken to practically
nothing, a state that would leave
earthlings exposed to high doses of solar
radiation.
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- Forces of
Change
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- How
all this erratic behavior occurs is not well understood.
But radical
changes seem to take place in as little as a thousand year's
time,
after which relative stability appears to reign for another 200,000
years or so. One thing is clear, researchers say: If something did not
continually regenerate the magnetic field, it would degenerate to zero
and our planet would be left naked to the hideous power of solar
radiation.
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- Gary A. Glatzmaier of the University of California, Santa
Cruz,
has some new ideas about how this magnetism is continually reproduced.
Scientists have typically assumed that the magnetism was all created in
Earth's core, and in fact an earlier rendition of Glatzmaier's computer
model -- which he's been working on for two decades -- helped to
illustrate
that process. But a new version of the simulation indicates
that the next
layer up, the mantle, can also play a role.
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- Glatzmaier and his
colleagues used a tremendous amount
of computing power to test a
relatively simple idea: Where the core and
the mantle meet, varying
rates of heat exchange might create changes in
the magnetic
field.
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- "They note that variations in heat flow at the boundary
induce a flow in the core, much like atmospheric circulation driven by
differential solar heating of the Earth's surface," says Bruce
Buffett
of the Department of Earth and Ocean Sciences at the University
of British
Columbia. "The calculations are still crude
approximations of the
real Earth, but they lend considerable support to
the argument that the
mantle has an important role."
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- Buffett, who was not
involved in the research, told space.com
that one of the most
significant results of the work is that Glatzmaier
and his colleagues
were able to simulate conditions that might cause a
magnetic
reversal.
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- While much about Earth's magnetism is mysterious, complete
reversals of polarity -- and the slightly less dramatic
"excursions,"
or wanderings -- are among the most intriguing
of terrestrial puzzles.
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- "The current results show how the temperature
patterns
on the lower mantle (above the core) can either increase or
decrease the
probability of magnetic reversals," Glatzmaier said.
He notes that
by feeding varying temperature differences into the
model, the duration
of the events and the intensity of the magnetic
field were altered.
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- Reversal of Fortune?
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- Earth's magnetic field does not
swap directions on any
set time frame. While reversals occur every
200,000 years on average, the
last one was 780,000 years ago,
Glatzmaier said, adding that "we're
overdue for one."
Excursions of the magnetic field, in which the poles
wander
significantly but do not reverse, occur more frequently.
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- Because it seems to
take centuries for a reversal to
occur, researchers agree that the next
one is not imminent (from the perspective
of a human lifetime, at
least). Geologically speaking, however, these things
happen in the
blink of an eye, and signals currently point to a change.
Earth's
magnetic field is weakening as you read this, part of a long-term
process that might be leading up to a complete magnetic reversal,
researchers
say.
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- If and when this occurs, the further erosion of the
protective
magnetic shield will allow higher doses of cosmic radiation
to bombard
the planet. How our species (and others) might adapt is an
open question.
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- "Clearly, our earliest ancestors have survived reversals
in the past," Buffett said. "The most obvious consequence is
that you'd need a new compass."
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