Magnetic North Pole Shifts(Forces
Runway Closures at Florida Airport)
By Jeremy A. Kaplan Published
January 06, 2011
|
FoxNews.com
Tampa International
Airport was forced to readjust its runways Thursday to account for the
movement of the Earth's magnetic fields, information that pilots rely upon
to navigate planes. Thanks to the fluctuations in the force, the airport has
closed its primary runway until Jan. 13 to change taxiway signs to account
for the shift, the Federal Aviation Administration said.
The poles are generated by movements within the Earth's inner and outer
cores, though the exact process isn't exactly understood. They're also
constantly in flux, moving a few degrees every year, but the changes are
almost never of such a magnitude that runways require adjusting, said Paul
Takemoto, a spokesman for the FAA.
The magnetic fields vary from place to place. Adjustments are needed
now at airports in Tampa, but they aren't immediately required at all
airports across the country.
So just how often is something like this necessary? "It happens so
infrequently that they wouldn't venture a guess," Takemoto told FoxNews.com.
"In fact, you're the first journalist to ever ask me about it."
Takemoto was quick to point out that the change, which also was
required at Tampa's smaller Peter O. Knight airport, will have no effect on
passenger safety.
"You want to be absolutely precise in your compass heading," he pointed out.
"To make sure the precision is there that we need, you have to make these
changes."
Kathleen Bergen, another spokeswoman for the FAA, explained that runway
designations and charting rely upon geomagnetic information. "Aviation is
charted using latitude and longitude and the magnetic poles," she told
FoxNews.com.
The busiest runway at Tampa International will be re-designated 19R/1L on
aviation charts. It had been 18R/36L, indicating its alignment along the
180-degree approach from the north and the 360-degree approach from the
south, explained an article in
the Tampa Tribune detailing the changes. Later this month,
the airport's east parallel runway and the seldom used east-west runway will
be closed to change signs reflecting their new designations as well.
"The Earth's poles are changing constantly, and when they change more
than three degrees, that can affect runway numbering," Bergen said.
While rejiggering the runways is a very extreme event, the fields are
constantly in flux and constantly being remapped, explained Lorne McKee, a
scientist with
the geomagnetism division of Natural Resources Canada.
he explained. The field has swung from approximately 10 degrees
east in the late 16th century to 25 degrees west in the early
19th century -- before returning to a current value of about 3
degrees west.
It wasn't immediately clear when or even if changes would be
required at other airports. And even the rate of change is inconsistent,
McKee said, noting that it's changing much more quickly at the poles
themselves.
Beyond just sliding around the planet, the magnetic north and south
poles have been known to
completely flip as well; these reversals, recorded in the
magnetism of ancient rocks, are unpredictable. The last one was
780,000 years ago. Are we overdue for another? No one knows.
FoxNews
Earth's Inconstant
Magnetic Field |
12.29.03
| |
Our planet's magnetic field is
in a constant state of change, say researchers who are beginning to
understand how it behaves and why.
Every few years, scientist Larry Newitt of the Geological Survey of
Canada goes hunting. He grabs his gloves, parka, a fancy compass, hops on a
plane and flies out over the Canadian arctic. Not much stirs among the
scattered islands and sea ice, but Newitt's prey is there--always moving,
shifting, elusive.
His quarry is Earth's north magnetic pole.
At the moment it's
located in northern Canada, about 600 km from the nearest town:
Resolute Bay, population 300, where a popular T-shirt reads
"Resolute Bay isn't the end of the world, but you can see it from
here." Newitt stops there for snacks and supplies--and refuge when
the weather gets bad. "Which is often," he says.
Right: The movement of Earth's north
magnetic pole across the Canadian arctic, 1831--2001. Credit: Geological
Survey of Canada. [More]
Scientists have long known that the magnetic pole moves. James Ross located
the pole for the first time in 1831 after an exhausting arctic journey during
which his ship got stuck in the ice for four years. No one returned until the
next century. In 1904, Roald Amundsen found the pole again and discovered that
it had moved--at least 50 km since the days of Ross.
The pole kept going during the 20th century, north at an average speed of
10 km per year, lately accelerating "to 40 km per year," says Newitt. At
this rate it will exit North America and reach Siberia in a few decades.
Keeping track of the north
magnetic pole is Newitt's job.
"We usually go out and check its
location once every few years,"
he says. "We'll have to make
more trips now that it is moving
so quickly."
Earth's
magnetic field is changing in other ways, too: Compass needles in
Africa, for instance, are drifting about 1 degree per decade. And
globally the magnetic field has weakened 10% since the 19th century.
When this was mentioned by researchers at a recent meeting of the
American Geophysical Union, many newspapers carried the story. A
typical headline: "Is Earth's magnetic field collapsing?"
Probably not. As remarkable as these changes sound, "they're mild
compared to what Earth's magnetic field has done in the past," says
University of California professor Gary Glatzmaier.
Sometimes the field completely flips. The north and the south poles swap
places. Such reversals, recorded in the magnetism of ancient rocks, are
unpredictable. They come at irregular intervals averaging about 300,000
years; the last one was 780,000 years ago. Are we overdue for another? No
one knows.
Above: Magnetic stripes around
mid-ocean ridges reveal the history of Earth's magnetic field for millions
of years. The study of Earth's past magnetism is called paleomagnetism.
Image credit: USGS. [More]
According to Glatzmaier, the ongoing 10% decline doesn't mean that a
reversal is imminent. "The field is increasing or decreasing all the time,"
he says. "We know this from studies of the paleomagnetic record." Earth's
present-day magnetic field is, in fact, much stronger than normal. The
dipole moment, a measure of the intensity of the magnetic field, is now 8 x
1022 amps x m2. That's twice the million-year average of 4 x 1022 amps x m2.
To understand what's happening, says Glatzmaier, we have to take a trip
... to the center of the Earth where the magnetic field is produced.
At the heart of our planet lies
a solid iron ball, about as hot
as the surface of the sun.
Researchers call it "the inner
core." It's really a world
within a world. The inner core
is 70% as wide as the moon. It
spins at its own rate, as much
as 0.2o of longitude per year
faster than the Earth above it,
and it has its own ocean: a very
deep layer of liquid iron known
as "the outer core."
Right: a schematic diagram of Earth's interior. The outer core is the source
of the geomagnetic field. [Larger
image]
Earth's magnetic field comes from this ocean of iron, which is an
electrically conducting fluid in constant motion. Sitting atop the hot inner
core, the liquid outer core seethes and roils like water in a pan on a hot
stove. The outer core also has "hurricanes"--whirlpools powered by the Coriolis
forces of Earth's rotation. These complex motions generate our planet's
magnetism through a process called the dynamo effect.
Using the equations
of magnetohydrodynamics, a branch of physics dealing with conducting
fluids and magnetic fields, Glatzmaier and colleague Paul Roberts
have created a supercomputer model of Earth's interior. Their
software heats the inner core, stirs the metallic ocean above it,
then calculates the resulting magnetic field. They run their code
for hundreds of thousands of simulated years and watch what happens.
What they see mimics the real Earth: The magnetic field waxes and
wanes, poles drift and, occasionally, flip. Change is normal,
they've learned. And no wonder. The source of the field, the outer
core, is itself seething, swirling, turbulent. "It's chaotic down
there," notes Glatzmaier. The changes we detect on our planet's
surface are a sign of that inner chaos.
They've also learned what
happens during a magnetic flip.
Reversals take a few thousand
years to complete, and during
that time--contrary to popular
belief--the magnetic field does
not vanish. "It just gets more
complicated," says Glatzmaier.
Magnetic lines of force near
Earth's surface become twisted
and tangled, and magnetic poles
pop up in unaccustomed places. A
south magnetic pole might emerge
over Africa, for instance, or a
north pole over Tahiti. Weird.
But it's still a planetary
magnetic field, and it still
protects us from space radiation
and solar storms.
Above: Supercomputer models of
Earth's magnetic field. On the
left is a normal dipolar
magnetic field, typical of the
long years between polarity
reversals. On the right is the
sort of complicated magnetic
field Earth has during the
upheaval of a reversal. [More]
And, as a bonus, Tahiti could
be a great place to see the
Northern Lights. In such a time,
Larry Newitt's job would be
different. Instead of shivering
in Resolute Bay, he could enjoy
the warm South Pacific, hopping
from island to island, hunting
for magnetic poles while auroras
danced overhead.
Sometimes, maybe, a little
change can be a good thing.
Feature Author:
Dr. Tony Phillips
Feature Production Editor:
Dr. Tony Phillips
Feature Production Credit:
Science@NASA
Core Concerns
The hidden reaches of
Earth are starting to reveal
some of their secrets
By RICHARD MONASTERSKY
Gary A. Glatzmaier gazed
down on the world he had created
and decided it was good. Peering
deep into the bowels of the
planet, he saw vast currents of
molten iron alloy swirling at
temperatures above 5,000
kelvins, nearly as hot as the
surface of the sun. He watched
for 40,000 years as the globe's
magnetic field pulsated like the
beating of a heart. Deeper
still, at the center, he beheld
a spinning orb made of solid
iron almost as large as the
moon.
This creation, forged from
numbers and equations, is a
virtual version of Earth's
metallic core. Glatzmaier, a
geophysicist at Los Alamos
(N.M.) National Laboratory,
constructed the extremely
sophisticated computer model to
simulate the magnetic dynamo
that churns away, unseen, far
below Earth's crust.
Five years ago, most
geophysicists considered such
representations poor stand-ins
for the real core -- the
scientific equivalent of a
tone-deaf Elvis impersonator. In
the last year, however, these
models have earned newfound
respect by showing striking
similarities to the real thing.
The simulation by Glatzmaier and
his colleague Paul H. Roberts of
the University of California,
Los Angeles scored a major coup
with its prediction that Earth's
solid inner core spins out of
sync with the rest of the planet
-- a feature verified 3 months
ago by seismologists (SN:
7/20/96, p. 36,
Putting a New Spin on Earth's
Core ).
Combined with recent
advances in seismology, the
computer models are opening
windows into Earth's hitherto
impenetrable iron heart. This
new access gives scientists hope
that they can finally tackle
what Einstein reputedly called
one of the five greatest
unsolved problems in physics:
the origin of the planet's
magnetic field.
Although theorists have
made great strides since
Einstein offered that challenge,
geomagnetists still lack a firm
understanding of how the field
forms and why it changes
direction every few hundred
thousand years or so. "The
mechanisms behind the magnetic
field and behind the reversals
are still really mysterious.
It's fair to say that this is
one of the grand intellectual
challenges -- not just in the
earth sciences, but, I think, in
all of the physical sciences,"
says Raymond Jeanloz, a
geophysicist at the University
of California, Berkeley.
A soft-spoken scientist
most at home among his
equations, Glatzmaier declines
any comparison with the creator
in Genesis. It's interesting to
note, however, that Glatzmaier
began his modeling work with the
sun, only later moving on to
model Earth.
Initially, Glatzmaier
simulated the sun's magnetic
field, which arises from the
motion of ionized hydrogen and
helium inside that star. The
branch of physics governing this
realm is called
magnetohydrodynamics, a mouthful
of a term that researchers often
shorten to MHD.
After the sun, Glatzmaier
studied Jupiter, the Kuwaiti oil
fires, and Earth's rocky mantle
before finally turning to
Earth's core. The recent model
-- a variation of the one
developed for the sun --
simulates in three dimensions
the currents of iron alloy
flowing within the core.
The planet's nucleus is
believed to have formed early in
Earth's 4.5-billion-year
history, when molten iron and
other heavy elements sank deep
into the planet. As this
metallic soup cooled over the
eons, crystals of iron froze at
the center, creating a solid
iron core inside the surrounding
liquid alloy.
Over time, this process
built an inner core 2,440
kilometers wide, about one-fifth
the diameter of the planet. The
outer core of liquid alloy spans
2,260 km from top to bottom and
is composed of 90 percent iron
and 10 percent lighter elements,
possibly oxygen and sulfur.
The slow cooling of the
core, which continues today, is
critical because it stirs the
iron alloy. Heat escaping from
the top of the outer core chills
the upper layers of the outer
core, causing the material to
sink. At the same time, iron
crystals freeze and adhere to
the surface of the inner core,
leaving behind material richer
in the lighter elements. This
alloy floats to the surface of
the outer core.
This movement of metallic
fluids gave birth to Earth's
geomagnetic field, according to
MHD theory. Basic physics
teaches that moving metals can
produce an electric current if
they pass through a preexisting
magnetic field. This principle
underlies most electric
generators, which use heat to
move turbines that carry wires
past magnets.
If magnetic fields were
common in the early solar
system, as scientists believe,
then convective flow in the
outer core must have created
electric currents in the fluid
iron. The process turned into a
self-sustaining dynamo, because
electric currents produce their
own magnetic fields. Once the
core started producing a
magnetic field, the continuous
movement of the iron alloy would
have maintained electric
currents in the outer core,
thereby sustaining the
geomagnetic field.
Physicists
had sketched out the general
picture of this dynamo model by
the late 1950s, but the details
of what goes on in the outer
core remain one of Earth's
deepest secrets. What little is
known about the outer core comes
from the portion of the
geomagnetic field that reaches
Earth's surface. With its
prominent north and south poles,
this field is roughly dipolar in
orientation, as if it came from
a huge bar magnet buried inside
the planet.
The simple exterior field
-- the one that guides Boy and
Girl Scouts, airliners, and
migrating birds -- is but a tiny
fraction of the magnetic field
writhing within Earth's core.
The portion one can sense at
Earth's surface comes only from
the uppermost layer of the outer
core. The much more complex
field generated deeper down is
trapped inside the outer core
and never reaches the planet's
exterior.
In fact, much of the field
created in the upper layer of
the outer core also remains
hidden. The toroidal portion of
the field -- which runs in
circular east-west bands within
the outer core -- does not leak
outside the core, so scientists
cannot measure it. Only the
poloidal element -- which loops
from one pole around to the
other pole -- extends to the
planet's surface and into space.
While Earth conceals most
of its field, a computer model
is less bashful. That's why
Glatzmaier and Roberts have
attempted to create a virtual
version of the geodynamo, which
they run at the Pittsburgh
Supercomputing Center and at Los
Alamos. They started by
specifying how quickly heat
leaks out of the core, and then
they let the MHD equations
govern how the liquid alloy
responds. The flow patterns, as
they established themselves,
generated electric currents and
a magnetic field.
"The question I wanted to
answer was whether convection in
the fluid core could actually
maintain the magnetic field -- a
field that looks like the
Earth's magnetic field," says
Glatzmaier. "People had assumed
it was happening that way, but
it was never really
demonstrated. What's encouraging
is that I'm getting a magnetic
field that looks a lot like
Earth's in its strength and its
structure."
The similarities extend
beyond mere looks. The
computer-fabricated field
migrates slowly to the west in a
manner similar to that of the
actual field, whose features
shift westward by roughly a
degree each decade.
The model represents a
step forward, says Glatzmaier,
because in most previous
attempts, researchers had
prescribed the flow patterns
rather than letting them evolve
in response to the magnetic
field. The earlier techniques
used a short-cut that simplified
the problem and guaranteed a
realistic outcome -- as if the
tone-deaf Elvis impersonator
only lip-synced instead of
actually singing.
"The less you specify in
the model, the more you are able
to learn. If you specify
everything, you can get
something that looks just like
the Earth, but you will not
understand why things happen
because you have specified the
solution," says Glatzmaier.
Glatzmaier and Roberts let
their model run through
millennia of simulated time,
watching the magnetic field
wither and then rebound, all the
while remaining dipolar. About
35,000 years into the simulation
(and after more than a year of
real time), the dipolar field
nearly disappeared. For 1,000
virtual years the field
languished, with a confusing
multitude of magnetic poles
popping up instead of fixed
north and south poles. When the
field eventually recuperated, it
was pointing in the opposite
direction.
Here was a real triumph
for Glatzmaier and Roberts.
Their MHD model had produced a
geomagnetic reversal entirely on
its own, without any provocation
from the experimenters.
"Our original motivation
was not to simulate magnetic
field reversals. That seemed too
much to hope for. So that was a
nice surprise," says Glatzmaier.
As the world turns
over: In this computer
simulation, the magnetic field
emanating from the core flipped
upside down. Before the
reversal, poloidal magnetic
field lines leave the north
magnetic pole, curve around the
planet, and dive back into the
south pole (left). During the
transition, the field becomes
disorganized (middle) for
roughly 1,000 years and then
reestablishes itself with the
opposite polarity (right). Lines
wrapping around the core in
east-west bands indicate the
toroidal magnetic field.
The two researchers
published their reversal data in
the Sept. 21, 1995 Nature.
Although the model simulations
have continued, with one now
exceeding 200,000 years in
duration, Glatzmaier and Roberts
have not witnessed a second
reversal.
That may be a good sign,
since reversals of the actual
geomagnetic field usually occur
only once every few hundred
thousand years and occasionally
much less frequently. Still,
with only one reversal under
their belts, the scientists
cannot yet draw many conclusions
about what causes the process.
The MHD model garnered
even more attention last July,
when two seismologists reported
that the solid inner core of the
actual Earth is spinning faster
than the rest of the planet.
Xiaodong Song and Paul G.
Richards of the Lamont-Doherty
Earth Observatory in Palisades,
N.Y., who made the discovery,
credited the MHD model for
stimulating their search.
Glatzmaier and Roberts had
predicted the core's quick spin
last year, after studying the
flow patterns of the iron alloy
within their model. The
simulation revealed
eastward-moving currents of
fluid at the bottom of the outer
core, roughly analogous to the
jet streams in the atmosphere.
These currents in the outer
core, the scientists realized,
would put a magnetic torque on
the inner core, forcing it to
spin slightly faster than the
mantle and crust.
One of Glatzmaier and
Roberts' chief competitors,
Jeremy Bloxham of Harvard
University, has documented a
similar torque within his MHD
model of the core. In the
Harvard simulations, which began
more recently than the Los
Alamos work, the core spins
faster than the rest of the
planet on average, but it slows
down for brief periods. "I
wouldn't be surprised if the
rate changes with time," says
Bloxham.
There may, however, be
explanations besides magnetic
torque for the core's fast spin.
Berkeley's Jeanloz notes that
the rotation rate of the entire
Earth is slowing as a result of
the friction caused by lunar and
solar tides. The deceleration of
the inner core, however, may lag
behind that of the rest of the
planet because the inner core is
separated from the mantle and
crust by the fluid outer core.
According to this theory, the
inner core is now rotating as
quickly as Earth's surface was
spinning 60,000 to 100,000 years
ago.
"We may be able to
distinguish if one or the other
of these ideas is correct over
the coming decades, if not
years," says Jeanloz. If
magnetic torques are causing the
discrepancy, seismologists
monitoring the inner core should
see the rotation rate vary with
time. If the slowdown of Earth
is to blame, then the rotation
rate should change little except
for an extremely slow
deceleration. Both of these
mechanisms may work together,
says Jeanloz.
As seismologists continue
to refine ways of detecting the
inner core's rotation,
Glatzmaier, Roberts, and others
work on improving the MHD models
of the core. At present, the
models take shortcuts in
simulating fluid flow in the
core. Because of computer
limitations, MHD models treat
the iron alloy as being orders
of magnitude more viscous than
the actual liquid core, which
scientists think flows about as
easily as water. "We're hoping
we're not doing too much harm by
making this approximation," says
Glatzmaier.
These and other
limitations had led many
geophysicists to disregard MHD
models. The recent successes,
however, have quieted critics
and forced them to start taking
the models seriously.
"The types of numerical
calculations being done today
are just beginning to provide us
with a set of tools that we need
to understand how the geodynamo
works," says Bloxham. "There is
just an enormous amount of work
that we need to do. But I think
it's a very exciting time."
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