November 18, 2003
Every so often - every 250,000 years on average
- the Earth’s magnetic
If such a reversal happened today, compass
needles would point south rather than north. Here, view a
computer-model-generated animation of our planet’s magnetic field,
and see what happens during a reversal.
To learn how Earth’s
magnetic field works, see insert below.
Watch a simulated reversal of Earth's magnetic field, from the first
signs of instability to the final, inevitable flip.
1 - At the simplest
level, the source of Earth's magnetic field can be
thought of as a giant bar magnet within the Earth. In
reality, the magnetic field is created by a complex
interaction involving a churning, electrically
conducting liquid metal outer core, the release of heat
at the surface of the solid inner core, and the spinning
motion of the Earth.
2 - The complex
interaction that gives our planet its magnetic field
results in a field that is likewise complex. This
computer-generated graphic is derived from a computer
model created by Gary Glatzmaier of UC Santa Cruz and
Paul Roberts of UCLA. To fashion their model, Glatzmaier
and Roberts made use of dozens of equations that
describe the dynamics of Earth's interior. As with this
snapshot from the computer model, the Earth's magnetic
field is not uniform. The intensity and direction of the
field changes not only from one location to another, but
over time as well.
3 - Another view
derived from the computer model illustrates the
direction of the magnetic field, with the blue areas
representing where the north-seeking side of a magnet
(or compass) would point and the orange areas showing
where the south-seeking side of a magnet would point.
The large oval represents the direction of magnetism at
the planet's surface; the small oval represents the
direction at the surface of the Earth's core.
4 - The study of
ancient lava flows in Oregon and elsewhere reveal that,
before a reversal, the magnetic field is erratic and
weakens drastically. The Glatzmaier-Roberts computer
model behaves in a way that agrees with this process.
Here, the core has already begun to fluctuate, with
anomalies, or areas of reversed polarity, appearing as
islands of blue and orange.
5 - In the
Glatzmaier-Roberts model, a reversal begins with
additional north and south poles appearing at the core.
These additional poles may not appear at the Earth's
surface—at least not initially—though these islands of
reversed polarity can weaken the overall magnetic field
strength. (One such island has appeared beneath the
South Atlantic Ocean.) In this simulation, a weak pole
of reversed polarity now exists at the core. This
anomaly has not made it to the surface—a compass near
this "area" would still point to true north—but the
strength of the magnetic field is as much as 30 percent
weaker at the surface. After a short period of
instability, the north and south magnetic poles switch
polarity. Scientists don't fully understand why this
6 - Here, the
entire animation plays without pausing. The strength of
our world's magnetic field has been diminishing for the
past 300 years. If the Glatzmaier-Roberts model
accurately simulates the processes that drive the
magnetic field, the loss of strength could be an
indication that a reversal is under way. This would be
no surprise. On average, reversals of the Earth's
magnetic field happen every 250,000 years. It's now been
about 720,000 years since our last reversal. Judging
from history, we know that a reversal is long overdue.
But there's no need to throw away your old compasses. A
reversal usually takes hundreds or thousands of years to
What Drives Earth’s Magnetic Field?
When an electric current passes through a metal wire, a magnetic
field forms around that wire.
Likewise, a wire passing through a
magnetic field creates an electric current within the wire. This is
the basic principle that allows electric motors and generators to
operate. In the Earth, the liquid metal that makes up the outer core
passes through a magnetic field, which causes an electric current to
flow within the liquid metal.
The electric current, in turn, creates
magnetic field - one that is stronger than the field that
created it in the first place. As liquid metal passes through the
stronger field, more current flows, which increases the field still
further. This self-sustaining loop is known as the geomagnetic
Energy is needed to keep the dynamo running.
This energy comes from
the release of heat from the surface of the solid inner core.
Although it may seem counterintuitive, material from the liquid
outer core slowly "freezes" onto the inner core, releasing heat as
it does so. (High pressures within the Earth cause material to
freeze at high temperatures.)
This heat drives convection cells
within the liquid core, which keeps the liquid metal moving through
the magnetic field.
Coriolis force also plays a role
in sustaining the geomagnetic dynamo. Our planet's spinning motion
causes the moving liquid metal to spiral, in a way similar to how it
affects weather systems on the surface.
These spiraling eddies allow
separate magnetic fields to more or less align and combine forces.
Without the effects caused by the spinning Earth, the magnetic
fields generated within the liquid core would cancel one another out
and result in no distinct north or south magnetic poles.
Earth’s weakening and moving magnetic shield
November 18, 2003
Like the plot of a sci-fi B movie, something weird is
happening deep underground where the constant spin of
Earth's liquid metallic core generates an invisible
magnetic force field that shields our planet from
harmful radiation in space.
Gradually, the field is
growing weaker. Could we be heading for a demagnetized
doomsday that will leave us defenseless against the
lethal effects of solar wind and cosmic rays? "Magnetic
Storm" looks into our potentially unsettling magnetic
Scientists studying the problem are looking everywhere
from Mars, which suffered a magnetic crisis four billion
years ago and has been devoid of a magnetic field, an
appreciable atmosphere, and possibly life ever since, to
a laboratory at the University of Maryland, where a team
headed by physicist Dan Lathrop has re-created the
molten iron dynamo at Earth's core by using 240 pounds
of highly explosive molten sodium.
The most visible
signs of Earth's magnetic field are auroras, which are
caused by charged particles from space interacting with
the atmosphere as they flow into the north and south
But the warning signs of a declining field are
subtler - though they are evident in every clay dish that
was ever fired. During high-temperature baking, iron
minerals in clay record the exact state of Earth's
magnetic field at that precise moment.
By examining pots
from prehistory to modern times, geologist John Shaw of
the University of Liverpool in England has discovered
just how dramatically the field has changed.
plot the results from the ceramics," he notes, "we see a
rapid fall as we come toward the present day. The rate
of change is higher over the last 300 years than it has
been for any time in the past 5,000 years. It's going
from a strong field down to a weak field, and it's doing
so very quickly."
At the present rate, Earth's magnetic field could be
gone within a few centuries, exposing the planet to the
relentless blast of charged particles from space with
unpredictable consequences for the atmosphere and life.
Other possibilities: the field could stop weakening and
begin to strengthen, or it could weaken to the point
that it suddenly flips polarity - that is, compasses begin
to point to the South Magnetic Pole.
An even older record of Earth's fluctuating field than
Shaw refers to shows a more complicated picture.
lava flows from the Hawaiian Islands reveal both the
strength of the field when the lava cooled and its
orientation - the direction of magnetic north and south.
"When we go back about 700,000 years," says geologist
Mike Fuller of the University of Hawaii, "we find an
incredible phenomenon. Suddenly the rocks are magnetized
backwards. Instead of them being magnetized to the north
like today's field, they are magnetized to the south."
Such a reversal of polarity seems to happen every
250,000 years on average, making us long overdue for
another swap between the north and south magnetic poles.
Scientist Gary Glatzmaier of the University of
California at Santa Cruz has actually observed such
reversals, as they occur in computer simulations (view
one in See a Reversal). These virtual events show
striking similarities to the current behavior of Earth's
magnetic field and suggest we are about to experience
another reversal, though it will take centuries to
Some researchers believe we are already in the
transition phase, with growing areas of magnetic
anomaly - where field lines are moving the wrong
way - signaling an ever weaker and chaotic state for our
Geophysicist Rob Coe, also of the University of
California at Santa Cruz, may have even found a lava
record in Oregon that charts the magnetic mayhem that
ensues during a period of reversal.
The picture that
emerges may not be up to Hollywood disaster standards,
but considering that human civilization has never had to
cope with such a situation before, it could be an
interesting and challenging time.