September 25, 2008
Why Earth’s Magnetic Field Flip-Flops
Every so often, Earth's magnetic field flips on its head,
turning the magnetic North Pole into the South Pole and vice versa.
It last happened 780,000 years ago, and is predicted to
occur again in about 1,500 years ... maybe. The overall frequency is hard to
predict - there was one period in Earth's history when the field didn't reverse
for 30 million years.
Why these flip-flops
happen at all is a great riddle, but a new hypothesis on the origins of the
magnetic field could shed light on the reason.
How it works
field is really two fields with two separate sources, argues paleomagnetist
Kenneth Hoffman of California Polytechnic State University in San Luis Obispo and
geochronologist Brad Singer of the University of Wisconsin, Madison, in a paper
published in the Sept. 26 issue of the journal Science.
One component of the field, the stronger part, is the
north-south pointing "axial dipole" magnetic field, which can be
pictured as the kind of field that would be created by a giant bar magnet
inside the Earth.
There is also a weaker field spread around the planet, not
positioned along the north-south axis. The researchers suggest this weak field
is created closer to the surface of Earth's outer
core, while the stronger north-south field is produced throughout the core,
including the deepest parts.
Both fields are thought to result from the movement of
electrons from hot iron atoms in the convective flow of the core, which is more
of a liquid than a solid. The movement of the charged particles through the
planet creates a magnetic field just as the movement of an electric current
through a wire does.
The researchers suggest that the sporadic reversal of
Earth's magnetic field occurs when the axial dipole field weakens, leaving the
weaker, more disperse field intact.
"The field is not always stable, the convection and the
nature of the flow changes, and it can cause the dipole that's generated to wax
and wane in intensity and strength," Singer said. "When it becomes
very weak, it's less capable of reaching to the surface of the Earth, and what
you start to see emerge is this non-axial dipole, the weaker part of the field
that's left over."
How can they know
To figure this out, Hoffman and Singer analyzed remnants of
lava that poured out of volcanoes in Tahiti and Germany between 500,000 and
700,000 years ago. The lava contains an iron-rich mineral called magnetite.
When the hot lava is erupted, the iron atoms' electrons spin in random
directions, but as it cools down the electron spins freeze pointing
in the direction of the planet's magnetic field lines.
At times when the Earth's magnetic dipole field was strong,
the spins in both Tahiti and Germany pointed toward the magnetic North Pole.
However, when the dipole field was weakening or preparing to reverse, the spins
were left to be guided by the strongest nearby magnetic field lines from the
weaker field that was left over, which were different in Germany compared to
The finding shows that when the main element of Earth's
magnetic field is weak, smaller local magnetic pulls kick in. By studying where
these local pulls are strongest, the researchers can map out this second layer
of field and learn how it differs from the stronger field.
Ultimately, scientists hope to have a better handle on these
issues by the time our planet's field flips
"The magnetic field is one of the most fundamental
features of the Earth," Singer told LiveScience.
"But it's still one of the biggest enigmas in science. Why [the flip]
happens is something people have been chasing for more than a hundred
When the next reversal comes, it probably won't inflict much
damage to life on Earth, Singer said. Though we could expect a slight increase
in damaging cosmic radiation, which is usually repelled by the magnetic field,
it shouldn't be enough to cause serious harm. And though magnetic north and
south poles will switch spots, the seasons and other cycles on the planet are
primarily due to Earth's position relative to the sun, which wouldn't change
significantly. All in all, the flip is nothing that our planet hasn't faced
many times before.
"The magnetic field has reversed itself hundreds, if
not thousands, of times," Singer said. "Right now, the axial dipole
is waning in strength, so we might experience a field reversal in the next two
millennia if it continues to weaken at the current rate."