Rick Groleau
November 18, 2003

from PBS Website

Spanish version


Every so often - every 250,000 years on average - the Earth’s magnetic poles reverse polarity.


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 happens.

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 complete.






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 its own 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 dynamo.

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.


The so-called 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

from PBS Website


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 future.

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 magnetic poles.

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.

"When we 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.


Ancient 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 unfold.

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 protective shield.

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.