CAN you point to the north pole? Almost certainly not. Even if you are armed with a compass, you could easily get it wrong. If you are in California, your needle might be a full 18 degrees out. “You need to take account of that, even if you’re just hiking – it can be the difference between going left and going straight on,” says at the British Geological Survey.
It isn’t just that your compass can be thrown off by local quirks in the magnetic field. The north pole itself isn’t what it used to be. In 1900, the pole was in Canada. A century later, it was near Greenland. In the past 18 years, it has raced eastwards at about 40 kilometres per year, and is currently heading for Siberia.
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The weird behaviour of Earth’s magnetic field doesn’t end there. It also occasionally reverses its polarity: there were times in our planet’s history when a compass needle pointed to what we call south. Even now, there are spots under the surface where a compass would point the wrong way. What is going on? The mystery has deep implications for technology and the future of our planet.
To address it properly, we would need to make like Jules Verne and journey to the centre of the Earth, where the field has its source. That isn’t exactly practical. Instead, inventive minds have sought out magnets frozen into the planet aeons ago and built giant spinning spheres of liquid sodium. All of which could help us better understand our planet, evade solar storms and perhaps take the right route home.
Although few of us have been anywhere near the North Pole, it feels as familiar a landmark as the Grand Canyon or Mount Everest. Children know it as the place where Santa Claus lives. But there is more than one pole on top of the world (see diagram). There is the geographic North Pole, which is where Earth’s spin axis contacts the ground. And there is the magnetic north pole, where the planet’s magnetic field points directly downwards. It is this spot that is on the move, as is its counterpart at the other end of the planet. The magnetic south pole isn’t in the middle of Antarctica as you might expect, but off the coast of an area called Victoria Land.
For centuries, how the magnetic field was generated was a mystery, exercising thinkers from René Descartes to Edmond Halley. Einstein himself once considered the origin of this field to be one of the greatest unsolved problems in physics. He toyed with the idea that it was somehow the result of a mismatch between the charges on two particles, the electron and the proton. But the idea that won out was first proposed 100 years ago by the Irish physicist Joseph Larmor. He suggested that the field was the result of a roiling, electrically conductive liquid at the centre of Earth, a geodynamo.
We have every reason to think that Larmor was right. We know from bouncing sound waves through our planet that it has a metal core of two parts. The inner core, at 6000°C, is a tad hotter than the surface of the sun, but remains solid because of the immense pressure. The outer core is molten iron with a volume seven times that of the moon.
The difference in temperature between the exterior and interior parts of the outer core means that the liquid constantly flows around in convection currents. Hot liquid iron rises towards the exterior surface of the outer core, then cools, becoming more dense, and descends again. The metal is crammed full of electrons and this moving flow of electric charge generates a magnetic field.
If the fluid’s motion depended on convection alone, Earth’s magnetic field might take a relatively simple shape. However, the planet’s rotation pushes the fluid around in a different direction, with a force that depends on where in the planet this fluid is. Then there is the variable viscosity of the liquid metal due to the different densities at different distances from the inner core. This all adds up to a complex pattern of turbulent flows in the outer core, creating a tangle of magnetic field lines.
Given all that, it might seem surprising that the magnetic north pole isn’t wandering more wildly. But the flowing happens in slow motion. “When we say the flow is turbulent, we mean on a timescale of tens of thousands of years,” says at the University of California, Santa Cruz.
“The north pole has been racing eastwards for the past 18 years, and is currently heading for Siberia”
The wandering of the poles can be explained by this slow-motion turbulence, then. But what happens next is unclear. Does the north pole continue to move, and how far might it go? Could it be on the verge of flipping entirely? These are questions worth asking. After all, Earth’s magnetic field deflects away charged particles in the solar wind, which would otherwise rain down on us. No one expects it to do anything drastic quickly. But if it did, it would cost us dearly.
One way to get a handle on the magnetic field’s future is to look at its past. When a magnetic rock formation such as feldspar gets hot and then cools again, crystals within it align themselves with the prevailing field of the time. “Certain crystals have magnetic inclusions within them and they are excellent recorders of the magnetic field,” says at the University of Rochester, New York. Reading these crystals provides a record of what the magnetic field did going back many millions of years.
And it is quite a record. It shows that Earth’s magnetic field has weakened and strengthened throughout history, occasionally even flipping entirely. We know that there have been 183 pole reversals in the past 83 million years, the last one 780,000 years ago. Tarduno and his colleagues have just analysed 565-million-year-old magnetic rocks from Quebec in Canada and found that , the lowest value ever measured. At this time, Earth went through a 75,000-year period in which the field was sputtering and stuttering, rapidly changing direction, its strength coming and going.
This doesn’t just confirm that Earth’s field has been shifting for aeons, it also reveals more of the story of early Earth. “It looks like the field was on the verge of collapse,” says Tarduno. If that had happened, it is unlikely that complex life would ever have arisen on Earth. Without a magnetic field, the atmosphere would have been blown away by the solar wind and any life forms bombarded by harsh radiation. What prevented this disaster? According to Tarduno’s interpretation of the magnetic records, it was the formation of Earth’s solid inner core.
It looks as though, at this point, the planet’s whole core was molten but beginning to cool significantly. This would have slowed the convection currents, initially weakening the magnetic field. But it would have also allowed the deepest part of the core to begin to crystallise. Lighter elements such as silicon and oxygen would have been ejected from the solid into the fledgling outer core, making the deeper parts of it less dense and adding a new power to the convection currents. “We believe that when the inner core started to form, it provided a new energy source for the magnetic field,” says Tarduno.
His studies have also revealed a patch of Earth’s surface stretching from Zimbabwe to Chile where the magnetic field today is extremely weak. So weak, in fact, that satellites with orbits that pass above it need protection. Without the shield of a normal magnetic field, their electronics are in danger of being fried by the solar wind. Tarduno and his colleagues have discovered that at one place within this patch, underneath southern Africa at the core-mantle boundary, the field is actually reversed. A compass down there would point south. “It’s an astounding thing,” says Tarduno.
He and his team see these anomalies in all sorts of places going back in time and have even begun to observe how they moved. “It’s getting to the point where we can track some of these anomalies in the core flow,” says Tarduno. They can even see how they occasionally evolved into full reversals. “Sometimes, these things grow large enough that the whole field suddenly reverses polarity,” he says. Which begs the question: do the shenanigans at the north pole signal an imminent reversal?
That is one of the things Glatzmaier and his peers are trying to find out. To do that, they have to go beyond measuring the past and create computer models of the magnetic field that can predict what it will do next. For instance, starting from the basic laws of physics as applied to a moving, conductive fluid, they can produce a “dipole”, a form of the field with two opposing poles. When they allow the models to run, they see variations in strength over time and location. They also see wandering poles and even get occasional reversals. It all matches what we see in the real world. In April, a simulation even managed to that the north pole has taken at intervals during its journey eastwards.
“When it comes to models of Earth’s core, there is nothing better than a 3-metre-wide ball of liquid sodium”
Impressive as this is, it doesn’t do the real magnetic field justice. “Our computer models are complicated, but not nearly as complicated as the reality down there,” says Glatzmaier. No one has the computing power to model the field realistically. “The flow structure is twisting and shearing the existing magnetic field, and generating more magnetic field in the process. We have to use a very crude approximation of this.”
Things get even more complicated if you want to predict the direction of the magnetic field at any given spot on the surface. Models of Earth’s geodynamo won’t do that perfectly because concentrated areas of magnetic rocks can impinge on the local magnetic field, creating declinations where the field lines veer away from the north-south vertical. That is why won’t point directly north in parts of California and many other places.
Perhaps we need a more realistic model – a physical, rather than digital one. As physical models of Earth’s core go, none is more impressive than the ball of liquid sodium under the care of at the University of Maryland.
Handle with care
As any chemistry teacher will tell you, sodium is a difficult metal to handle, even in small amounts. It will catch fire when wet, sometimes spontaneously igniting in moist air, and is usually kept safely embalmed in oil in the school chemical cupboard.
Lathrop isn’t dealing with small amounts of the stuff. He is rotating a ball of sodium encased in stainless steel that is 3 metres across. The whole thing weighs 20 tonnes. At its centre, there is a solid metal core 1 metre across, which can be rotated independently up to 15 times a second. An array of 31 magnetometers distributed around the outer surface measure the generated field. Because this model core is so much smaller than the real thing, the Maryland team compensate with a better conducting metal than iron and faster rotation: the sphere rotates four times per second when cranked up to full speed.
Lathrop and his team have shown that the turbulent flow of a liquid metal will amplify and sustain a magnetic field, demonstrating that Larmor’s geodynamo hypothesis really does fit the facts. As yet, though, they haven’t managed to get a magnetic field to appear spontaneously as, presumably, Earth’s must have done. Instead, they apply a seed field, which is amplified by the sloshing sodium. It is promising but frustrating: we can’t show that Earth’s magnetic field arises from nothing more than the movement of the conducting fluid, and without being sure of the model’s validity, neither can we use the sphere to predict when the field will undergo reversals.
Lathrop’s experiment might be missing a key ingredient. The team has spotted a weak flow in the metal that seems to be caused by precession. This is a kind of extra rotation, like the way gently pushing a spinning top away from upright will cause it to slowly spin around the vertical axis as well as spinning on its own axis. Earth’s core is certainly subject to this, being tugged by the gravity of the moon as it orbits the sun. But Lathrop’s set-up doesn’t have an explicit mechanism for creating precession. The weak flows that Lathrop saw seem to be produced by the way Earth itself is rotating under his experiment.
We might learn more from a new and improved project under construction in Dresden, Germany. Last year, researchers at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) lab calculated that . Their calculations also mimicked the way Earth precesses at 23.5 degrees from the vertical of its orbital plane.
The practical implementation of this idea is not for the faint-hearted. It will involve a cylinder filled with 8 tonnes of liquid sodium. This will rotate 10 times per second on its long axis, and once per second on a slightly different axis. Safety is a paramount concern. “We have a building dedicated to experiments with liquid sodium, and there is a second building within the building for containment,” says at the HZDR. “Nobody will be in the building during experimental runs.”
These spectacular investigations are worth the effort. A better understanding of Earth’s field could avert a much bigger danger. If we can understand how our magnetic field varies, we can do more to protect our power grids from solar storms that can smash their way through a weakened field, or avert the possibility of cosmic rays harming people. In the extreme, we could lose the field altogether, as happened to Mars about 4 billion years ago. That would loosen Earth’s grip on its atmosphere and eventually make our home as uninhabitable as the Red Planet. It may be flaky and its poles may wander, but let’s not take our magnetic shield for granted.
