Is Earth's Magnetic Field Failing?
New Scientist, August 22, 2001
If you can't journey to the centre of the Earth, why not bring the churning heart of the planet to your lab? It's scary stuff, but Adrian Cho thinks it might tell us why we're still alive. Take 14 tonnes of highly explosive metal, melt in a large vessel and stir vigorously. Stand well back. Intrepid researchers at the University of Maryland plan to try out this recipe, and, needless to say, the fire marshal is already having sleepless nights. But it will be well worth the trouble if they solve the long-standing puzzle of how the Earth produces its magnetic field. It might even be a matter of life or death. The Earth's field is one of nature's great gifts, shielding us from lethal cosmic radiation and possibly stopping our atmosphere being stripped away by the ravages of the solar wind. If our magnetic field were to switch off entirely, the Earth could become as sterile as Mars. Our protective shield is unlikely to fail permanently, but a temporary shutdown may be imminent. It could happen within as little as 2000 years. Measurements of the Earth's field show that it is getting weaker, and suggest that we are heading for a field reversal, in which the north and south magnetic poles will swap. When the reversal is in full swing, there will be a time when the field sinks almost to zero before cranking up again. This unprotected period might only last for a few years, or it could go on for thousands. To know for sure, we'll need a very precise model of the Earth's core.
The core is a ball of iron 6960 kilometres across, at a temperature of more than 5000 °C. The outer 2260 kilometres are liquid, the inner part is squeezed solid. Convection roils the outer portion of the core, as cooler, denser fluid sinks under the pull of gravity, while hotter, less dense liquid rises to take its place. So how could this swirling molten metal create a magnetic field? Magnetism, electricity and motion are like a three-for-two special offer: if you have two of them, the third one comes free. In a bicycle light dynamo, for example, a magnet and the spinning rear wheel of your bike generate electricity. In the Earth's core, researchers believe that the magnetism of a "seed field" from, say, a nearby star, works with the motion of the churning metal to generate electric currents. The electricity in turn feeds the magnetic field. Given the right conditions for this "magnetic dynamo", the seed field will stretch, twist and grow as the molten metal moves. Eventually, the field will become strong enough to influence the motion of the fluid, effectively controlling its own growth. Once at this point, the magnetic dynamo can produce a stable, self-sustaining field.
However, this is still a matter of faith among physicists-they can write the equations that describe the motion of a conductor and the evolution of a magnetic field, but they can't explain exactly how it reaches a steady state. That's mainly because the fluid flow inside the Earth is turbulent, teeming with whorls and eddies. "We don't have enough computer memory and power to resolve the really small eddies," says Gary Glatzmaier, a computational physicist at the University of California in Santa Cruz. And so models must rely on simplifications and approximations. What they need is something real they can use to refine their computer models-a turbulent core they can play with. Several research groups are now building them. To capture the effects of turbulence, they have to make devices that allow liquid metal to flow freely. Researchers in Cadarache, France, have built a small device that will be filled with 330 litres of molten metal, and another team at the University of Wisconsin, Madison, will soon rev up a spherical mock-up of the Earth's core 1 metre in diameter.
But Daniel Lathrop, Daniel Sisan and Woodrow Shew at the University of Maryland have by far the most ambitious plan. For the moment they are working with a pair of small devices, but they are drawing up plans for a ball 3 metres across that will contain 14 tonnes of sodium. It will be heated to more than 110 ¡C to melt the metal, and propellers will churn the liquid to simulate the effect of convection in the core. The entire ball will spin seven times a second to mimic the Earth's rotation. If you know your chemistry, alarm bells should be ringing by now. Sodium may be a wonderful conductor of electricity, but it is also rather reactive. Chemists keep the metal in oil to avoid contact with air or water-otherwise it can burn or even explode. When just 100 kilograms of sodium exploded at the French nuclear research centre in Cadarache in 1994, a worker was killed. To ensure safety in Maryland, the entire device will sit inside a big metal box. "That makes the fire marshal and the safety officer feel a whole lot better," says laboratory technician Donald Martin.
Despite the risk, the sphere really does need to be as big as possible. Size matters because the magnetic fields need space to stretch, twist and grow. Field lines confined to a small space tend to resist this sort of deformation. Researchers in Riga, Latvia, and in Karlsruhe, Germany, have generated magnetic fields in somewhat smaller vessels, but only by forcing sodium to flow along helical paths. This doesn't mimic the more complicated workings of the Earth's core, says Agris Gailitis at the University of Latvia. "It is really low turbulence", he says. In the Earth, as in any free-flowing dynamo, the fluid will be highly turbulent. So the only way to get anywhere close to mimicking the Earth's core is to have a huge volume of madly churning molten metal. The faster it goes, and the bigger the volume of the fluid, the more the field will twist, stretch and grow towards a steady state. So far, no one has yet managed to persuade such a freely churning fluid to generate a magnetic field. But a sphere 3 metres across might do the trick.
Theorist David Sweet, working with Lathrop and his colleagues at the University of Maryland and the Los Alamos National Laboratory, has shown how this giant ball of sodium should produce a self-sustaining magnetic field (Physics of Plasmas, vol 8, p 1944). They studied how churning liquid metal responds to a magnetic "seed" pulse that kick-starts a self-sustaining field. At a low flow speed, the field inside the liquid decays as soon as the pulse is turned off. But the rate of this decay decreases as the flow increases. Eventually, it won't decay at all. When the experimenters subject their giant ball of churning sodium to brief blasts of a magnetic seed field, the dynamo should spring to life. But it won't be steady straight away-the dynamo starts up like a sputtering old lawnmower, says Sweet. His calculations show that the field comes on full blast, drops to zero, and then returns to full blast later. These bursts are common to all turbulent magnetic dynamos, Sweet says, and are the signs that Lathrop and his colleagues will look for to see if they've created one. As the flow speed increases further, the field will eventually stop bursting.
The researchers will also try to observe "saturation", when the flowing fluid does not just produce a magnetic field, but the field in turn controls the flow of the fluid-this is what allows the field to sustain itself. Getting this right will require careful stirring, warns Cary Forest, a physicist at the University of Wisconsin in Madison. The flow has to have a particular character in order to generate a self-sustaining field. "If the flow is not right you're not going to get a dynamo," he says. Get the flow wrong and you could end up simulating the core of the wrong planet. Earth and Venus are similar in size and basic composition, yet Earth has a field while Venus doesn't. No one knows why, but flow might be the key. They may not know the precise recipe for successful flow, but theorists believe there are two essential ingredients. The first appears to be differential rotation, which will stretch any stray magnetic field lines around and around the axis-like a kid stretching a wad of chewing gum round and round his finger. The second ingredient is flow parallel to the spin axis, creating loops of magnetic field bulging out of the tightly spiralling lines-imagine the kid pulling a single strand of the wound-up gum towards the end of his finger. As the fluid continues to rotate, these loops of magnetic field can twist off, the two ends joining to form independent field lines.
Lathrop believes the required flow probably arises out of the interplay between turbulence and steady rotation. "The rotation tends to organise the turbulence," he says. Unlike the Earth, Venus's crust hasn't split into tectonic plates. This reduces the effectiveness of the planet's convection cooling system and suppresses any turbulence. Venus may also rotate too slowly to calm and organise any turbulence that does arise. Whichever is lacking, something in the flow seems to stop Venus's core generating a field. Only by building mock-ups of the Earth's core will we find out what's really going on. Meanwhile, there's another, more urgent question that needs addressing. If Lathrop's experiment does produce bursts of magnetic field, rather than a steady field, does that mean we are lucky enough to be living in the middle of a burst of the Earth's dynamo? Could it be about to cut out? That's a worry, because the Earth's field deflects high-energy particles crashing in from space. These cosmic rays can cause cancer and other diseases. The field also deflects the solar wind, the torrent of ionised gas streaming from the Sun. This ill wind may have blown away most of Mars's atmosphere when the Red Planet lost its magnetic field roughly 4 billion years ago (New Scientist, 10 February, p4). The Earth's dynamo appears to be operating beyond the bursty turn-on transition, Glaztmaier says. If he's right, the field won't cut out entirely-at least, not until the planet has cooled for a few billion years, slowing the convection. But without a more thorough understanding of the role of turbulence in generating the field, it's hard to be entirely sure. ...