Is Earth’s magnetic field failing?

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.

Whorls and eddies

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, p 4).

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.

Sinister portent

What’s more, Earth’s field has a well-known penchant for reversing its poles every
now and then. These reversals are recorded in the magnetism of ancient rocks. And
measurements of the field show that its strength is decreasing at the moment.

Interpreting that decline is difficult, says Sten Odenwald, a researcher on NASA’s
IMAGE project to investigate the Earth’s magnetosphere, the region of space
dominated by the planet’s magnetic field. “We don’t really know if the decline is just a
natural ripple, or a portent of something far more sinister.”

If we’re heading for a field reversal, then for a while the Earth will be hit by much more
radiation than it currently receives. “There’s going to be a long period of time-possibly
many generations-when we’re going to have to find a way to deal with all this extra
energy,” says James Green, another IMAGE researcher. “I don’t know that anyone’s
done a proper scientific investigation of what will happen. It’s certainly one of the
things we should be looking into.”

If all goes well, Lathrop and his colleagues intend to have their giant sodium ball up
and spinning within two years. Meanwhile, Forest intends to roll out his 1-metre ball at
Wisconsin this summer, and believes he will be first to generate the dynamo effect in
a freely flowing fluid.

Whoever wins the race, says Glatzmaier, these experiments should give physicists
benchmarks against which to test their dizzyingly complicated programs. “We’ll be
able to apply our computer models to the experiments instead of a planet or a star,
and see if we can match them,” he says. This, it seems, could be the start of
something big. After decades of quiet research, dynamo physics might be about to
explode-metaphorically speaking, of course.