杏吧原创

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鈥檚 field is one of
nature鈥檚 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鈥檚 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鈥檒l need a very precise model of the Earth鈥檚 core.

The core is a ball of iron 6960 kilometres across, at a temperature of more
than 5000 掳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鈥檚 core, researchers believe that the magnetism of a 鈥渟eed field鈥� 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 鈥渕agnetic dynamo鈥�, 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鈥攖hey can
write the equations that describe the motion of a conductor and the evolution of
a magnetic field, but they can鈥檛 explain exactly how it reaches a steady state.
That鈥檚 mainly because the fluid flow inside the Earth is turbulent, teeming with
whorls and eddies. 鈥淲e don鈥檛 have enough computer memory and power to resolve
the really small eddies,鈥� 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鈥攁 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鈥檚 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
掳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鈥檚 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鈥攐therwise 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. 鈥淭hat makes the fire marshal and the safety officer feel
a whole lot better,鈥� 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鈥檛 mimic the more complicated workings of the
Earth鈥檚 core, says Agris Gailitis at the University of Latvia. 鈥淚t is really low
turbulence鈥�, 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鈥檚 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 鈥渟eed鈥� 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鈥檛 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鈥檛
be steady straight away鈥攖he 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鈥檝e created one. As the flow speed
increases further, the field will eventually stop bursting.

The researchers will also try to observe 鈥渟aturation鈥�, when the flowing fluid
does not just produce a magnetic field, but the field in turn controls the flow
of the fluid鈥攖his 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. 鈥淚f the flow is not right you鈥檙e
not going to get a dynamo,鈥� 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鈥檛. 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鈥攍ike 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鈥攊magine 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. 鈥淭he rotation tends to organise the
turbulence,鈥� he says. Unlike the Earth, Venus鈥檚 crust hasn鈥檛 split into tectonic
plates. This reduces the effectiveness of the planet鈥檚 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鈥檚 core generating a field. Only by building mock-ups of
the Earth鈥檚 core will we find out what鈥檚 really going on.

Meanwhile, there鈥檚 another, more urgent question that needs addressing. If
Lathrop鈥檚 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鈥檚 dynamo? Could it be about to cut out?

That鈥檚 a worry, because the Earth鈥檚 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鈥檚 atmosphere when
the Red Planet lost its magnetic field roughly 4 billion years ago
(New 杏吧原创, 10 February, p 4).

The Earth鈥檚 dynamo appears to be operating beyond the bursty turn-on
transition, Glaztmaier says. If he鈥檚 right, the field won鈥檛 cut out
entirely鈥攁t 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鈥檚 hard to be entirely sure.

Sinister portent

What鈥檚 more, Earth鈥檚 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鈥檚 IMAGE project to investigate the Earth鈥檚 magnetosphere, the region of
space dominated by the planet鈥檚 magnetic field. 鈥淲e don鈥檛 really know if the
decline is just a natural ripple, or a portent of something far more
蝉颈苍颈蝉迟别谤.鈥�

If we鈥檙e heading for a field reversal, then for a while the Earth will be hit
by much more radiation than it currently receives. 鈥淭here鈥檚 going to be a long
period of time鈥攑ossibly many generations鈥攚hen we鈥檙e going to have to
find a way to deal with all this extra energy,鈥� says James Green, another IMAGE
researcher. 鈥淚 don鈥檛 know that anyone鈥檚 done a proper scientific investigation
of what will happen. It鈥檚 certainly one of the things we should be looking
颈苍迟辞.鈥�

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. 鈥淲e鈥檒l be able to apply our computer models to the experiments instead
of a planet or a star, and see if we can match them,鈥� he says. This, it seems,
could be the start of something big. After decades of quiet research, dynamo
physics might be about to explode鈥攎etaphorically speaking, of course.

Recreating the turbulence of the Earth鈥檚 core in the laboratory may reveal
that the Earth has magnetic poles sticking out in all sorts of directions.
Computer simulations suggest the complex flow of the molten core produces an
equally complex field. But these subtleties are hidden by the thousands of
kilometres of rocky mantle that separate the Earth鈥檚 core from its surface. Only
the familiar north-south portion of the field is stable and strong enough to
reach to the surface and beyond.

The experiments might also provide insights into how the Earth鈥檚 field
reverses every few hundred thousand years. Computer models suggest that the
poles regularly try to swap places, in response to field changes initiated by
the churning liquid metal in the outer core. But they only rarely succeed,
because the solid inner portion of the core keeps the field steady.