Geophysicists compare the behavior of Earth’s outer core to a liquid,
with about the same flow as water. As the big hot ball of melted metal spins
and roils about, it creates the planet’s magnetic field, which changes
with the core’s own chaotic behavior. Modeling that behavior — and
the incipient flip-flops of the magnetic field’s orientation that sometimes
accompany it — has proven difficult. But a team of researchers has made
a small step toward doing so, using one of the largest supercomputers on the
planet to run the most realistic model yet.
The magnetic field, which protects the planet from bombardment by cosmic radiation,
is constantly changing both in strength and configuration. Although researchers
consider it unlikely to be imminent, some suggest that the magnetic field may
shift polarity sometime in the next several millennia, so that the positive
and negative poles flip between north and south.
Geophysicists have tried to predict the core’s, and therefore the magnetic
field’s, behavior using models that function much like weather models do,
but without “the enormous benefit” of being inside the fluid being
modeled, says David Stevenson of Caltech in Pasadena. “We actually know
what’s going on” for the atmosphere and oceans, Stevenson says, but
not in Earth’s core. Atmospheric and climate modelers can use direct observations
so they can omit viscosity from their models, but models of the core must include
viscosity, a flow characteristic represented by a parameter known as the Ekman
number.
Using the smallest Ekman values yet for “turbulent viscosity” of the
liquid outer core, a team of scientists at the Japan Aerospace Exploration Agency
in Kanagawa and the Tokyo Institute of Technology used the Japanese supercomputer
Earth Simulator to model the magnetic field. It took several thousand hours
to model about 200,000 years of motion in Earth’s core. The Japanese team
published their results in Science on July 15, noting that their model
expresses a magnetic field that looks realistic, with nodes of magnetic variations
that flow and merge over time until the entire field has switched its polarity.
Such patches might be precursors to a reversal event, they say.
Although other models produce realistic-looking magnetic fields as well, says
Gary Glatzmaier, a modeler at the University of California in Santa Cruz, “no
model is yet as realistic as we would like it to be” for any kind of conclusions
regarding magnetic field somersaults. Nevertheless, Glatzmaier says, the Japanese
researchers “are certainly on the right track” in using the smaller
values for turbulent viscosity.
The smaller Ekman number gets closer than ever to what geophysicists think is
reality in the core, where the extremely low viscosity makes for very small-scale
turbulence that is quite difficult to model on a computer of any size. Glatzmaier
compares it to trying to simulate every small gust of wind down to the human
scale while running a model of the planet’s atmosphere, to determine the
global weather and future climate. That means that for even more realism, values
for the Ekman number should be smaller, probably by several orders of magnitude,
Glatzmaier says.
The new model tends to reverse its polarity too often, with reversals occurring
every 5,000 years, rather than once in tens to hundreds of thousands, or even
millions of years, as they do in the geologic record, Glatzmaier and Stevenson
say. Plus, “based on the smooth, large-scale patterns of the magnetic fields”
that the team reported, “the model’s simulated fluid flows may still
be too laminar compared to the strong turbulence expected in the Earth’s
fluid core,” Glatzmaier says.
Still, the team’s results using more realistic viscosity values support
the “essential idea” that rotation and the behavior of the magnetic
field are dominant in theoretical calculations, Stevenson says. Although this
model “should not be regarded as the final word,” he says, “this
is a significant step forward,” in a pursuit that sees major improvements
only every decade or so.
Naomi Lubick
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