One of the great puzzles of geophysics is how the Earth generates and maintains its magnetic field. The general thinking is that when conducting material in Earth’s outer core moves due to convection, it generates electric currents, and these create a magnetic field.
But this field is also influenced by the Earth’s rotation, and this influences the convection currents of electrically conducting materials in the core. The result is a powerful feedback process that leads to hugely complex behavior.
To better understand these processes, physicists have built increasingly complex physical models of the Earth’s core to explore this question. Most experiments simulate the conductive core using liquid metals rotating in a plane in an external magnetic field.
But these experiments have a significant limitation. Liquid metal is opaque, so it is not possible to see how convection currents evolve inside them, particularly when the motion is usually in a plane. Neither are computer simulations much help—the physics is so complex and the feedback effects so strong that even the best simulations cannot solve the resulting equation at the required level of detail.
The result is that neither physical models nor computer simulations have been able to reproduce the observed behavior of the Earth’s field.
What’s needed is a different model that can better capture the complex processes at work, and preferable one that can reveal the way convection currents arise and evolve.
Today, Kelig Aujogue at Coventry University in the U.K. and a few pals unveil an experimental model based on a rotating hemisphere filled with a transparent electrolyte that does this. And they say their model reveals for the first time how the magnetic field drastically changes the structure of convective plumes inside the Earth.
First some background about the forces at work in the Earth’s core. The principle phenomena are: buoyancy, which drives fluid motion; the Coriolis force from the Earth’s rotation; and the magnetic force that arises from the interaction between induced electric currents and magnetic fields.
Geophysicists characterize the way these forces interact using a quantity known as the Ekman number—the ratio of viscous forces in a fluid to the forces that arise from planetary rotation. When the Ekman number is small, disturbances within the fluid are able to propagate but this propagation process is hugely complex.
The Ekman number in the Earth’s core is tiny, around 10-15. The best computer models can simulate Ekman numbers in the region of 10-5 but even these results have never been calibrated against experimental results using liquid metals because the flow cannot be seen in these setups.
Enter Aujogue and co. Their apparatus consists of a hemispherical glass dome filled with sulfuric acid, heated at its center by a cylindrical heating element and cooled on the outside.
While sulfuric acid is a reasonable conductor, it is around four orders of magnitude less good than liquid metals. The team compensate by placing the entire apparatus in a hugely powerful magnetic field of up to 10 Tesla, which is 100 times higher than is possible with conventional electromagnets.
There is only one place on the planet capable of producing magnetic fields of this strength, the Grenoble High Magnetic Field Laboratory in France, which is where the team set up their gear.
The entire set up has to be rotated inside this field. This means that all the components have to be made out of nonmagnetic materials to avoid the induced currents this would create.
Handling sulfuric acid is no walk in the park either. The components have to be chemically resistant and the experiment carefully designed to ensure the safety of the scientists involved. The apparatus also has to be designed so that the data from the experiments can be easily collected.
To view the flow within the core, the team uses a technique called particle image velocimetry. This involves firing a laser into the fluid and recording how it reflects off small particles or bubbles within it. By tracking their movement, it is possible to build up a detailed 3-D picture of the flow.
These are a challenging set of constraints. Nevertheless, the outcome is impressive. “For the first time, the [principle] forces can be produced and precisely controlled in a flow that can also be fully mapped by means of optical visualization techniques,” say Aujogue and co.
And the results are something of a surprise. “The magnetic field has a spectacular effect on the structure of convective plumes,” say the team.
This doesn’t just apply to Earth but to any planet or moon with a magnetic field and a liquid core, such as Mercury or Ganymede.
And there is plenty of scope for future work. The team says it is possible to easily vary the size of the core and the temperature difference it creates so different regimes can be investigated.
Of course, more work needs to be done to see how closely these results reflect what is going on inside the Earth. But this is a fascinating step along the way to even better models that fully describe Earth’s strange magnetic field.
Ref: arxiv.org/abs/1606.01780: Little Earth Experiment: an Instrument to Model Planetary Cores
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