Saturday, February 18, 2006


Earth's magnetic field

As a member of the American Geophysical Union I receive a copy of Physics Today every month. I have to admit that I don't usually read it. I made an exception this month, and I'm glad I did. There was a really interesting article on an experimental set up at the University of Wisconsin-Madison that acts as an analog for Earth's magnetic field. Unfortunately the article is subscription only, but the research group that's discussed in the article has a pretty nice website (The University of Wisconsin-Madison Dynamo Experiment".

The earth's outer core is composed of molten iron, and motion in this liquid outer core generates the earth's magnetic field. There's still quite a lot that's not known about the processes that generate the magnetic field, and those processes can be hard to study. There are two main approaches that geophysicists have taken: 1) using computer models, and 2) trying to simulate the conditions that exist in the outer core.

There are quite a few geophysicists who use computer models to try to figure out what's going on in the interior of the earth. Most of the ones I know try to simulate the mantle, although there are some that deal with the inner and outer core. The mantle is around 2900 km thick, while the earth's crust is a measly 70 km thick at most, and is usually a lot thinner. So people who model the mantle have to deal with a really substantial volume of the earth. This can be a problem for computer models. The larger and more complex the model becomes the more computation time is needed. This isn't too much of a problem in the mantle because while the mantle is really big most of the features in the mantle that people are interested in are really big too. Things like mantle convection cells, mantle plumes, and subducting plates are all at least 100's of kilometers in size. That's definitely not too complicated to work with given the power of modern computers. Unfortunately for computer modelers the story is more complicated in the outer core. The Physics Today article points out that the magnetic structures in the outer core are at least a few hundreds of kilometers in size, small scale turbulence within those structures (on the order of 10m in size). This means that a realistic model would have to have 10^15 grid cells (10^12 = 1 trillion, for way of comparison). Even with supercomputers that number of grid cells would result in models that have computation times that are too long to work with. This is where the people who build machines to simulate outer core conditions really come into play. It's not possible to build machines that completely mimic the outer core, which is composed of incredibly hot and incredibly dense liquid iron. The temperature at the top of the outer core is something like 3000-5000 degrees C, while the pressure is ~100+ GPa (1,000,000 times the pressure at the surface of the earth). Fortunately there are things that behave like molten iron but at much lower temperatures. The properties of liquid sodium at 98 degrees C are much like the properties of iron in the outer core. That temperature is definitely low enough to work with.

The team at UW built a hollow sphere 1 meter in diameter that they fill with liquid sodium (as an aside, sodium doesn't react well with water, so it can be a tricky material to work with). They use propellers inside the sphere to simulate turbulence in the outer core. Fill the sphere and turn it on, and presto, you've got a working analog of the earth's core. The cool thing is that turbulent flow in the sphere can generate a magnetic dipole, which means a magnetic field with a north and a south pole, which is just like Earth's magnetic field. The UW team hasn't performed enough experiments to fully understand what's going on inside their sphere, but it's a really promising start.

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