Surrounding the beautiful planet we call home, a powerful but invisible shield acts as a barrier to protect us from the sun.  The magnetosphere, reaching 40,000 miles beyond the earth on the sunward side, blocks high levels of solar particles that would otherwise harm life.

The Earth's magnetic field is formed by the rotation of molten iron in the core.  The movement of the iron works like a power plant dynamo, building an electrical current and producing a local electromagnetic field--only in this case the dynamo is planet-sized. The field turns the earth into a giant magnet, with north and south poles.

However, in the 1960's, scientists testing the magnetism of igneous rocks along the Mid-Atlantic Ridge found that the polarity of the rocks frequently reversed--an indication that the earth's magnetic poles have reversed several times in the past.  The question was, why?

Well, our knowledge of the earth's inner currents is still too sketchy to offer a complete answer. But scientists in recent years have come up with some cool ways of testing the relationship between the earth's core and magnetosphere.  In 1999, for example, a team of researchers in Latvia demonstrated that spinning a cylinder of liquid sodium produced a magnetic field, although in their field the poles did not change.

Last week news broke of another experiment using a cylinder full of sodium.  Michaël Berhanu and his Parisian team of scientists created an earth core simulation by rotating two disks--the ends of the cylinder containing the molten metal--in opposite directions.  By adjusting the speeds of the disks, they figured out how to induce the magnetic poles to flip.

When the disks were spun at equal but opposite rates, this field stayed constant. But if the rotation rates were different, the behaviour was more complex . . .

 Under these conditions the magnetic field switches polarity apparently at random, typically every minute or so. It happens just as it happens to the Earth, with the field declining slowly to zero and then reappearing quickly in the opposite orientation. They find that one field orientation persists more often than the other, presumably because of a bias introduced by the Earth's magnetic field.

  The reversals seem to happen when the energy needed to spin the driving disks — which varies because the turbulence of the liquid produces a variable amount of friction — is lower than average.  -Nature

Although many theories of polar reversal point to movement in the mantle as a trigger, Berhanu's group believes that all the ingredients needed for reversal are contained in the core.  As far as I can tell though, the experiment does not rule out the possibility of external influences.

*To give credit where credit is due, I must add that my uncle, Martin Devine, a science teacher currently teaching anatomy at Indiana University, thought of this "spinning cylinder" experiment idea all by himself one day a couple months ago, when he and I were discussing the problem of polar reversal over lunch. His idea, though, was to use an actual sphere.  He says that MRI could possibly be used to map the current patterns as they spin inside. Oh the things we could do if we had the time and money. . .

(ScienceDaily coverage) 

Image credit: NASA Marshall Space Flight Center (NASA-MSFC)