Pedagoguery

The earth's magnetic field is something we take pretty much for granted. When you take out a compass, you pretty much expect the needle to point north. However, the geologic record provides evidence that this constancy is an illusion. Hundreds of times over the last 150 million years, the polarity of our magnetic field has reversed, flipping the north and south magnetic poles. How does this field arise and what causes it to reverse? While we have a good general idea of the first question, answers to the second are somewhat elusive. We are getting closer to finding the answers, however.

When Maxwell formulated his laws of electromagnetism, one of the fundamental principals was the fact that electricity and magnetism are inextricably linked. Take and electric charge and move it, it gives rise to a magnetic field. Similarly, a magnetic field will cause the motion of a charged particle to bend. It is this principal that allows the dynamo within the earth to produce a magnetic field. The field is produced in the region of the outer core, that area within the earth from 2900km to 5100km below the surface which is composed primarily of molten iron and nickel. Iron and nickel are, of course, conductive. This is the first requirement for a planetary dynamo: a conductive fluid.

The necessary condition second is a supply of energy to move the fluid. This is supplied from a number of sources. There is still some residual heat from the creation of the earth trapped in the core. Then there is the heat released by the decay of radioactive elements. This means that the core is hotter at the bottom than it is at the top. The hotter fluid at the bottom is less dense than the overlying fluid, making it buoyant, so it rises. When it gets to the top, it cools down, becoming denser, and therefore it sinks. This process is called thermal convection, and it drives the motion of the core fluid. The constant release of heat from the inner core to the mantle also means that as the outer core cools, iron crystallizes out onto the inner core, causing the inner core to gradually get larger. As the iron crystallizes, it releases its latent heat, providing more heat to the surrounding outer core.

The third necessary condition for a dynamo is the rotation of the earth. As the earth spins, the Coriolis effect, the same thing that causes hurricanes to spin, deflects the rising fluid into a helical path, as if it were following the coils of a giant spring. It is this constant spiral motion of the conductive inner core fluid that produces the earth's magnetic field, at least in the broad terms. However, computer simulations of the core flow are known to be deficient. They do a good job of modeling large scale smooth, or laminar flow, but it is thought that much of the flow is actually chaotic or turbulent, which is much more difficult to model. And it is that turbulence that probably serves as the engine that periodically causes the polarity of the field to reverse.

Picture if you would the magnetic field of the earth. Magnetic field lines stream out of the southern hemisphere, curve around the earth and plunge back in in the north. If you could look at the field down at the inner core-mangle boundary, however, you would see something somewhat different. There would be patches in both hemispheres where the magnetic flux is reversed from what it is elsewhere in that hemisphere. These reversed flux patches arise when turbulent flow pushes magnetic field loops up above the outer core. As the loops rise, the twist around in a helical path, and when they break the surface, they cause the patches because each loop will contain an outward and an inward pointing section of magnetic flux. Normally, these patches stay buried in the mantle, but if the effect becomes large enough, they can break the surface of the earth, and if they become large enough they can cause a reversal of polarity. This is accomplished by a general weakening of the existing field by the reversed patches, which themselves eventually grow to replace the prior field.

How long would it take for such a reversal to complete? Computer models seem to indicate that, from the surface, the process would take from about 1000 to 3000 years. During the transition period, the surface field would be chaotic and shifting – not at all suitable for navigation. Although the surface field would settle down by then, it would take longer for the field at the core to complete its transition, and until it did, we would observe minor shifts in the field at the surface. Whether these simulations are accurate enough to reliably depict the reality is an open question.

So are we due for a reversal? It is not clear. The current period of stability is longer than average for the last 30 million years or so. Satellite observations of the magnetic field indicate that it is possible, however. In 1980 observations of the earth's magnetic field showed a large reverse flux patch in the southern hemisphere. It started in southern South America, catching southern Brazil, Uruguay, Argentina, and Chile and stretched across the Antarctic Sea to just catch the southern tip of Africa. In 2000, those observations showed that the patch had grown, becoming roughly Y-shaped by merging with a smaller patch over Antarctica and taking in more of South Africa. It is unclear, however, whether this is the start of a reversal, and it may not become clear for hundreds of years.

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