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What causes the magnetic field to reverse its polarity?

Lorenz diagram showing chaotic orbit around two attractor points.

Nothing. That answer might surprise you, but the fact that the field occasionally reverses is simply a property of the continuous, on-going behavior of the Earth's dynamo. There is no "cause" per se. 

With respect to the physics of the process itself, some lessons can be learned from the laboratory. It is possible, for example, to design an electrical-magnetic-mechanical dynamo consisting of spinning metal disks and coils of wire, which, when supplied with mechanical energy, sustains its own magnetic field. Depending on the details of the apparatus, the magnetic field can be steady, with no time dependence at all, or it can reverse periodically, like the Sun's magnetic field does every eleven years, or it can reverse randomly, bouncing back and forth in an orbit around two preferred states (opposite polarities) like the Earth's magnetic field does.

It is also possible to develop mathematical equations that describe the behavior of this laboratory system - the equations describe what is popularly known as "chaos," and even though the laboratory system is relatively simple, its equations have some similarity to those describing the dynamics of the Earth's core. 

In summary, nature allows for different kinds of dynamos, some of which just simply have the property that they undergo occasional random reversals. Earth's core happens to be one of those dynamo types.

Tags: Geomagnetism, Magnetic Field, Monitoring, Sun, Core, Declination, Polarity