What causes the magnetic field to reverse its polarity?
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. In answering the previous question we discussed the phenomenology of polarity reversals, what they are and how they might affect a (hypothetical) compass, but with respect to the physics of the process itself, some lessons can be learned from the laboratory. It is possible, for example, to design a machine, 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
