Causes
The magnetic field of the Earth, and of other planets that have magnetic fields, are generated by dynamo action in which convection of molten iron in the planetary core generates electric currents which in turn give rise to magnetic fields. Most scientists believe that reversals are an inherent aspect of this process. In simulations, it is observed that magnetic field lines can sometimes become tangled and disorganized through the chaotic motions of liquid metal in the Earth's core. For example, Gary Glatzmaier and collaborator Paul Roberts of UCLA have made a numerical model of the electromagnetic, fluid dynamical processes of Earth's interior. Their simulation reproduced key features of the magnetic field over more than 40,000 years of simulated time. Additionally, the computer-generated field reversed itself. Global field reversals at irregular intervals have also been observed in the laboratory liquid metal experiment VKS2.
In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation. This scenario is supported by observations of the solar magnetic field, which undergoes spontaneous reversals every 9–12 years. However, with the sun it is observed that the solar magnetic intensity greatly increases during a reversal, whereas reversals on Earth seem to occur during periods of low Earth field strength.
Read more about this topic: Geomagnetic Reversal