The magnetic field of the Earth has reversed its polarity several hundred times during the past 160 million years. Polarity reversals are known to be strongly irregular and chaotic, and the reversal durations are relatively short (typically a few thousand years) compared with the constant polarity intervals between reversals.
This image shows a simple deterministic model illustrating the geomagnetic reversals. The model is based on the non-linear interaction between two magnetic modes (dipole and quadrupole) and one velocity component of the Earth's core flow, and the image shows typical trajectories in the 3D phase space. The corresponding strange attractor reproduces irregular reversals between two symmetrical states.
While the behavior in a given polarity is strongly chaotic and seems random, the path followed by trajectories during a reversal is always the same: during a reversal, the magnetic field changes shape (from dipolar to quadrupolar structure), rather than simply vanishing.