One of the most fascinating natural phenomena on Earth is the flipping of its magnetic field, which has occurred hundreds of times in the last 160 million years. When the magnetic field flips, the North Pole becomes the South Pole and vice versa. The last time this happened was some 780,000 years ago, so we could be heading for another reversal soon. Now, physicists in Italy have found that the frequency of these polarity reversals is not random as previously thought but occurs in clusters, revealing some kind of "memory" of previous events (physics/0603086).
Although a full geomagnetic polarity reversal can take thousands of years to complete, it does have implications. As well as affecting the migration trajectories of birds and other animals, the disruption to the Earth’s magnetic field could expose the Earth to hazardous cosmic rays. Geoscientists believe that our planet’s internal magnetic dynamo is responsible for pole reversals, but the actual mechanism is not well understood.
Previous analyses assumed that the number of times the poles have reversed over last 160 million years follows a Poisson distribution, implying that the events are random. The Poisson distribution tells you the probability of a number of events occurring in a fixed time if the events are independent and the average rate is known. A good example of the Poisson distribution in physics is the likelihood of unstable radioactive nuclei decaying in a certain period.
Now, a team of physicists led by Vincenzo Carbone of the University of Calabria have discovered that the sequence of polarity reversals can be well described by a Lévy distribution instead. In contrast to Poisson statistics, the Lévy distribution describes stochastic processes that are characterised by the presence of “memory” effects — or long-range correlations between the events in time. Lévy distributions are widely used to study many critical phenomena, such as earthquakes, and also when analysing financial data. The researchers obtained their results by careful statistical analysis of different sets of paleomagnetic data containing estimates of when the Earth’s poles reversed.
“The result means that polarity reversals are not random events that are independent of each other,” explains team member Fabio Lepreti. “Instead, there is some degree of memory in the magnetic dynamo processes giving rise to the reversals,” he says. “We hope that our work will serve as a useful reference point for models that aim to describe the phenomenon of pole reversal.” The Italy team now plans to build new dynamic models to describe the field reversal sequences in a simple way, so that the physical mechanisms that trigger pole reversals can be more easily explained.