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Geomagnetic polarity reversals


 

 

The Earth’s magnetic field has not always been oriented as it is today. It has reversed is polarity many hundreds times during its geological history. Although the geomagnetic polarity reversal is one of the most remarkable phenomena, the mechanisms in the Earth’s core that cause it are little understood, leaving reversals one of the most enigmatic geophysical phenomena.
From a theoretical point of view the possibility that the geomagnetic field could change its polarity was well known. Indeed, the magnetohydrodynamic description of the geodynamo is insensitive to the sign of the magnetic field: i.e. the induction equation is invariant under magnetic field parity transformations. Therefore, the geodynamo can have a polarity opposite to the directly observable normal polarity as an equally viable solution. So if a mechanism for a transition between these two states exists, we should expect normal and reversed polarity states with equal probability.

In the past, several hypotheses were put forward on the possible mechanisms responsible for polarity reversals. All these hypotheses can be grouped into two main classes. In the first case reversals are considered as the results of magnetohydrodynamic instabilities triggered by finite amplitude perturbations of an otherwise stable dynamo. Conversely, in the second case polarity reversals are considered to be due to irregular oscillations of nonlinear dynamo.
Although, the conditions that give rise to the geomagnetic polarity reversals are not completely understood, it is known that the reversal sequence of the Earth’s magnetic field contains information about Earth’s core processes. In particular from the analysis of the length distribution of the polarity intervals it is possible to try to model the reversal mechanism while from the analysis of the rate of occurrence of these reversals it is possible to try to gain information on gradually changing conditions at the core-mantle boundary. Indeed, it is generally accepted that changes in the frequency of field reversals reflect the dynamic evolution of the fluid core and that the analysis of long-term trends in geomagnetic reversal frequency might provide important information on core-mantle coupling.

Modern reversal chronologies show more than 300 successful reversals in the past 166 Ma. Indeed, it has been found that the direction of the dipole component reverses, on an average, about every 300,000 to 1,000,000 years. These reversals are very sudden on a geologic timescale (about 5,000 years) and the time between reversals is highly variable, sometimes occurring in less than 40,000 years and at other times remaining steady for as long as 35,000,000 years. No regularities or periodicities have yet been discovered in the pattern of reversals. Indeed, a long interval of one polarity may be followed by a short interval of opposite polarity.

The geomagnetic polarity timescale since the late Jurassic
(figure from Windows to the Universe)

A time interval in which the geomagnetic field is predominantly of one polarity is known as a chron: typical lengths range from of the order of 0.1-1 Myr, with those rare extremely long intervals dominated by one polarity being designated superchrons. The polarity chrons are interrupted at irregular intervals by shorter polarity subchrons (originally called events) lasting for 20-50 ka. At times the polarity record shows large departures of the magnetic pole from normal or reversed polarity, but the polarity does not change completely; the pole wanders into equatorial latitudes but returns to its initial location on the rotation axis. The departure is short-lived, lasting less than 10 ka, and the phenomenon is called a magnetic excursion.

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