Geomagnetic reversal

A geomagnetic reversal is a change in the Earth's dipole magnetic field such that the positions of magnetic north and magnetic south are interchanged (not to be confused with geographic north and geographic south). The Earth's magnetic field has alternated between periods of normal polarity, in which the predominant direction of the field was the same as the present direction, and reverse polarity, in which it was the opposite. These periods are called chrons.

Reversal occurrences appear to be statistically random. There have been at least 183 reversals over the last 83 million years (thus on average once every ~450,000 years). The latest, the Brunhes–Matuyama reversal, occurred 780,000 years ago^[1] with widely varying estimates of how quickly it happened. Some sources estimate the most recent four reversals took on average 7,000 years to occur.^[2] Clement (2004) suggests that this duration is dependent on latitude, with shorter durations at low latitudes and longer durations at mid and high latitudes.^[2] Others estimate the duration of full reversals to vary from between 2,000 to 12,000 years.^[3]

There have also been episodes in which the field inverted for only a few hundred years (such as the Laschamp excursion^[4]). These events are classified as excursions rather than full geomagnetic reversals. Stable polarity chrons often show large, rapid directional excursions, which occur more often than reversals, and could be seen as failed reversals. During such an excursion, the field reverses in the liquid outer core but not in the solid inner core. Diffusion in the outer core is on timescales of 500 years or less while that of the inner core is longer, around 3,000 years.^[5]

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