Strength of Earth's dipole magnetic field throughout the Brunhes normal chron.
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Image courtesy of the USGS.

Earth's Magnetic Field Strength - Past 800,000 Years

The strength of Earth's magnetic field fluctuates over time. Occasionally the magnetic field flips, with the North and South Magnetic Poles trading places. Those "flips" are called magnetic reversals. The last reversal was 780,000 years ago. Are we due for another reversal soon?

The graph shows the strength of the dipole portion of Earth's magnetic field over time. The strength of the dipole moment is given in units of 1022 Amperes-meters2. The last reversal of Earth's magnetic field was 780,000 year ago. The period of "normal" (meaning "like it is today") polarity of Earth's field since that reversal is known as the Brunhes normal chron. The Brunhes normal was proceeded by the Matuyama reversed chron, during which time Earth's magnetic poles were opposite their current orientation. The graph shows how the strength of Earth's dipole field has risen and fallen numerous times throughout the Brunhes normal chron.

The dashed line at a dipole strength of 4 x 1022 A m2 is a threshold for magnetic reversals noted by some scientists. In the past, magnetic reversals have sometimes occured when the dipole field strength dropped below this critical value. Note, however, that the field strength has fallen below 4 several times during the Brunhes normal without ushering in a reversal.

Some people have speculated that we are currently on the verge of another reversal. Although it is true that the strength of the dipole field has been declining for many years, one can see on the graph that the field has been considerably weaker than its current value (around 6) numerous times during the Brunhes normal... without triggering a reversal. Even though it is indeed possible that we are in the midst of a reversal, it seems more likely that this is just another temporary reduction in dipole field strength like so many that preceded it.

Last modified September 25, 2009 by Randy Russell.

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