Centuries of human observation, as well as the geologic record, show that the Earth's magnetic field changes dramatically in its strength and structure over
time.
Some 800,000 years ago, a magnetic compass' needle would have pointed south because the Earth's magnetic field was reversed. These reversals typically
happen every several hundred thousand years.
Besides the reversals, there is a secondary pattern of geomagnetic "wobble," known as paleomagnetic secular variation, or PSV, that may be a key to
understanding why some geomagnetic changes occur.
In a paper published in Earth and Planetary Science Letters, researchers reported identifying the pattern by studying two high-resolution sediment cores
from the Gulf of Alaska that allowed them to develop a 17,400-year
reconstruction of the PSV in that region, then compared those records with
sediment cores from other sites in the Pacific Ocean to capture a magnetic
fingerprint, which is based on the orientation of the magnetite in the sediment
that acts as a magnetic recorder of the past.
The Earth's magnetic field does not align perfectly with the axis of rotation, which is why "true north" differs from "magnetic north."
In the Northern Hemisphere, this disparity in the modern field is driven by regions of high geomagnetic intensity that are centered beneath North America
and Asia.
When the magnetic field is stronger beneath North America, or in the "North American Mode," it drives steep inclinations and high intensities in the North
Pacific, and low intensities in Europe with westward declinations in the North
Atlantic.
"What we have not known is whether this snapshot has any longer-term meaning - and what we have found out is that it does," said Joseph Stoner, an Oregon
State University (OSU) paleomagnetic specialist and co-author on the study.
The alternate "European mode" is in some ways the opposite, with shallow inclination and low intensity in North Pacific, and eastward declinations in the
North Atlantic and high intensities in Europe.
The magnetic field is generated within the Earth by a fluid outer core of iron, nickel and other metals that creates electric currents, which in turn
produce magnetic fields. The fact that it changes is well known, the reasons why
have remained a mystery.
The simplest form of magnetic field comes from a dipole: a pair of equally and oppositely charged poles, like a bar magnet.
"We've known for some time that the Earth is not a perfect dipole, and we can see these imperfections in the historical record," said Maureen Walczak, a
post-doctoral researcher at OSU and lead author on the study.
"We are finding that non-dipolar structures are not evanescent, unpredictable things. They are very long-lived, recurring over 10,000 years, persistent in
their location throughout the Holocene."
According to the researchers, the common magnetic signal found in the cores now covers an area spanning from Alaska to Oregon, in the U.S. Pacific
Northwest, and over to Hawaii, a U.S. state in the Pacific Ocean.
"Magnetic alignment of distant environmental reconstructions using reversals in the paleomagnetic record provides insights into the past on a scale of
hundreds of thousands of years," Walczak said.
"Development of the coherent PSV stratigraphy will let us look at the record on a scale possibly as short as a few centuries, compare events between ocean
basins, and really get down to the nitty-gritty of how climate anomalies are
propagated around the planet on a scale relevant to human society."
"As it turns out, the magnetic field is somewhat less complicated than we thought," Stoner was quoted as saying in a news release from OSU.
"It is a fairly simple oscillation that appears to result from geomagnetic intensity variations at just a few recurrent locations with large spatial
impacts. We're not yet sure what drives this variation, though it is likely a
combination of factors including convection of the outer core that may be biased
in configuration by the lowermost mantle."
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