Earth's magnetic field

Reverse, reverse

To predict a cataclysmic future, scientists explore Earth’s geomagnetic past

A new study finds a 700-year-old geomagnetic anomaly.

NASA's Goddard Space Flight Center/Duberstein

In the South Atlantic, stretching from Chile to South Africa, there’s a wide span where the Earth’s magnetic field isn’t quite in line with the rest. Here, at what’s called the South Atlantic Anomaly, the field is weak. It’s so weak that it affects the satellites passing over; they’re bombarded by solar radiation and space debris that are usually held off by the field.

And this patch is growing. Underneath the surface, at the core-mantle boundary, it appears there are patches where the field has actually flipped polarity — the field is reversed. That’s why these are sometimes called reverse flux patches.

Some are concerned that if the patch grows too big, it could trigger a sudden collapse of the field or even a flip of Earth’s magnetic poles. If this were to happen, electrical grids and satellite technologies would be at risk. Experts aren’t panicking yet, but that doesn’t mean researchers aren’t doing all they can to study the causes and consequences of geomagnetic anomalies in the past, in order to inform what we might expect in the future.

In a new study published in the Proceedings of the National Academy of Sciences, paleomagnetists — researchers who study the history of Earth’s changing geomagnetic field — found evidence that a sharp directional change and intensity dip occurred around 1100–1300 C.E. in what is today Cambodia. This means the area may have experienced a geomagnetic “twitch” not unlike the South Atlantic Anomaly today.

What is Earth’s magnetic field?

At its simplest, Earth’s magnetic field acts as if there’s a giant bar magnet inside — it’s why compasses point toward the North Pole — thanks to the liquid iron in its core. But the field is more complicated than that. There are spots where it’s stronger or weaker than the bar magnet would explain, and spots where the direction is out of line.

As these patches wax and wane, they affect the field as a whole — sometimes even causing the magnetic north and south poles to flip entirely, which happens every million years or so.

Are we headed for a flip now? It’s not out of the question. To understand Earth’s future, researchers look to its past. To do this, paleomagnetists look for materials that were heated to high temperatures and then cooled — like lava, or clay pots fired in a kiln. When materials are brought to a high enough temperature, the magnetic molecules inside lose their magnetism. As they cool back down, the molecules not only become magnetized proportionally to the strength of the Earth’s magnetic field, but also are more likely to point in the direction of the field.

This, combined with radiocarbon dating, allows researchers to measure the intensity and direction of the past field from artifacts and rocks.

What’s new — In the new study, researchers from the Scripps Institution of Oceanography at the University of California, San Diego, teamed up with archaeologists excavating a site near the ancient city of Angkor, the capital of the Khmer Empire, in Cambodia.

The study team partnered with archaeologists excavating a site near the ancient city of Angkor in Cambodia.Michel RENAUDEAU/Gamma-Rapho via Getty Images

The area was once host to ancient iron-smelting operations centered around a huge hematite deposit, Phnom Dek, which in Kmerian means “iron mountain.” The researchers excavated a site called Tonle Bak that was active around the time when Angkor was starting to rise and the Khmer Empire was starting to gain power, between the 11th and 13th centuries.

The archaeological team was interested in the interplay between the widespread iron smelting and the rise of the empire. But the excavations also provided a unique opportunity to reveal information about the Earth’s geomagnetism.

“Culturally, this site was amazingly rich,” co-author Mitch Hendrickson, an archaeologist from the University of Illinois at Chicago, tells Inverse “But it also gave us this great ability to test these geological questions as well.”

Iron production sites are valuable to geomagnetic research since they once produced massive amounts of waste that was heated. Finding a smelting furnace is better than something like a clay pot, since it’s attached to the landscape — meaning it’s still facing the same direction it was when it was heated and then cooled, so the direction of the magnetic field when it was heated is preserved.

But furnaces are hard to find since they’re routinely broken apart as part of the smelting process. At Tonle Bak, they found three — the first ever found in Cambodia.

What is happening to Earth’s magnetic field?

This is the first time researchers have uncovered paleomagnetic data from this area and one of the first data points from near the equator.

Most existing records of the Earth’s past magnetic field are from heavily studied areas like Europe and the Levant, which is why researchers like lead author Shuhui Cai at the Chinese Academy of Sciences, an expert paleomagnetist in China and surrounding areas, are working to expand these records into new parts of the globe.

The researchers found that the rate of directional change from 1200 to 1300 C.E. was about 10 times what’s happening today, and the intensity of the geomagnetic field dropped sharply between 1100 and 1300 C.E. It’s reminiscent of today’s South Atlantic Anomaly. There’s no evidence that the patch caused any problems for society at the time.

An unusually weak spot in Earth’s magnetic field, known as the South Atlantic Anomaly, is being tracked by NASA scientists.

“What’s exciting about this paper is that it basically points out that there may be other areas of the globe … where the archaeomagnetic record shows one of these reverse flux patches,” John Tarduno, a paleomagnetist at the University of Rochester who was not involved in the study, tells Inverse. “So that’s really exciting.”

Reverse flux patches, or spots where the Earth’s geomagnetic field is wonky, are something scientists are still working to understand — especially when it comes to how they can affect the field as a whole. The strength of Earth’s magnetic field has already been dropping over the past 1,000 years, and if the South Atlantic Anomaly grew and took over, we’d have a field reversal, study co-author Lisa Tauxe, a paleomagnetist at Scripps, tells Inverse.

“How fast can that happen? What do we expect when it happens?” she asks. “Before I can speculate, I need to know what it’s done in the past. That’s why I’m interested in building a database from which we can address this issue.”

Abstract: Extensive spatial and temporal distribution of high-quality data are essential for understanding regional and global behaviors of the geomagnetic field. We carried out chronological and archaeomagnetic studies at the Angkor-era iron-smelting site of Tonle Bak in Cambodia in Southeast Asia, an area with no data available to date. We recovered high-fidelity full-vector geomagnetic information from the 11th to 14th century for this region, which fill gaps in the global distribution of data and will significantly improve the global models. These results reveal a sharp directional change of the geomagnetic field between 1200 and 1300 CE, accompanied by an intensity dip between 1100 and 1300 CE. The fast geomagnetic variation recorded by our data provides evidence for the possible existence of low-latitude flux expulsion. Related discussions in this paper will inspire a new focus on detailed geomagnetic research in low-latitude areas around the equator, and exploration of related dynamic processes.
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