High above Earth at an altitude of around 480km (300 miles), roughly over Brazil, is something strange. A spot where satellites go haywire, where the Hubble Space Telescope can’t collect data and where even the astronauts on the International Space Station (ISS) can’t go on spacewalks.
You can’t see it, but it’s there. And it’s nicknamed the Bermuda Triangle of Space.
Officially known as the South Atlantic Anomaly, or SAA, the strange behaviour of electronics in this region is caused by a dent in Earth’s magnetic field.
While the magnetic field around the planet helps keep us safe from radiation in the form of charged particles, the field is much weaker in this area – and that means more of these dangerous particles.
“In this region, charged particles routinely travel closer to the surface of Earth than they do elsewhere,” says Dr Ashley Greeley, a NASA heliophysicist. These particles get trapped in this dent and can hang around there for years, causing chaos to spacecraft.
“If an energetic proton strikes sensitive electronics, it can cause a temporary issue, like data loss or bit flip, or a permanent loss, where the electronic component stops working entirely. Or charged particles can accumulate on the surface of the spacecraft and cause spacecraft charging, which can, in turn, damage electronic components.”
Space agencies and private companies make sure to turn their satellites off if they’re going to pass through the SAA and they take extra care not to schedule any spacewalks there either. But the presence of the anomaly isn’t something we have any control over. Nevertheless, we need to keep track of it because it’s moving.
A sinkhole in space
The SAA is a weak point in the Van Allen belts, the pair of protective belts of radiation around Earth’s equator held in place by its magnetic field.
These two doughnut-shaped belts (one inner and one outer) sit hundreds of miles above Earth’s surface and are full of energetic particles that can be trapped there, travelling at high speeds, for thousands of years.
But they’re not symmetrical, and where the field is weaker, the particles come closer to Earth. These electrons (in the inner belt) and protons (in the outer belt) can get as close as 200km (125 miles) from the surface – putting them squarely within low Earth orbit, where many satellites and the ISS live.
That might make the SAA seem like a huge annoyance, but in fact, it’s a vital part of keeping the space around us safe from radiation.
“If there was no SAA, then there would be harsh radiation right above the atmosphere,” explains space weather expert Prof Yuri Shprits of the German Research Centre for Geosciences.
That’s because the SAA acts like a sink, drawing in these energetic particles and keeping the rest of the orbit clear. This one region is dangerous, but it makes the wider environment safer.
It's all about the core
While the effects of the anomaly are felt high up in space, the reason for it is located deep within Earth’s core, where temperatures reach thousands of degrees. A layer of liquid iron and nickel around 2,253km (1,400 miles) thick makes up the outer core, and as this liquid metal flows around, it creates the planet’s magnetic field.
This liquid metal is moving due to convection – the same reason that hot air rises, says geomagnetism researcher Dr Monika Korte. “Hot material rises and cooler material sinks. Then this convective motion becomes more and more turbulent, and becomes mixed up by the force of Earth’s rotation.”
This twisting, convective movement of liquid metal creates a kind of dynamo, producing a huge magnetic field that stretches out far beyond the planet. But it isn’t perfectly even and there’s something about these processes in the core that create the SAA.
While the planet’s magnetic field is generally a dipole field – it has a magnetic pole near the north pole and another near the south pole – the field isn’t uniform and there are areas where the field differs.
“What we find, in particular under the SAA, are that in some regions we have patches where the magnetic flux has the opposite sign compared to this large dipole direction,” says Korte.
There’s one of these reverse flux patches at the boundary between the core and the mantle under the SAA, but similar patches also appear elsewhere, near the northern magnetic pole for example. How these patches relate to the SAA is something researchers are still trying to understand, however.
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An anomaly on the move
What really makes the SAA odd though, is the fact that not only is it growing, it’s also moving.
“It moves fast,” says Prof Ricardo Trindade, who researches the history of the SAA. “Every five years, you see a difference. And over centuries it really changes a lot.”
To look back in its history, researchers search for stalagmites in caves. These mineral deposits are formed over centuries by water dripping from above and accumulating on the cave floor. By cutting them into slices and running the slices through a magnetometer, experts can plot the history of the Earth’s magnetic field.
It’s like looking at the rings of a tree to determine its age.
This kind of research has shown that the SAA isn’t new and can be traced back as far as 11 million years. But it has also shown that the anomaly has shifted over time, and that it didn’t start out over the South Atlantic at all.
“The anomaly was born in Western Africa, in Namibia, more or less,” says Trindade. It started out small around the year 1500, but then started to grow and migrate toward the west. “Then in the beginning of the 20th century, it was already very close to South America.”
The location and movement of the anomaly isn’t random, but rather seems to be related to how it’s generated within Earth.
The boundary between the outer core, which is liquid metal, and the mantle, which is solid rock, means there’s a big difference in movement and heat conduction between the two layers. And there seems to be some relationship between the differences in the mantle beneath Africa and the generation of the anomaly.
“We think it’s related to plate tectonics,” said Trindade. “It’s the fact that Africa has no plates subducting around it.” With no plate edges nearby, there’s no way for cooler material to move down deeper into Earth, so this portion of the core-mantle boundary remains hot.
But in South America, the view under the surface is quite different. There, the Nazca plate carries material from the surface straight down to the core-mantle boundary, making the boundary cooler.
It’s this difference in temperature that could be the cause of the anomaly, and the explanation for how and why it moves over time. It also seems to not just be a one-off phenomenon, but something that’ll happen over and over again.
“[When looking back at the historical record] what we could see, is that every time an anomaly is born in Africa, 250 years later the same anomaly reaches South America,” says Trindade.
“The anomaly must be born in Africa because of this anomaly in the mantle and then it moves to South America. Then somehow, after some time a new anomaly appears in Africa and the anomaly in South America disappears.”
If this is true, then the anomaly must be millions of years old, coming and going over time in an endless cycle.
A dramatic reversal
Given the apparently cyclical nature of the anomaly, people have wondered if it’s related to another cycle of Earth’s magnetic field, in which the magnetic poles occasionally reverse.
Over the history of the planet, scientists have observed that the magnetic field can drop to a very low strength and then its magnetic north and south can switch places.
“That would be a big thing, because if the field flips, we’re going to get a field that’s much weaker than the field nowadays,” says Trindade, comparing the results to having an SAA over the entire planet.
“So all satellites are going to be in danger, all communications, all our space missions are going to have to deal with that. And that’s a huge problem.”
Both he and other experts agree that this isn’t in danger of happening any time soon, however. The average strength of the magnetic field is falling, but it still appears far above the level that would trigger a reversal.
It’s hard to predict, though, because the periods over which this reversal happens aren’t regular. “We have times where it changed several times within a million years, and then we have this time in the Cretaceous, for example, where over nearly 40 million years, it stayed stable in one direction,” says Korte.
“We see that on average, it has changed two to four times per million years. But that’s really just an average.”
Some people think that this reversal could be related to the anomaly, because both feature an unusually low level of magnetic field strength, and both seem to be related to the movement of material in the core.
“There are people who would suggest that eventually, when you cool down the surface of the core-mantle boundary, you reduce the effectiveness of the convection into the core, and then you’re more prone to have a flip,” Trindade explains.
But from current data, the historical reversals that we do know about didn’t happen when the anomaly was at one particular position. It also suggests the weakened field of the SAA doesn’t indicate a reversal happening soon.
The question is still very much open, however. “No one really understands yet how these events happen, why they happen and what the relation between them is,” says Korte.
Critical to life
All this talk of weakening magnetic fields and satellites under threat from anomalies might sound alarming, but we don’t need to worry about a reversal just yet. The planet’s magnetic field has strengthened and weakened throughout history, and it’s at a stronger-than-average level right now.
For a reversal to take place, a significant change within Earth’s core is required, which can’t happen that fast.
“I don't really like to say that the poles flip, because that sounds like it’s something that can happen very quickly,” Korte says.
The process isn’t like flipping a light switch and it isn’t on the horizon: “It’ll definitely not happen within our life spans and the very earliest would be in 600 or 700 years – and even that, I would think, is quite unlikely.”
The SAA is a powerful demonstration of the importance of the invisible but vital magnetosphere around our planet, and the way that it protects us from brutal space radiation. Some scientists have even stated that without the magnetosphere, life could never have evolved on Earth.
“If we didn't have the magnetosphere, our atmosphere would be eroded away,” Trindade says. That means losing water and carbon dioxide, and eventually there would be no water cycle left.
“The water cycle is something that I think everybody agrees is essential for life. So indirectly, the magnetic field is very important for keeping life on a planet.”
About our experts
Dr Ashley Greeley is a NASA heliophysicist and research scientist in the Energetic Particles Lab. She has been published in journals such as Journal of Geophysical Research: Space Physics, Geophysical Research Letters and Journal of Atmospheric and Solar-Terrestrial Physics.
Prof Yuri Shprits is a researcher at the German Research Centre for Geosciences. His work is published in various journals including Space Weather, Journal of Geophysical Research: Space Physics and Scientific Reports.
Dr Monika Korte is a geomagnetism researcher at the Helmholtz Centre for Geosciences. She is published in Applied Computing and Geosciences, Physics of the Earth and Planetary Interiors and Frontiers in Earth Science to name a few journals.
Prof Ricardo Trindade is a professor of geophysics at the University of São Paulo. His work has been published in a variety of journals like Journal of Geophysical Research: Solid Earth, Earth Planets and Space and Chemical Geology.
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