A spin in the Earth's magnetic field may be in the making. And if it is, an electromagnetic bubble in southern Africa is likely ground zero for change.
New research using burnt clays in cleaning rituals by Iron Age farmers finds that over the past 1,500 years, an electromagnetic anomaly in the Southern Hemisphere has risen and fallen, with the magnetic field in the region weakening and strengthening. This oddity may herald a gradual reversal in the magnetic field, such that magnetic north moves toward the South Pole and vice versa. (Such a flip-flop last occurred 780,000 years ago.)
The study suggests that the magnetic field in southern Africa may not be rare today, study co-author John Tarduno, who researches Earth's magnetism at the University of Rochester in New York, told Live Science. It can be a long-standing hotspot for changes in the global magnetic field.
"This may be the place where reversals began in the last few million years," Tarduno said.
The planet's magnetic field is generated by the churning of liquid iron in the core. Without the field, life on the planet would be very different, if not impossible: this invisible shield protects the Earth's surface from deadly cosmic radiation.
Right now, the field is experiencing weakness, and no one is sure why. The South Atlantic Anomaly, a region of the magnetic field stretching from South Africa to Chile, is particularly weak, Tarduno said, so scientists have been interested in figuring out what might be going on in the core below that area.
The problem is that prior to about 160 years ago, with the advent of magnetic observatories and (eventually) satellite observations, there weren't many records of what the magnetic field looked like in the southern hemisphere, Tarduno said. Ninety percent of the data that exists comes from the northern half of the planet. To begin to rectify that disparity, Tarduno and his team excavated clays from the Limpopo River Valley in Zimbabwe, South Africa and Botswana. In times of drought, hundreds or thousands of years ago, Bantu-speaking farmers burned their clay huts and grain deposits in ritual ceremonies. Unbeknownst to these ancient farmers, the fire heated the magnetic minerals in the clay and set in their place a record of the strength and orientation of the field at that time. Now, researchers can study those properties to find out what the magnetic field was doing at the time.
Locked in clay
Excavations unearthed these burned clays as early as 425 AD. C., Tarduno said, providing the longest record to date of the magnetic field in southern Africa. The data shows that the magnetic field underwent sudden directional changes between A.D. 400 and 450, and then again between D.D. 750 and 800. Enter approximately D.D. 1225 and 1550, the field was noticeably weakened. The first two shifts could also indicate a weakened field, Tarduno said, but more research is needed to determine the magnetic intensity in those time frames. The researchers reported their findings on February 15 in the journal Geophysical Research Letters.
What these changes suggest is that what is happening in the southern hemisphere's magnetic field today may have happened earlier, Tarduno said.
The field changes may have to do with underlying processes moving deep below the Earth's surface, Tarduno said. In recent years, scientists have documented a strange patch of magnetic field below southern Africa at the core-mantle boundary, where the polarity of the field is reversed.
"That patch may be largely responsible for the decreasing magnetic field," Tarduno said.
The patch is like a whirlpool in a stream, he said. As for what causes the eddy, it may be something strange about the mantle just above the core at that location, he said. The mantle under southern Africa is unusual, and possibly both warmer and denser than the surrounding mantle, he said.
"We think it's causing changes in the flow of iron [in the core] as it enters this region," Tarduno said.
That could mean that southern Africa is the source of magnetic field reversals, Tarduno said, although there is no guarantee the field will change now, the weakening could also dissipate, as it has in centuries past.
Even if the field is not reversed, however, the weakening itself could have social implications, Tarduno said.
“They are not in the nature of disaster movies. That's not the point, ”he said. Instead, a weakening field could allow more cosmic radiation to hit Earth, making infrastructure like the power grid more susceptible to geomagnetic storms and even changing atmospheric chemistry so more UV rays can creep in, causing greater risk. of skin cancer in humans.
"It's definitely something we have to take care of, take a look," Tarduno said.
Original article (in English) Live Science.