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16 July 2014updated 17 Jul 2014 8:49am

Get ready for the Earth’s magnetic field to flip over

New satellite data has indicated that the Earth's magnetic field is weakening, ahead of a rare - but regular - event.

By Ian Steadman

It’s still somewhat surprising to think that plate tectonics has only been an accepted and established for roughly 50 years. That is, our understanding of the structure of atoms was more sophisticated, earlier, than our understanding of the planet we live on. Though, that’s maybe unfair; as much as scientists could infer the existence of plate tectonics from, for example, the similarities between the coastlines of Africa and South America, gathering evidence to prove that the Earth has a crust floating on a molten mantle was somewhat difficult. (And it meant that the person credited as the modern theory’s originator, Alfred Wegener, was considered a crank for years before the establishment changed its mind.)

After the Second World War, scientists were given the chance to use submarine-detecting devices for peaceful research missions, mapping the ocean floor. In the early 1960s a series of papers were published on the magnetic properties of the undersea world, with some surprising findings – it appeared that some places appeared to have ocean floor with a magnetic field that was the reverse of the Earth’s. This chimed with the experience of fisherman living off volcanic islands, who have known for centuries that some patches of ocean can cause the north point of a compass to suddenly switch and point south; and it was consistent with magnetic rocks on land that also seemed to have the “wrong” magnetic polarisation.

Yet what became clear when those patches of irregularity were mapped, across the whole of the Atlantic and Pacific, was that they weren’t randomly allocated patches – rather, it became clear that the ocean floor was laid out in long, symmetrical stripes of magnetism, emanating from faults like the Mid-Atlantic Ridge. It was clear that magma was coming up from inside the planet, magnetised with a certain polarity, and then cooling and forming new rock as two tectonic plates moved apart from each other; and that every time the Earth’s magnetic field flipped, it reversed the polarisation of the magma that was becoming new sea floor. It was a key discovery in confirming the theory of plate tectonics.

It might be a surprise to hear that the Earth’s magnetic field flips over, but it does – every few hundred thousand years, the field weakens, then suddenly (which is relative in this context, on the order of a few hundred years) what was north-facing becomes south-facing, and what was south-facing becomes north-facing. We monitor the Earth’s magnetic field with satellites these days, and the European Space Agency’s Swarm array has noticed it weakening more significantly than expected:

Measurements made over the past six months confirm the general trend of the field’s weakening, with the most dramatic declines over the Western Hemisphere.

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But in other areas, such as the southern Indian Ocean, the magnetic field has strengthened since January.

The latest measurements also confirm the movement of magnetic North towards Siberia.

Rune Floberghagen, the mission manager for Swarm, told Live Science that this new data could mean that a flip is due within the next few hundred years, contrary to earlier estimates of around 2,000 years from now.

We don’t yet know whether this is signifying a “proper” flip like the Brunhes–Matuyama reversal of nearly 800,000 years ago, or a more temporary one like the one that occured roughly 40,000 years ago during the last ice age, but which only lasted for slightly more than 400 years. This is in part because we still aren’t totally sure why these reverses even happen at all. The poles wobble anyway, and it could be that the molten core of the Earth is a bit like a spinning top, occasionally changing its pattern of movement and falling over; or it could be because of large chunks of mantle nearer the surface “fall” into the centre periodically, causing turbulence that throws things off.

Regardless, the changing of the magnetic field is not a cause for alarm, even if the magnetic field is largely responsible for protecting us all from the worst effects of cosmic radiation. This is because the magnetic field does not entirely disappear – it just weakens. There are some who theorise that these events have been linked with mass extinction events, but the evidence is tenuous at best.