Earth's Magnetic Fields

Earth's Magnetic Fields

On magnetic signatures to find their way across thousands of miles to the very beaches where they hatched. Now, researchers reporting in the journal Current Biology on May 6 have some of the first solid evidence that sharks also rely on magnetic fields for their long-distance forays across the sea.

"It had been unresolved how sharks managed to successfully navigate during migration to targeted locations," said Save Our Seas Foundation project leader Bryan Keller, also of Florida State University Coastal and Marine Laboratory. "This research supports the theory that they use the earth's magnetic field to help them find their way; it's nature's GPS."

Researchers had known that some species of sharks travel over long distances to reach very specific locations year after year. They also knew that sharks are sensitive to electromagnetic fields. As a result, scientists had long speculated that sharks were using magnetic fields to navigate. But the challenge was finding a way to test this in sharks.

"To be honest, I am surprised it worked," Keller said. "The reason this question has been withstanding for 50 years is because sharks are difficult to study."

Keller realized the needed studies would be easier to do in smaller sharks. They also needed a species known for returning each year to specific locations. He and his colleagues settled on bonnetheads (Sphyrna tiburo).

"The bonnethead returns to the same estuaries each year," Keller said. "This demonstrates that the sharks knows where 'home' is and can navigate back to it from a distant location."

The question then was whether bonnetheads managed those return trips by relying on a magnetic map. To find out, the researchers used magnetic displacement experiments to test 20 juvenile, wild-caught bonnetheads. In their studies, they exposed sharks to magnetic conditions representing locations hundreds of kilometers away from where the sharks were actually caught. Such studies allow for straightforward predictions about how the sharks should subsequently orient themselves if they were indeed relying on magnetic cues.

If sharks derive positional information from the geomagnetic field, the researchers predicted northward orientation in the southern magnetic field and southward orientation in the northern magnetic field, as the sharks attempted to compensate for their perceived displacement. They predicted no orientation preference when sharks were exposed to the magnetic field that matched their capture site. And, it turned out, the sharks acted as they'd predicted when exposed to fields within their natural range.

The researchers suggest that this ability to navigate based on magnetic fields may also contribute to the population structure of sharks. The findings in bonnetheads also likely help to explain impressive feats by other shark species. For instance, one great white shark was documented to migrate between South Africa and Australia, returning to the same exact location the following year.

"How cool is it that a shark can swim 20,000 kilometers round trip in a three-dimensional ocean and get back to the same site?" Keller asked. "It really is mind blowing. In a world where people use GPS to navigate almost everywhere, this ability is truly remarkable."

In future studies, Keller says he'd like to explore the effects of magnetic fields from anthropogenic sources such as submarine cables on sharks. They'd also like to study whether and how sharks rely of magnetic cues not just during long-distance migration but also during their everyday behavior.

Our planet's gassy atmosphere does a good job of promoting life and protecting us from the odd smallish asteroid. But without the Earth's magnetic field, the solar wind — a constant gale of high-energy particles shooting out of the Sun at upwards of 400 kilometres per second — would make life a lot like hell.

If those high-speed particles (mostly protons and electrons) reached our atmosphere they'd make short work of stripping it away, leaving us vulnerable to full-on solar radiation. Death would be fairly quick and crispy.

But thankfully, as well as being covered in gas, Earth has got its own magnetic field that extends way out beyond our atmosphere.

Magnetic fields don't go unnoticed by charged particles, and the solar wind gets deflected around Earth and heads further on out to the cosmic shade. (The odd bit of solar wind that sometimes gets through has the decency to put on a nice show — auroras are caused by those high-speed particles smashing into high altitude gases around the poles).

The Earth's magnetic field (the geomagnetic field to the experts) has been shielding our atmosphere for upwards of 3 billion years. Much searching has found no evidence of either a giant bar magnet skewering the planet, or the kind of huge coils of current-filled wire that would generate a field like Earth's. So what exactly is it that turns our semi-solid spinning planet into a magnet?

All magnetic fields are generated by moving charged particles. Every time a charged particle moves it generates its own magnetic field, and if a bunch of them can be organised so their individual fields don't cancel each other out, you've got yourself a magnet.

You can turn any piece of wire into a magnet at the flick of a switch. Current in a wire is caused by electrons moving in a nice orderly fashion, each with their own tiny magnetic field. (More about how electricity works). If you can align the moving electrons so their fields add together (by coiling the wire into a loop) you've got yourself an electromagnet.

Permanent magnets — like bar magnets and fridge magnets — can only be made from materials that have got unpaired electrons whizzing around their atoms. Those unpaired electrons each provide a mini magnetic field (actually, all electrons have their own magnetic field thanks to their spin, but when they're paired their fields cancel each other out). But only materials like iron, cobalt and nickel have atoms that can be arranged so the magnetic fields from their unpaired electrons add together like a zillion tiny compasses to give an overall magnetic field. You can't attach finger paintings to fridges with anything less.

The origin of our geomagnetic field is a bit more complicated. The magnet-making action goes on in the liquid outer core of the planet — a layer about 2500 kilometres thick, made mostly of molten iron with a bit of nickel and the odd lighter element.

Because the Earth's core is a tricky place to visit, no one is exactly sure what goes on down there. But geoscientists think that two effects working together could be responsible for producing our magnetic field.

The moving charge comes courtesy of all that molten iron, which is so hot that it's ionised, so there are positive ions and negative electrons moving around. But getting the tiny magnetic fields surrounding all those charges to work together relies on giant currents and massive cyclones of molten metal. In fact, the same effects that drive our weather — convection currents and the Coriolis effect — also turn all that molten iron into a giant self-perpetuating liquid magnet.

Convection is about less dense things — like hot air — rising. In the liquid outer core, the denser iron freezes onto the solid inner core, leaving less dense material behind and giving off latent heat. The combination of the heat and buoyancy means the less dense material rises up through the layer, and a convection current is underway.

Those convection currents heading up from the entire surface of the inner core are enough to generate some bigger scale magnetic fields. But if that was all that was going on, the different magnetic fields generated by the different currents would cancel each other out.

Luckily, the spin on our planet is fast enough for the Coriolis effect to kick in. The Coriolis effect is what makes cyclones spin clockwise in the southern hemisphere and anti-clockwise up north. But down in the planetary basement, the Coriolis effect makes the convection currents twist in spirals, which line up roughly north/south. That twisted lining up of the convection currents makes their individual magnetic fields align instead of cancelling out, so they form an overall planet-wide magnetic field with a decent north and south pole. Hooray for appalling internal weather!

The system is called a geodynamo, and, being based on such convoluted interdependent effects, it's no wonder the magnetic poles have a tendency to wander and occasionally flip out.

We can't see inside the planet, so we can't directly confirm how the geodynamo works. But some heavy-hitting computer simulations have confirmed that the convection and Coriolis effect could make the liquid outer core act like a magnet. Not only that, the simulated magnetic field has the same kind of wandering poles as the real field, and it even pulls one of the planetary field's most famous tricks — the old magnetic pole reversal.

We know from ancient rocks that the magnetic poles have randomly switched places throughout history. (Iron-rich minerals in magma line up with the Earth's magnetic field, so when the magma cools the orientation of the minerals gives a permanent record of the Earth's magnetic field at the time the rocks formed).

And we know from the internet that it's easy to freak people out with the idea that we could be left defenceless against solar wind/satellites/stray compasses if we lose our magnetic field during one of the reversals. Better still, the poles flip on average every 200,000 years, and we haven't had one for over 700,000 years, so all the ingredients are there for a good thriller.

The good news is, while we don't know when a pole reversal will happen, we know it won't happen overnight, or even over a lifetime. They take a thousand years or more. If the computer models are anything to go by, we'd notice a weakening of the magnetic field over a millennium or two and see a whole lot of juicy stuff like mini poles turning up here and there. We'd never be without a magnetic field altogether, and even while the field was weakening our compasses would still point north. Then finally, the field would begin to strengthen again with the poles in reversed locations, and the N on your trusty compass would be in need of some serious re-branding.
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