The Poles, they are a-changing …

The ups and downs of Earth’s magnetic field

By Bart Braun

It looks as if Earth’s magnetic field is about to be reversed – but will it really reverse itself? And if so, what will happen? “Your descendents will survive”… but our satellite technology probably won’t.

A compass points to the north – at least, here in the Netherlands its does. However, if you were in South Africa, your compass would deviate to the west and if you were in Siberia, more to the east, because magnetic north and geographic north are not exactly the same.

It’s nothing to worry about: sailors have been allowing for this declination for centuries and have even measured it in their attempts to determine their location at sea more accurately. But historians, delving into the Navy archives, have noticed changes in the declination measurements in the course of the past centuries: the earth’s magnetic field is shifting. The changes are only tiny: two or three degrees per century, and more if you are closer to the Poles. But if you ever discover an old treasure map with north marked on it, you would need to take this information into account if you want to find the treasure.

On the geological time scale, which ranges across millions or even billions of years, it is evident that the North Magnetic Pole has switched back and forth across the planet quite a number of times. Sometimes, the magnetic north simply flips over towards the geographic South Pole.

To understand how it works, you will need more than the standard image of Earth. At school, you will have been taught that you could regard the Earth as a gigantic bar magnet, but in reality, things are more complicated. Geologists know, from measuring earthquakes, that the core of the Earth is solid and covered by a thick, liquid layer of iron, heated to thousands of degrees: it flows around at a rate of twenty kilometres per year. The iron’s movement produces a magnetic field; much the same happens in a dynamo: its movement produces a current to light your bike lamp. Electricity and magnetism are closely interrelated.

This liquid magnet is situated three thousand kilometres below the earth’s surface but can still interact with the upper parts of the Earth´s crust. “There are periods during which the Earth’s magnetic field reverses its polarity and periods during which nothing really happens”, explains Professor Cor Langereis, Professor of Paleomagnetism at Utrecht University.  “There is a kind of system to this reversal: the continental drift due to plate tectonics seems to be related to periods of many, or very few reversals, of Earth’s magnetic field.”

The last reversal occurred around 780,000 years ago, but it is possible that another one is imminent: since we have been able to gauge the strength of Earth’s magnetic field, it has grown a little weaker. It is something that occurs during a reversal: Earth’s magnetic field drops to ten per cent of its usual strength, and new magnetic South and North Poles pop up.  The word “pop up” should be read here in the patient, geological sense of the word: a reversal takes a total of three thousand years to come about.

“Even if a reversal were imminent, the soonest it would occur is in 1,500 years time”, continues Langereis. “In geological terms, that’s tomorrow, of course.”

The Earth’s magnetic field is not just handy for reading compasses: it also works as an invisible force field. The universe in general, and more specifically the sun, continually fire enormous amounts of small particles at us, which the magnetic field intercepts and deflects away from Earth. A small number end up at the Poles and crash into the atoms in the atmosphere, giving us the Northern lights.

If the magnetic field becomes weaker, as it does during a reversal, many of those particles will not swerve past, but will head straight for earth. Could that cause any harm? “Your descendents will survive”, says Langereis reassuringly. “After all, it has happened before and mankind has survived. When the magnetic field is at only ten per cent of its strength, the increased radiation will be comparable to the levels to which you are exposed if you live in a place built on granite.” Granite is actually is tiny bit radioactive, but it hasn’t yet decimated the inhabitants of say, Ireland, which is mainly granite.

Neither will it harm animals that use the magnetic field for orientation. Langereis explains: “If you demagnetize a pigeon, it will circle for a little longer, but it will find its way eventually. Animals also use sight to navigate, or currents – like sea turtles do. But it will have consequences for all our electronic gear”, he adds. The sun’s magnetic storms could seriously damage them. A diminishing magnetic field around the Earth would make our satellites particularly vulnerable. In 1989, a sun storm destroyed a Canadian nuclear power station’s transformer. That should give your descendents something to worry about.

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