Why is the Earth's magnetic field north-south?

Why don't compasses point to East or West?
31 January 2017


"The Blue Marble" is a famous photograph of the Earth taken on December 7, 1972, by the crew of the Apollo 17 spacecraft en route to the Moon at a distance of about 29,000 kilometres (18,000 mi). It shows Africa, Antarctica, and the Arabian Peninsula.



Why is the Earth's magnetic field north-south?


Stuart - OK, first thing’s first, you might be interested to know that the earth has more than one north and south pole.

Graihagh - um, que?

Stuart - Yeah! It has geographic poles – these as the two points on the surface of the globe which the earth rotates around. But it also has magnetic poles. These are where the Earth’s magnetic field lines flow in and out of the earth’s surface.

But the magnetic and geographic poles aren’t the same – they’re a few hundred kilometres apart.

Graihagh -  OK so where do the magnetic poles come from, and why are they different?

Stuart - Our current best theory is that the earth’s magnetic field is caused by its core. By studying vibrations from earthquakes as they pass through the earth, scientists estimate that the core is made of a solid metallic centre, surrounded by a layer of molten metal.

It’s the movement of this molten layer that’s thought to create the earth’s magnetic field.
So that’s why the magnetic north and south poles line up with the geographic north-south, rather than say east-west.

But as well as rotating, the layer also has convection currents, a swirling of the metal caused by intense heat.

It’s the combination of both rotation and convection that is responsible for the earth’s magnetic field – this is known as Dynamo Theory. This complicated movement, influenced by other factors, mean the magnetic and geographic poles don’t quite line up. And in fact, throughout the earth’s
history they’ve continually switched places!

Graihagh -  Fortunately for us, this happens not very often – can you imagine the calamity and confusion if our compasses were all wrong? On average, the poles switch every 250 thousand years. And we know this...

Stuart - By looking at patterns in different rock layers, it’s possible to work out that the earth’s magnetic field has flipped direction many times in the past. Current researchers use supercomputers to try model and understand this still mysterious behaviour…

But what it does mean is that if the poles were to flip again, any compasses would be left pointing in completely the wrong direction…
Graihagh - Stuart Higgins turning our world upside down there. Next time, we’ll be tackling this:


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