Mysterious magnetism

The Earth has a magnetic field and that's why the needle of a compass always points North. People have been using compasses to find their way around for over 2000 years, but we still don't know where the Earth's magnetic field comes from and how exactly it behaves. "It's unbelievable that people are living on the Earth and not understanding why the compass needle points North," says Emmanuel Dormy, a mathematician who is currently co-organising a research programme about the topic at the Isaac Newton Institute for Mathematical Sciences (INI) in Cambridge. Only the most cutting edge of maths and science questions get discussed at the INI — which goes to show just how mysterious the Earth's magnetism still is. The Earth is a dynamo As a very crude approximation, the Earth's magnetic field is like that of the familiar bar magnet, which has poles at either end and generates magnetic field lines all around it.

The details of the Earth's field, though, are a lot more complex. Scientists think that it is generated by electrical currents running through the liquid, molten iron that forms the outer layer of the Earth's core. In effect, the Earth behaves like a massive dynamo, not unlike the dynamos that power bicycle lights (you can find out more in this article). But because the liquid in the Earth's outer core is continually sloshing about, the Earth's magnetic field displays tangles a simple bar magnet can't rival, and it also changes over time — it can even flip, reversing its North and South magnetic poles. The details of what's going on, and the theory explaining the dynamo process, are far from understood.

“Of course we all know technical dynamos, but they work only because there's a complex arrangement of wires, sliding contacts, and so on," says Ulrich Christensen, who organises the INI programme alongside Dormy. "[By contrast] the Earth's iron core is an unstructured ball of fluid and it's much more difficult to understand how in such a system a magnetic field can be generated."