The Science of Discworld Revised Edition

forces, which are like centrifugal forces but a bit more subtle, caused by the Earth’s rotation on its axis. Roughly speaking, the twisting tangles the original, weak magnetic field like spaghetti being twirled on to a fork; then the magnetism bubbles upwards, trapped in the rising parts of the iron core. As a result of these motions, the magnetic field becomes a lot stronger.
    So, yes, the Earth does behave a
bit
as though it had a huge bar magnet buried inside it, but there’s rather more going on than that. Just to paint the picture in a little more detail, there are at least seven other factors that contribute to the Earth’s magnetic field. Some of the materials of the Earth’s crust
can
form permanent magnets. Like a compass needle pointing north, these materials align themselves with the stronger field from the geomagnetic dynamo and reinforce it. In the upper regions of the atmosphere is a layer of ionized gas – gas bearing an electrical charge. Until satellites were invented, this ‘ionosphere’ was crucial for radio communications, because radio waves bounced back down off the charged gas instead of beaming off into space. The ionosphere is moving , and moving electricity creates a magnetic field. About 15,000 miles (24,000 km) out lies the ring current, a low-density region of ionized particles forming a huge torus. This slightly reduces the strength of the magnetic field. The next two factors, the magnetopause and the magnetotail, are created by the interaction of the Earth’s magnetic field with the solar wind – a continual stream of particles outward bound from our hyperactive sun. The magnetopause is the ‘bow wave’ of the Earth’s magnetic field as it heads into the solar wind; the magnetotail is the ‘wake’ on the far side of the Earth, where the Earth’s own field streams outwards getting ever more broken up by the solar wind. The solar wind also causes drag along the direction of the Earth’s orbit, creating a further kind of motion of magnetic field lines known as field-aligned currents. Finally, there are the convective electrojets. The ‘northern lights’, or aurora borealis, are dramatic, eerie sheets of pale light that waft and shimmer in the northern polar skies: there is a similar display, the aurora australis, near the south pole. The auroras are generated by two sheets of electrical current that flow from magnetopause to magnetotail; these in turn create magnetic fields, the westward and eastward electrojets.
    Yes,
like
a bar magnet – in the sense that an ocean is like a bowl of water.
    Magnetic materials found in ancient rocks show that every so often – about once every half a million years, but with no sign of regularity – the Earth’s magnetic field flips polarity, reversing magnetic north and south. We’re not sure exactly why, but mathematical models suggest that the magnetic field can exist in these two orientations, with neither of them being totally stable. So whichever one it’s in, it eventually loses stability and flips to the other one. The flips are rapid, taking about 5,000 years; the periods between flips are about a hundred times as long.
    Most of the other planets have magnetic fields, and these can be even more complicated and difficult to explain than that of the Earth. We’ve still got a lot to learn about planetary magnetism.
    One of the most dramatic features of our planet was discovered in 1912 but wasn’t accepted by science until the 1960s, and some of the most compelling evidence was left by those flips in the Earth’s magnetism. This is the notion that the continents are not fixed in place, but wander slowly over the surface of the planet. According to Alfred Wegener, the German who first publicized the idea, all of today’s separate continents were originally part of a single supercontinent, which he named Pangea (‘All-Earth’). Pangea existed about 300 million years ago.
    Wegener surely wasn’t the first person to speculate along such lines, because he got the idea – in part, at least – from the curious similarity between the shapes of the coasts of Africa and South America. On a map the resemblance is striking. That wasn’t Wegener’s only source of inspiration, however. He wasn’t a geologist; he was a meteorologist, specializing in ancient climates. Why, he wondered, do we nowadays find rocks in regions with cold climates that were clearly laid down in regions with warm climates? And why, for that matter, do we