Wandering Poles The Strange Science of Earth's Magnetic Shield

Nearly three thousand kilometers below your feet, an ocean of liquid iron is moving. It is not water, and it is nothing like a sea you could sail. It is a shell of molten metal about twenty-two hundred kilometers thick, hotter than the surface of some stars, wrapped around a solid iron ball about two-thirds the width of the Moon. The metal churns and spirals, and as it churns it produces something that reaches out through the rock, through the oceans, through the sky, and far into space. We call it the magnetic field, and without it, life on the surface would look very different.

Scientists call the machine that makes this field the geodynamo, and the name is apt, because it works on much the same principle as a generator. Move an electrical conductor through a magnetic field, and you induce a current. That current produces a magnetic field of its own. Under the right conditions, the arrangement feeds itself. Liquid iron is an excellent conductor, and the outer core is in constant motion, driven by convection. Heat escapes from the deep interior, and as the solid inner core slowly grows, it releases lighter elements into the liquid above it. Warmer, lighter fluid rises, cooler, denser fluid sinks, and the whole mass keeps overturning.

Earth's rotation gives that overturning its shape. Because the planet spins, the rising and sinking fluid is twisted into columns and spirals aligned roughly with the rotation axis. Those flows stretch and wind up the existing magnetic field lines, regenerating the field that is already there. No one has ever seen this directly, and no one can drill into the outer core. What researchers have is physics, the faint signals of seismic waves that pass through the planet, and computer simulations running on some of the largest machines ever built.