Field notes on things that run themselves
There Is No Magnet Inside the Earth
Hold a compass anywhere on Earth and the needle settles, patiently, into line — one end swinging north as if tugged by a thread. It is easy to picture what it points at: a great bar magnet buried in the planet, its poles roughly under the Arctic and the Antarctic. There is no such magnet. There cannot be. A permanent magnet is a solid whose atoms have been frozen into alignment, and it surrenders that alignment the instant it gets hot — iron gives up its magnetism entirely at about seven hundred and seventy degrees Celsius. The deep Earth is thousands of degrees. Whatever the needle is answering to, it is not a magnet in the ordinary sense; it is something with no fixed magnetization at all, no stored north and south — a field with, in a real way, no object underneath it.
The field is made, right now, about eighteen hundred miles beneath your feet. Down there, under the cool rock of the mantle, the planet turns liquid: an ocean of molten iron and nickel some fourteen hundred miles deep, wrapped around a solid ball of iron at the very center. This is the geodynamo — dynamo because, like the alternator turning in a car, it makes a magnetic field out of sheer motion, and geo because the moving part is a planet’s worth of metal. The iron down there is far too hot to hold any magnetism of its own. What it can do is flow, and carry electric current as it flows, and that turns out to be enough.
The trick is a loop that feeds itself. Set a parcel of that molten metal moving — heat lifts it, the planet’s spin twists it — and because liquid iron is an electrical conductor sweeping through whatever faint magnetic field already happens to be there, the motion drives electric currents through it. Those currents, in turn, wrap themselves in a magnetic field of their own. And that fresh field is exactly what the next moving parcel of iron needs in order to drive the next round of current. Motion makes field; field organizes motion into more of the same. Geophysicists call it a self-exciting dynamo: a magnet that generates the very thing it needs in order to go on being a magnet, bootstrapped out of nothing but a stirred, conducting fluid. There is no permanent core to it and no stored charge — still the stirring, and there is nothing left for a compass to find.
So what does the stirring? Two engines, mostly, and both come down to heat trying to get out. The whole core is slowly cooling, so hot iron near the bottom rises while cooler iron sinks — ordinary convection, the overturn of a pot on a stove scaled up to a world. But there is a subtler pump underneath it. At the very center, the liquid iron is freezing, grain by grain, onto the solid inner core, and as it freezes it leaves its lighter impurities behind — a broth of lighter elements that floats up through the heavier melt like foam rising in a glass. That slow buoyant rise stirs the outer core from below, and the planet’s rotation combs all the churning into tall, coherent, corkscrewing columns. The magnetic field we live inside is just the neat outward signature of that hidden, roiling weather of iron.
Here is how we know the field is nothing but process. Left to itself — the iron stilled, the currents no longer driven — a magnetic field this size does not simply sit there; it bleeds away, its currents dragging against the metal’s own electrical resistance, losing more than half their strength roughly every fifteen thousand years and fading to nothing within a few tens of thousands. On the clock of a planet, that is a struck match, out almost as soon as it’s lit. And yet rocks that froze the field’s direction into themselves as they cooled record it running for at least three and a half billion years, and very possibly longer still. A pattern that should erase itself in twenty thousand years has instead held for billions. There is only one way to read that: it is being remade the entire time — not a thing that endures but a thing that is poured, without pause, into the same shape.
And because it is poured rather than fixed, it wanders. The magnetic poles drift year by year; the whole field breathes stronger and weaker; and every so often, on no schedule anyone can forecast, it does what no bar magnet ever could — it sags toward nothing and returns inside out, north and south changed ends. These reversals are not a clock. They have come a few times per million years of late, but that average hides everything: the last full flip was about seven hundred and eighty thousand years ago, and there have been stretches of tens of millions of years with no reversal at all. The field keeps no time and promises no particular arrangement. The only thing it keeps is itself — one shape held steady while an ocean of iron pours through it and nothing solid ever stays.
One loop I’m watching
Next: the Sun runs the very same trick on a wilder scale — a magnetic field conjured out of nothing but moving fluid — except its fluid is plasma, not iron, and unlike Earth’s aimless field the Sun’s keeps a remarkably good clock, turning its whole magnetic self inside out about every eleven years. A dynamo that reverses on a schedule.
Tip: the ← and → arrow keys move between issues.
Prefer feeds? Follow by RSS or JSON Feed.
Want something playable between issues? Thought Toys has its own email list and RSS feed.
New to The Standing Wave? Start here →