Nib asked
Why does the Earth have a magnetic field?
Earth is not a giant bar magnet: its core is far too hot to hold a permanent magnet. The field is made continuously by a dynamo, churning liquid iron in the outer core, twisted by Earth's spin, whose electric currents sustain their own…
7 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the bar magnet
Textbook pictures show Earth's field looping out of one pole and into the other, exactly like the field of a bar magnet. From that, it is easy to imagine a giant magnet sitting in the middle of the planet. The picture of the field is fine. The explanation hiding inside it is not.
Earth's field looks like a bar magnet's, but nothing like a bar magnet makes it.
a common mix-upEarth does not have a giant permanent magnet buried in its core.
2too hot
A magnet works because the tiny magnetic regions inside the iron line up. Heat shakes them out of line, and above the Curie temperature, 770 degrees Celsius for iron, the magnetism disappears completely. Earth's outer core is at least 4,000 degrees Celsius, five times hotter. So the magnetic field cannot be stored in the core: it has to be made, all the time.
Iron stops being a magnet at 770 degrees Celsius; the core is over 4,000.
Why can't a permanent magnet exist in Earth's core?
Show the answer
It is far above iron's 770 degree Celsius limit, so the iron cannot hold magnetism.
3the core
Earth's center has two parts. The inner core is a solid ball of iron and nickel, about 1,200 kilometers in radius, kept solid by enormous pressure. Around it lies the outer core, roughly 2,200 kilometers thick, where the same metals are liquid. Liquid metal conducts electricity, and it can move. Both facts matter.
The outer core is a deep layer of liquid, electrically conducting metal.
4the churning
The inner core is hot, and the outer core loses heat upward into the mantle. Liquid that is warmed near the bottom becomes lighter and rises; liquid that cools near the top becomes heavier and sinks. This convection keeps the liquid iron in constant motion, at speeds of tens of kilometers a year, which is fast for something geological.
Heat escaping the core keeps the liquid outer core churning.
5the spin
Because the planet turns, moving liquid is deflected sideways by the Coriolis effect, the same effect that spins hurricanes. In the core it bends rising and sinking flows into corkscrews, long spiraling columns parallel to the axis. This gives the motion the organized, twisting shape a dynamo needs.
Earth's rotation twists the core's flows into organized spirals.
What does Earth's spin do to the flow in the outer core?
Show the answer
It twists the rising and sinking liquid into corkscrew columns along the spin axis.
6the dynamo
Moving a conductor through a magnetic field drives an electric current in it. In the outer core, that current produces a magnetic field of its own, and the field acts back on the moving iron. As long as the liquid keeps churning, the loop keeps itself going, which is why it is called a self-sustaining dynamo. A weak seed field is enough to start it; the surface field of about 25 to 65 microtesla is the result.
Churning conductor makes current, current makes field, field helps make current: a dynamo.
a common mix-upThe field is not stored in the core; it is made continuously, only as long as the core keeps moving.
7the whole idea
Earth's magnetic field exists because the planet has a hot, liquid, electrically conducting outer core. Heat from the inner core makes it convect; Earth's spin organizes the flow into spirals; and the electric currents it generates sustain the field in a feedback loop. Because the field is made by flowing metal rather than stored, it can weaken, wander and even reverse. The last full reversal was about 780,000 years ago.
Heat, spin and a feedback loop in liquid iron make Earth's field.
Deep under your feet, a sea of liquid iron is quietly running a generator that has shielded Earth for billions of years.