Introduction
Right now, at this very moment, a stream of high-energy charged particles is hurtling toward Earth from the Sun at somewhere between 400 and 800 kilometres per second. This is the solar wind — a continuous outflow of protons, electrons, and other particles blasted off the Sun's surface. It never stops. Day and night, every day of the year, for billions of years, it has been washing over our planet.
We are alive, in large part, because we have a shield. Not a physical wall — something far more elegant: a magnetic field that deflects most of those particles before they ever reach the upper atmosphere. The particles are flung around Earth and continue on into space, never touching the surface. This shield is what makes Earth fundamentally different from a cold, barren rock.
What generates this magnetic field? The answer lies thousands of kilometres underground — in the liquid iron of Earth's outer core, churning in slow but powerful convection currents, organised by the spin of the planet into a pattern that sustains a self-reinforcing electromagnetic loop. This is the geodynamo.
In Lesson 1.1.1, we established that the outer core is liquid iron-nickel — proved by the S-wave shadow zone. In Lesson 1.1.3, we saw that the core provides heat that drives mantle convection. Now we look at what the core does for the planet's surface: it generates the field that keeps Earth habitable. This lesson covers the geodynamo, the magnetosphere, and one of the most remarkable pieces of geological evidence ever discovered — the record of Earth's magnetic history written in the rocks of the ocean floor.
Key Terms
The self-sustaining process by which convection of electrically conducting liquid iron in Earth's outer core, organised by Earth's rotation, generates and maintains the global magnetic field. It is powered by the same heat budget that drives mantle convection.
The region of space around Earth dominated by Earth's magnetic field. On the sunward side it extends roughly 10 Earth radii into space; on the night side it stretches into a long tail. It deflects the solar wind and protects Earth's atmosphere from erosion.
A continuous stream of charged particles — mainly protons and electrons — emitted by the Sun at hundreds of kilometres per second. Without a planetary magnetic field, the solar wind erodes planetary atmospheres over geological time, as it has done to Mars.
An event in which Earth's north and south magnetic poles switch positions. Reversals have occurred hundreds of times over geologic history, at irregular intervals, and are permanently recorded in the magnetic properties of rocks that solidified at the time of each reversal.