The Dynamo Effect: How Planets Make Their Own Magnetic Field

Two numbers that decide whether a dynamo runs

Magnetohydrodynamics (MHD) merges the fluid equations with Maxwell's equations through the induction equation, ∂B/∂t = ∇×(v×B) + η_m∇²B, where η_m = 1/(μ₀σ) is the magnetic diffusivity. Whether a moving conductor can sustain a magnetic field comes down to the magnetic Reynolds number, Rm = vL/η_m: when Rm ≫ 1, advection by the flow dominates over resistive decay and the field is effectively "frozen" into the fluid, stretched and folded by convection until it amplifies itself. When Rm ≪ 1, diffusion wins and any field simply dies away.

Earth's geodynamo

Earth's geomagnetic field is maintained by convective motion of liquid iron in the outer core — a shell 1,500-3,500 km in radius at 4,000-5,000 K. That convection is driven by three effects working together: secular cooling of the whole core, compositional buoyancy as light elements (oxygen, sulfur, silicon) are released and rise as the solid inner core grows, and latent heat released at the inner-core boundary as iron freezes onto it. The resulting flow of electrically conducting fluid generates electric currents that sustain the field in a self-amplifying feedback loop — remove the convection, and the field would decay within tens of thousands of years.

Polarity reversals

The geodynamo is not a fixed, steady process — it reverses polarity irregularly, on average roughly every 300,000 years, with the most recent reversal about 780,000 years ago. Transitions take 1,000-10,000 years, during which field intensity drops by around 75% before settling into the new orientation. Paleomagnetic evidence, recorded in sea-floor spreading and rock samples as they cooled through their Curie temperature, documents hundreds of such reversals across Earth's history.

The Sun's dynamo runs on a different clock

The Sun's dynamo is driven not by core convection but by differential rotation — the equator completes a rotation in about 25 days, the poles in about 35 — combined with helical convection in the outer layers. That shearing motion produces the well-known 11-year sunspot cycle, which is actually half of a 22-year full magnetic cycle, since the field's polarity flips at each sunspot minimum. The same underlying physics — a conducting fluid, differential motion, and Rm ≫ 1 — explains dynamos as different as Earth's iron core and the Sun's plasma envelope.

Frequently asked questions

Try it live

Everything above runs in your browser — open Dynamo Effect, drag rotation rate, magnetic Reynolds number and convection drive, and watch a self-sustaining field emerge — with occasional polarity reversals.