The Sun is not only a heat source; it is also a rotating magnetic engine. Its plasma outer layers turn at different speeds at the equator and the poles, and this differential rotation twists and folds the magnetic field lines. At certain points the twisted bundles break through the surface and form sunspots — the most visible markers of solar magnetic activity.

Sunspot numbers are not constant. They rise and fall on a roughly eleven-year cycle, with the magnetic poles flipping at the start of each one. During active phases the Sun produces more flares and coronal mass ejections, driving geomagnetic storms, auroras, and satellite disruptions on Earth. During quiet phases the surface is almost spotless.

When this cycle began is tied to when the Sun's interior reached its present configuration. Once the core settled onto the hydrogen-burning main sequence — around 4.5 billion years ago — differential rotation and the dynamo that drives the cycle were also in place. Historical anomalies like the Maunder Minimum show that the cycle can briefly stall; but with such exceptions, the Sun has kept the same magnetic rhythm across geological time.