Sunspot formation is governed by the Sun’s magnetic field and differential rotation, combined with its gaseous nature. The magnetic field lines of the Sun run from pole to pole beneath the surface. Because the Sun rotates faster at the equator than at the poles (25 days versus 35 days), the field lines become twisted around the Sun (Fig. 4). Convection serves to distort the field lines even more, until they become so skewed that they break through the photosphere, resulting in sunspots. Sunspots appear dark because they are cooler than the photosphere around them, as the intense magnetic fields from which they arise do not allow the effective transfer of heat. When the polarity of the Sun’s magnetic field reverses during sunspot maximum, the twisted field lines are gradually straightened out again, thanks to the different rotational rates of the Sun’s surface. This cycle of the twisting and untwisting of the magnetic field lines occurs within every sunspot cycle.

Figure 4
The Sun’s magnetic field lines, which initially run from north to south, are continually twisted around the body of the Sun like a corkscrew as the sunspot cycle progresses. This twisting is responsible for the sunspot’s migration toward the equator.