Origin of Sunspots

Sunspots are dark because they are cooler than the rest of the Sun's surface. But that is only a partial explanation. Why are they cooler? The answer is the strong magnetic fields associated with the sunspots.

Recall that in the region below the photosphere, convective cells are largely responsible for vertical motion of large packets of gas and that this bubbling activity carries heat from the interior to the solar surface. Magnetic fields exert forces on charged particles and, because this solar material is highly ionized, the magnetic fields influence the convective motion. Detailed considerations indicate that the magnetic forces hinder the convection of heat to the surface by making it harder for the hot gases to rise. Thus, the region in sunspots having strong magnetic fields tends to be cooler than the surrounding region and appears darker than the surrounding regions at higher temperature.

The Babcock Model

The most consistent explanation that we have for the origin of sunspots, prominences, and related solar activity is called the Babcock dynamo model. Its essential ingredients are the strong solar magnetic field and the differential rotation of the Sun (faster at the equator than at higher latitudes). The Babcock model is illustrated in the following figure.

As the Sun rotates, because the equator rotates faster the lines of the magnetic field get distorted. Over many rotations, this has the effect of wrapping the field lines multiple times around the Sun. Sunspot pairs then develop when these wrapped field lines tangle because of underlying convective motion and penetrate the surface in loops.

In this interpretation, prominences (which are great loops above the solar surface) are just such loops between sunspot pairs seen on the limb of the Sun against the blackness of space. Later we will see that eruptions from the solar surface (flares) probably result from loops in the field lines that reconnect suddenly, releasing large amounts of energy.

Origin of the Butterfly Diagram
The Maunder butterfly diagram is explained by the Babcock model: the field lines wind up and tangle first at higher latitudes, so sunspots form there initially. Then, as the cycle progresses and the field winds up, the tangling progresses to latitudes nearer the equator and sunspots form closer to the equator.
Finally, the diagram given above indicates that the winding has little effect at latitudes nearer the poles and at latitudes very near the equator. This explains the scarcity of sunspots in those regions.

Explanation of the Solar Magnetic Cycle

The Babcock model explains the basics (though not all the details), of the solar magnetic cycle. When the magnetic field gets too wound up and tangled, it breaks and reorders into a simpler pattern and the differential winding begins again. This explains the sunspot cycle.

Further, when this happens the magnetic field reverses its polarity; this explains the alternating magnetic polarities of sunspot pairs in successive cycles. This also explains the twenty-two-year magnetic cycle: it takes two successive eleven-year sunspot cycles, one with one polarity and the next with reversed polarity before returning to the original polarity. Thus the full Babcock cycle is twenty-two years.