Although common sense might indicate that the Sun is pretty much always the same, it undergoes regular cycles of rising and falling activity, lasting roughly 11 years. The solar cycles are characterized by changes in the output of visible and UV light, and the number of sunspots, with sunspots and visible light peaking together. Although the impact of an individual cycle is difficult to detect in the Earth's climate, extended periods of high or low activity have occurred, producing events like the Little Ice Age, and our most recent cycle has seen a long period of low sunspot counts.

We've observed sunspots for centuries, and know how the darkened areas occur, as intense local magnetic fields block the flow of material on the sun's surface, allowing cooler, darker material to remain on the surface of the sun. What we haven't figured out, however, is why their numbers vary so much from cycle to cycle. Some computer modeling, however, has now suggested that the flow of material between the pole and equator deep within the sun may dictate the strength of solar cycles that occur years afterward.

At the surface of the sun, we can detect a poleward flow (it goes in opposite directions in the two hemispheres) that averages about 20 meters a second, although there are significant variations around that average. This material has to be replaced, and a flow from the poles towards the equator is thought to take place in deeper regions, beneath the areas we're currently able to observe. That flow must also vary to match the differences seen at the surface.

This deep flow is thought to be a key driver of the sunspot cycling, since it is thought to carry the magnetic disturbances that generate sunspots from the Sun's magnetic poles. But there seems to be little direct correlation between the current rate of flow and the state of the solar cycle.

To get a better sense of how variations in this flow of material could influence sunspots, the authors used a dynamo model in which they could vary the rate of flow from between 15 and 30 meters a second. They then ran over 200 cycles in which they switched the rate of flow to a new value at every minimum. The model produced patterns of sunspots that are consistent with past observations.

With the equivalent of nearly 1,800 solar years produced by the model, they started looking for correlations between the flow rates and the number of sunspots. The simplest potential explanation didn't hold up: "Unexpectedly, we find that there is no correlation between the flow speed at a given minimum and cycle overlap (or the number of spotless days) during that minimum."

Instead, it's the flow speed during the cycle before that seems to dictate the number of sunspots. Having a fast flow from the poles while a cycle is ramping up, followed by a slow flow during its decline, results in a very deep minimum. In addition, a larger difference between the rates at these two times resulted in fewer sunspots.

The models were run with an abrupt shift between flow rates, so the authors ran these conditions while allowing the rate to change smoothly. The results didn't change substantially. They also looked at the most recent solar cycle, which has been characterized by low numbers of sunspots, and found that it has some of the characteristics of the behavior they see in their model.

Overall, it's a compelling explanation, but it faces a significant problem: it takes 11 years just to have the Sun perform a single experiment for us, so this isn't going to be the simplest thing to verify. Our historic data also doesn't have the sorts of details we'd need to match up what the Sun was doing with the details produced by this model. But that may change soon, as the authors note the NASA's new Solar Dynamics Observatory has instruments that can give us a better view of the plasma flows deeper in the Sun's interior.

Nature, 2011. DOI: 10.1038/nature09786 (About DOIs).