Sunspots are cool, dark patches on the surface of the Sun. They are thought to be the result of a temporary suppression of convection by the internal writhings of the Sun's magnetic field. That's why the spots are cooler than their surroundings.
The size and number of sunspots famously follows an 11-year cycle, a phenomenon first pointed out by Heinrich Schwabe, a German amateur astronomer in 1843. But in 1967, Mstislav Gnevyshev at the Russian Academy of Sciences, pointed out that many of these cycles appear to have two peaks.
Today, Katya Georgieva at the Space and Solar-Terrestrial Research Institute in Sofia, Bulgaria, says she knows why. Her thinking implies that all cycles have two peaks but that these often coincide making them difficult to tease apart.
Over the years, astronomers have built a remarkably detailed picture of the strange patterns associated with sunspots.
For example, the spots tend to form in pairs with opposite polarities.
This is explained by the idea that the Sun's magnetic field forms into small loops called flux tubes that writhe like twisted rubber bands and occasionally burst through the Sun's surface. Sunspots form at the points of exit and entry of these loops, which is why they form in pairs of opposite polarity.
The spots also tend to form at high latitudes at the beginning of the solar cycle. Then, as the cycle progresses, they form closer to the equator.
Astronomers think the reason for this is that the Sun's magnetic field tends to oscillate over the 11-year cycle. So the field tends to stretch alternately in the north-south direction and then in the east west direction, like a ringing bell.
This oscillation pushes the flux tubes through the surface near the poles at the beginning of the cycle and then closer to the equator as the oscillation progresses.
What seems clear is that the spots are intimately linked with the complex interactions between the Sun's magnetic field and the patterns of convection in its outer layer.
The simplest pattern of convection is that heat moves in a cell-like cycle, towards the poles along the surface and then back to the equator under the surface. (This pattern is then complicated by factors such as the Coriolis force.)
Sometimes, however, this pattern can be undermined by currents that short circuit the cycle, travelling straight down from the surface to the lower layers, rather than following the full cycle to the poles.
Georgieva says that the flux tubes follow a similar pattern. Some follow the full equator-to-pole cycle while others short circuit this process. These different patterns of behaviour tend to push flux tubes through the surface at different times.
It is this difference in behaviour that leads to the double peaks in sunspot cycle, says Georgieva. "These two parts of the flux, when transformed by the differential rotation at the base of the convection zone, give rise to two peaks of sunspot activity," she says.
That seems to explain this strange aspect of sunspot activity. There are plenty of other curiosities, however. Georgieva's work is far from finished.
Ref: arxiv.org/abs/1103.4552: Why The Sunspot Cycle Is Double Peaked