The Sunspot Cycle

What sunspots are

A sunspot is a patch of the Sun's surface where its magnetic field is so concentrated that it suppresses the churning convection that normally carries heat up from below. Starved of that heat, the patch cools to about 3,700 degrees, against roughly 5,800 degrees for the surface around it. It is still searingly hot and bright, but because our eyes judge brightness by contrast, the cooler patch looks like a dark blot. The darkest core is the umbra, ringed by a lighter, feathery penumbra. A big sunspot group can be larger than the whole Earth.

The 11-year rhythm: the Schwabe cycle

If you count the spots month after month, the total rises and falls in a slow swell that repeats, on average, every 11 years (anywhere from about 9 to 14). At solar maximum the Sun's face is freckled with dozens of spots and frequent storms; at solar minimum it can go days at a time with a clean, blank disk. The German astronomer Heinrich Schwabe stumbled on this pattern in 1843, after seventeen years of patiently sketching the Sun while hunting for a planet inside Mercury's orbit that he never found. Today the rise and fall is tracked continuously; you can follow the current cycle on the United States Space Weather Prediction Center.

The 22-year magnetic cycle: the Hale cycle

There is a deeper rhythm hiding underneath the count. Early in the 20th century George Ellery Hale showed that sunspots are intensely magnetic, and that they come in pairs with opposite magnetic poles, a leading spot and a following one. Here is the surprising part: the polarity of those pairs is reversed in the northern and southern hemispheres, and it flips from one 11-year cycle to the next. So the Sun's magnetism only truly returns to its starting state after two sunspot cycles. That full magnetic cycle, about 22 years, is the Hale cycle. In the view above, the spots are tinted by polarity, as a magnetogram would show them, and you can watch the colors swap when the next cycle begins.

The butterfly diagram and Spörer's law

The spots do not appear just anywhere. At the start of a cycle they break out at middle latitudes, around 30 to 35 degrees north and south of the solar equator. As the cycle runs on, fresh spots emerge closer and closer to the equator, reaching about 8 degrees by the time the cycle fades. This steady equatorward march is called Spörer's law. Plot the latitude of every spot against time, year after year, and the two drifting bands trace a shape like a pair of butterfly wings, which is why it is known as the butterfly diagram (the right-hand view draws it as you scrub).

Why it matters: space weather

An active Sun is a stormy one. Near solar maximum, tangled magnetic fields snap and release solar flares and coronal mass ejections, huge clouds of charged particles. When these sweep past Earth they light up the auroras, but they can also disturb radio communication, knock satellites off course, and, in the strongest cases, strain power grids. The Sun's mood even has a long memory: during the Maunder Minimum, from about 1645 to 1715, sunspots all but vanished for seventy years, a lull that overlapped the coldest stretch of Europe's "Little Ice Age," although how much the quiet Sun contributed is still debated. We are now in Solar Cycle 25 (counted from 1755), which began in December 2019 and reached its maximum around 2024. You can find the solar cycle alongside every other rhythm on the cycles by length page.