Solar cycle

Why does the Sun's spot count rise and fall on a roughly eleven-year beat?

Count the dark spots on the Sun each month for two centuries and you get a ragged but unmistakable rhythm: a crowded face, then a nearly blank one, then a crowded face again, with about eleven years between crests. The Sun is a ball of hot gas with no moving parts, no seasons, and nothing obviously periodic about it. Where does a beat come from?

The answer usually offered — "the Sun's magnetic field flips every eleven years" — is true and explains nothing. It restates the rhythm in different words. What we want to know is what is doing the counting.

Start with what a spot is, because that decides the whole question. A sunspot is a place where a bundle of magnetic field thousands of times stronger than Earth's surface field breaks through the solar surface. The field stiffens the gas and chokes off the convection that normally delivers heat from below, so the patch cools — around 3,800 kelvin against the 5,800 of its surroundings — and looks dark only by comparison. Counting spots is therefore counting magnetic flux arriving at the surface. Whatever the cycle is, it is a magnetic cycle.

So where would a large-scale field come from, and what would happen to it? The Sun is a fluid electrical conductor, and field lines in such a fluid are dragged along by the flow. Does the Sun rotate as a solid body? It does not: the equator comes round in about 25 days while the high latitudes take about 34. Picture a field line running north to south through that fluid. What must the shear do to it? Within a few years it is wrapped around the Sun, stretched into strong bands running east and west. A weak pole-to-pole field is converted, quickly and reliably, into a strong wound-up one.

Wound-up field cannot stay put. A tube of field carries its own pressure, so to balance the gas around it, it holds less gas — it is lighter, and it floats. Bundles rise, break the surface, and leave a pair of footpoints of opposite polarity: a spot pair. Is this really one field wrapped round the Sun rather than a scatter of local accidents? Throughout a cycle, in a given hemisphere, the leading spot of nearly every pair has the same polarity; the other hemisphere is mirrored; and at the next cycle both reverse. Local accidents do not organise themselves like that. A single wound band does.

So we have half a cycle. The hard half is the return: how do you rebuild the pole-to-pole field with the sign turned over?

Look closely at the erupting pairs and one detail matters enormously — they emerge systematically tilted, the leading spot slightly nearer the equator. As the region decays and its flux spreads, that tilt biases the outcome. Leading-polarity flux drifts equatorward and cancels against the other hemisphere's leading flux. Trailing-polarity flux is carried poleward — and it is opposite in sign to the pole it is heading for, so it first erases the old polar field and then rebuilds it backwards. Around each spot maximum, the Sun's poles duly change sign.

The beat now falls out. Two half-cycles of opposite magnetic sign make a complete magnetic cycle of about 22 years. But a spot count is blind to polarity: it sees only "flux emerging" and peaks once per half-cycle. Eleven years is 22 halved by the crudeness of the measurement.

That answers "why two beats per magnetic cycle." It does not answer "why 22 and not 6 or 60," and here honesty is required: the period is not derived from first principles. In one family of models it is set by the slow meridional circulation — surface gas creeping poleward at ten to twenty metres per second, with a return flow at depth that has never been measured reliably — and in another by how fast turbulent motions diffuse the field through the convection zone. Which governs is actively argued. The clock is also a poor one: cycles have run from roughly nine to fourteen years, amplitudes vary by factors of several, and between about 1645 and 1715 — the Maunder Minimum — spots very nearly vanished for seventy years, though radiocarbon in tree rings shows the magnetic cycle went on ticking weakly beneath the blank surface.

Three clarifications hold the reasoning together. First, eleven years and twenty-two years are not rival numbers. The magnetic cycle is 22; the spot cycle is 11 because spot counts throw polarity away. The apparent contradiction dissolves once you notice which quantity is being counted.

Second, the Sun's brightness is not what varies. Total output changes by only about a tenth of a percent over a cycle, since darkening from spots is largely offset by bright faculae elsewhere. What varies enormously is magnetic activity — flares, coronal mass ejections, ultraviolet output — and it is that, not warmth, which reaches Earth as space weather.

Third, the honest caveat: the account above is the Babcock-Leighton picture, a leading framework rather than a settled result. It reproduces the polarity laws and the migration of spots from about thirty degrees latitude toward the equator across a cycle. It does not yet predict the period from the Sun's parameters, and it does not explain why the rhythm sometimes stops altogether.

• Why the Sun's differential rotation exists at all, given that the gas is free to slide.
• What actually happened during the Maunder Minimum, and whether such lapses can be forecast.
• How solar cycles are predicted a decade ahead from the strength of the polar field near minimum, and why that prediction works better than it has any right to.

Every term the collection defines is gathered in the glossary.