Astronomy & Celestial Mechanics
the sky, computed
Concept

Joys law — where it appears

The observation that the leading part of a sunspot group lies closer to the equator than the following part, by a tilt growing with latitude. Hale and Joy published it in 1919. The scatter is large, and this tilt is what builds the polar field in the Babcock–Leighton mechanism.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

One step of memory, and it is kept at the poles. Two predictors of a solar cycle's amplitude, each tested against the seven cycles for which both quantities exist, with amplitudes scaled so that cycle 21 is one. Left: the polar field measured at the minimum before the cycle begins, which correlates with what follows at r = 1.00. Right: the amplitude of the previous maximum, which correlates at r = 0.34 — that is, not at all. The cycle is therefore not a pendulum with momentum; it is a process with exactly one state variable, and the variable is the poloidal field that the decay of the previous cycle's spots leaves behind at high latitude. That is the observational core of the flux-transport picture: spots emerge tilted, their trailing polarity drifts poleward, and what accumulates there is the seed the next cycle's shear will wind up. It is also the only prediction in solar physics with a lead time of years, and it was what said, correctly and unpopularly, that cycle 24 would be the weakest in a century.

One step of memory kept at the poles

The amplitude of a solar maximum tells almost nothing about the next one. The polar field measured at the minimum in between tells a great deal — so the cycle is not a pendulum with momentum but a process with exactly one state variable, and the variable is a field of a few gauss at latitudes nobody can see well.

stars · Solar cycle How much of a region's dipole survives to reach the poles. The share of a bipolar region's initial axial dipole that is left in the final polar field, against its latitude of emergence, a Gaussian of width 6, 9, 12 degrees. At 30 degrees, near the start of a cycle, almost nothing survives; at 10 degrees, near the end, about 54 per cent does at 9 degrees' width. Multiplied by Joy's-law tilt, which rises with latitude, the most effective latitude is about 9 degrees. Flux emerging far from the equator cancels with its own kind before crossing it.

One region that can change the next cycle

The Sun's polar field at the end of a cycle predicts the strength of the next, and it is built from the slight tilts of thousands of active regions. They do not count equally. A region's contribution depends sharply on how near the equator it surfaces, so the last few years count most, and a law with more scatter than signal fixes the tilt. So a few large regions at low latitude, some with the wrong sign, decide what the next eleven years will be like, and nothing can say in advance which they will be.

stars · Solar cycle

Named alongside it

The objects these essays reach for when they reach for this one.

Polar fieldSolar cycleActive regionBabcock leighton dynamoBabcock leighton mechanismDynamoFlux-transportMaunder minimumMeridional circulationPredictionSunspotTachocline

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