Astronomy & Celestial Mechanics
the sky, computed
Concept

Active region — where it appears

A patch of the Sun's surface where strong magnetic field has risen through the convective zone, usually as a pair of opposite polarities with spots in it. Flares and coronal mass ejections come from such regions. As each decays, its flux is carried poleward and helps to build the next cycle's polar field.

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

4 cycles of wings, and the polarity reverses at every boundary. Sunspot latitude against date, one mark per spot group, over 4 cycles of 11 years. The pattern is the reason the plot is called a butterfly diagram, and both of its features are laws with names. Spörer's law is the downward slope: spots emerge near ±28° at the start of a cycle and near ±7° at the end, so each wing narrows towards the equator and never crosses it. Hale's law is what the two mark shapes say: the leading spot of a pair has one magnetic polarity in the north and the other in the south, and both reverse when a new cycle starts — so a diagram that repeats every 11 years in appearance repeats only every 22 in magnetism. The wings overlap: the first high-latitude spots of a cycle appear about 1.6 years before the last low-latitude spots of the one before, which is why counting spots gives a cycle length slightly different from measuring one between polarity reversals. The dot density in time is the sunspot number itself, drawn from the standard skewed fitting function — the rise to maximum takes about four years and the decline about seven, in every cycle ever recorded.

A magnetic clock read off a butterfly

Plot sunspot latitude against date and the marks form wings that open at thirty degrees and march to the equator. The polarities reverse between wings, so the magnetic period is twenty-two years and the famous eleven is an artefact of counting spots rather than fields.

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 cycleBabcock leighton mechanismButterfly diagramCosmogenic isotopeDifferential rotationHale polarity lawJoys lawMaunder minimumSolar dynamoSpoerer lawSunspot number

All concepts