What Are Sunspots? Magnetism Cooling Patches of a Star

The sun's visible surface, the photosphere, is around 5,500 degrees Celsius and is in constant convective motion, with hot plasma rising, cooling and sinking in cells visible as granulation. A sunspot forms where a bundle of magnetic field lines, generated deep inside, breaks through the surface. The field there reaches several thousand gauss, thousands of times the earth's field, and it is strong enough to suppress convection, because moving plasma is tied to the field lines and cannot cross them freely. With the upward transport of heat blocked, the area cools to around 3,500 to 4,000 degrees, which makes it perhaps a fifth to a half as bright as its surroundings and therefore appears black against them. A spot has structure: a dark central umbra where the field is nearly vertical and strongest, surrounded by a lighter penumbra of filamentary structure where the field is more inclined. Spots usually appear in pairs or groups with opposite magnetic polarity, being the two ends of a single loop that has risen through the surface.

The number of spots rises and falls with a period averaging about eleven years, and the pattern is more structured than a simple count suggests:

• •At the start of a cycle spots appear at high latitudes, around thirty degrees, and over the following years emerge progressively closer to the equator, producing the butterfly diagram when plotted against time
• •The magnetic polarity of leading spots is opposite in the two hemispheres and reverses at each new cycle, so the true magnetic cycle is about twenty-two years
• •The sun's overall magnetic field flips polarity around each solar maximum, so its north and south poles swap roughly every eleven years
• •Cycle strength varies considerably, and prediction of the next cycle's amplitude remains unreliable
• •Extended quiet periods occur, most famously the Maunder Minimum from roughly 1645 to 1715, when very few spots were recorded at all
• •The underlying mechanism is a dynamo driven by the sun's differential rotation, since the equator rotates faster than the poles and winds up the field, combined with convective motions that twist it

Sunspots matter because they mark magnetic activity rather than because the dark patches themselves do anything. Where fields are strongest, energy stored in twisted field lines is released suddenly when the lines reconnect, producing solar flares, which emit radiation across the spectrum and arrive at earth in eight minutes, disturbing radio communications and navigation. Coronal mass ejections throw billions of tonnes of magnetised plasma outward at high speed, and when one strikes the earth's magnetosphere it causes a geomagnetic storm, producing aurorae far from the poles, inducing currents in long conductors and, at sufficient strength, damaging power grids and transformers. The Carrington event of 1859 remains the benchmark, having set telegraph offices on fire and produced aurorae visible near the equator, and a repeat would be considerably more disruptive now. A storm in 1989 blacked out Quebec for nine hours. Satellite operators, airlines flying polar routes and grid operators all monitor solar activity for exactly this reason.

Telescopic sunspot observations begin in 1610 with Galileo, Christoph Scheiner and others, which makes the sunspot number the longest continuous quantitative record in astronomy, extended further back by measuring carbon fourteen in tree rings and beryllium ten in ice cores, since solar activity modulates the cosmic rays that produce them. The Maunder Minimum coincided with a cold period in Europe sometimes called the Little Ice Age, which has generated a long-running argument about solar influence on climate. The current assessment is that the sun's total output varies by only about a tenth of a percent across a cycle, which is far too small to account for the temperature change observed over the past century, and that volcanic activity contributed substantially to the seventeenth century cooling. Solar variation is included in climate models and is measured directly by satellites, and over recent decades solar activity has been flat or slightly declining while temperatures have risen, which rules it out as the driver.

A sunspot is a region where a strong magnetic field breaks through the photosphere and suppresses convection, blocking heat from below so the area cools to around 3,500 degrees and looks dark beside its 5,500 degree surroundings. Spots follow an eleven-year cycle, drifting from high latitudes towards the equator, with polarity reversing each cycle so the magnetic period is twenty-two years. They mark the activity that produces flares and coronal mass ejections, which disturb radio, navigation and power grids.