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Sunspots are dark areas of irregular shape on the surface of the Sun. They are often large enough to be seen with the naked eye, and Chinese records of major sunspots go back to at least 28 BCE.¹¹

Sunspots are much easier to observe than any other solar phenomena because of their dark appearance against much brighter surroundings. This is because they are typically about 1,500°C cooler than the rest of the photosphere – hence their darker appearance – although sunspots are actually fairly luminous. The spots vary in diameter from a mere 1,000 km to one million km. Many of them are larger than Earth, and easily visible to the naked eye under the right conditions.

Spectroscopic studies show that they are associated with strong, concentrated magnetic fields. A sunspot usually consists of a circular, dark core (the umbra), surrounded by a lighter region called a penumbra (Figure 2.25). The umbra is usually associated with a strong, vertical magnetic field, whereas the penumbra tends to have a weaker, horizontal field.

The penumbra displays light and dark linear features, known as filaments, which radiate away from the umbra. These filaments are the result of radial outward flows of gas guided by the nearly horizontal magnetic field.

In close proximity to sunspots, the strength of the magnetic field may increase more than 3,000 times. Field strengths are directly related to their physical size, ranging from 1,000 gauss in smaller examples to more than 6,000 gauss in giant sunspots. (This compares with an average 0.5 gauss for Earth's surface magnetic field.) These powerful fields partially block the flow of plasma and heat from below, causing the sunspot's temperature to drop more than a thousand degrees compared with its surroundings.

Figure 2.24 At the start of the 11‐year sunspot cycle, the Sun's magnetic field resembles a large bar magnet, with two magnetic poles. However, because the equatorial region rotates much faster than the poles, the magnetic field gets progressively wrapped around the Sun, becoming stretched as it nears the equator. After many rotations the magnetic field becomes extremely complex. The twisted field lines wrap around the Sun, generating areas of intense magnetic field that are visible at the surface as sunspots. The tangling process ends when the dynamo readjusts, recreating a dipole field.

(UCAR)

Sunspots are the most visible features associated with active regions, often seen alongside faculae and plage (see “The Chromosphere”). They almost always occur in groups or pairs of opposite polarity, where magnetic fields project through the photosphere from below: in the center of a sunspot the magnetic field lines are vertical. Isolated sunspot pairs tend to be aligned in an east–west direction.

Magnetograms show that the polarities of sunspot pairs located in the northern and southern hemispheres are reversed. In one hemisphere, the sunspot with a negative magnetic polarity almost always leads the sunspot which has a positive polarity (with respect to the westward apparent motion due to solar rotation). The behavior is similar in the other hemisphere, except for reversed magnetic polarities. This pattern is a direct result of the internal dynamo that generates the overall magnetic field.

Some sunspots survive only a matter of hours, but many persist for days, weeks, or even months in the case of the very biggest. Each spot resembles a whirlpool, where hot gas near the surface converges and then dives into the interior at speeds of up to 4,000 km/h. The sinking gas in the immediate vicinity of the sunspot reaches depths of only a few thousand kilometers. However, the descending flow removes the heat that accumulates beneath the spot and eventually brings it to the surface, far from the sunspot, where it is radiated into space.

The first evidence that sunspots are likely to form comes when sound waves deep in the convective zone begin to accelerate through a particular region. Within half a day, intense magnetic fields shoot upwards like a fountain, traveling at 4,500 km/h. A 20,000‐km wide column of hot gas, confined by an intense, rope‐like magnetic field, reaches almost to the visible surface.

Figure 2.25 Sunspots have a darker, central region (the umbra) surrounded by a lighter outer section (the penumbra) with a linear filamentary structure. Around the sunspot are thousands of granules, the tops of convectional cells 1,000–2,000 km in diameter. Sunspots appear dark because they are cooler than their surroundings. A large sunspot might have a temperature of about 4,000°C compared with about 5,500°C for the nearby bright photosphere. This sunspot was observed with the Swedish 1‐m Solar Telescope on La Palma.

(Institute for Solar Physics, Royal Swedish Academy of Sciences)

At a depth of 4,000 km, it may separate into strands that make their own way towards the surface, eventually forming smaller sunspots around the main spot complex. The intense magnetic fields prevent the normal upward flow of energy from the interior, leaving the sunspot much cooler than its surroundings. Immediately below the main spot is a cushion of cooler, less intensely magnetized gas. Each major sunspot complex is associated with a separate magnetic column in which the polarity is often aligned in the opposite direction.

Since more sunspots appear at solar maximum, the solar irradiance reaching Earth during that time might be expected to decrease. However, satellite radiometer measurements show that, while sunspots cause a decrease in the solar irradiance on time scales of days to weeks, the long‐term solar irradiance actually increases by about 0.1% as sunspot (magnetic) activity increases. The source of this additional irradiance has been traced to the bright faculae near the limb of the Sun (see above).

Figure 2.26 Sunspots observed by the Hinode spacecraft in February 2014. (Top) Visible light image. (Bottom) Magnetic field strength. Field strength varies from weak (purple, dark blue) to strong (green, yellow, red). Red indicates a strength of more than 6,000 gauss (600 mT), one of the highest figures ever recorded. Surprisingly, the strongest field was not in the umbra, as would be expected, but in a bright region between two umbrae. Horizontal gas flows from one umbra compressed the fields near another umbra, enhancing the field strength to over 6,000 gauss.

(NAOJ/JAXA)