Sunspot

Sunspots are temporary darkened regions visible on the Sun’s photosphere, representing some of the most prominent and scientifically significant solar phenomena. Though observed as surface features, their influence extends throughout the layers of the solar atmosphere. They form where intense concentrations of magnetic flux inhibit convective energy transport, resulting in reduced surface temperatures relative to the surrounding photospheric material. Their appearance, behaviour, and evolution provide critical insights into solar magnetism and the broader dynamics of the solar cycle. Sunspots typically occur in magnetically active regions and often appear in bipolar pairs reflecting opposite magnetic polarity. Their numbers vary systematically with the approximately eleven-year solar cycle, during which they emerge, evolve, and decay over periods ranging from days to several months.

Historical observations

The earliest known references to sunspots appear in Chinese texts such as the I Ching, completed before 800 BC, which describes visible darkenings on the solar disc. Chinese astronomers continued to record sunspots systematically, with deliberate observations documented from 364 BC and regular entries appearing in imperial records by 28 BC. In the Hellenistic world, the philosopher Theophrastus provided the earliest Western mention of sunspots around 300 BC. Medieval European interest is evidenced in a sunspot drawing created by John of Worcester in 1128.
The telescopic study of sunspots began in the early seventeenth century. Thomas Harriot observed them in December 1610, followed shortly by Johannes and David Fabricius in 1611. Their work was soon supplemented by independent observations from Christoph Scheiner and Galileo Galilei, who began systematic documentation of sunspots by 1612. During the following decades, astronomers such as Johannes Hevelius contributed to the growing corpus of records, including observations made during the early Maunder Minimum (1653–1679), a period notable for unusually low sunspot activity.
In the nineteenth century, William Herschel proposed a correlation between sunspot frequency and variations in terrestrial climate, using wheat prices as an indirect indicator. Although later analyses by figures such as Richard Carrington and John Henry Poynting refuted such correlations, the hypothesis contributed to the emerging recognition of solar variability. By the late nineteenth and early twentieth centuries, a more rigorous understanding of sunspots and their magnetic nature was established, culminating in George Ellery Hale’s demonstration in 1908 that sunspots are associated with strong magnetic fields.

Morphology and structure

Sunspots exhibit a characteristic two-part structure: a dark central umbra and a surrounding penumbra. The umbra is where the magnetic field is most intense and nearly vertical, suppressing convection and reducing temperatures to about 3000–4500 K, in contrast to the surrounding photosphere at approximately 5780 K. Despite their dark appearance against the solar surface, a typical sunspot would shine more brightly than the full moon if viewed in isolation, owing to its substantial inherent luminosity.
The penumbra consists of radially extended filaments with more inclined magnetic fields. In many sunspot groups, several umbrae may share a continuous penumbra, producing highly complex structures. Features known as light bridges—bright intrusions that divide or penetrate umbrae—have been observed during the formation or decay of spots. These regions possess weaker and more inclined magnetic fields, and gas pressure temporarily dominates over magnetic pressure, permitting limited convective activity. The Wilson effect, which refers to geometric foreshortening of the umbra when sunspots approach the solar limb, demonstrates that sunspots lie in shallow depressions in the photosphere.

Formation and development

Sunspots represent the photospheric intersections of magnetic flux tubes that rise buoyantly through the convective zone. When magnetic fields become sufficiently concentrated, they inhibit convective heat transport, causing the overlying plasma to cool. This results in the formation of a dark pore, an initial stage of a sunspot lacking a penumbra. As pores grow and coalesce, they eventually reach a threshold size at which a penumbra forms, signifying the transition to a mature sunspot.
Multiple processes continue to shape sunspots as they travel across the solar surface. They can exhibit proper motions of several hundred metres per second, particularly during their emergence. Their sizes vary widely, with diameters ranging from values below typical observational thresholds to those large enough to be seen with the naked eye under safe viewing conditions. Emergence is usually accompanied by other active-region features such as coronal loops, prominences, and magnetic reconnection events, indicating strong magnetic activity in the region.

Decay and internal dynamics

Although magnetic pressure should theoretically promote rapid dispersal of magnetic concentrations, sunspots persist for extended periods due to stabilising subsurface flows. Helioseismic studies conducted with instruments aboard the Solar and Heliospheric Observatory (SOHO) in 2001 revealed that powerful downdrafts beneath sunspots form rotating vortices that help maintain the coherent magnetic structure. As magnetic flux gradually disperses into the surrounding photosphere, the sunspot decays, its penumbra disintegrates, and the umbra diminishes until the region returns to the background solar state.

Sunspot cycles

Sunspot numbers rise and fall according to the solar cycle, a periodicity averaging about eleven years but varying between roughly ten and twelve years. The cycle is characterised by a rapid increase in sunspot numbers leading to solar maximum, followed by a slower decline towards solar minimum. Early in each cycle, sunspots appear at higher solar latitudes and gradually migrate towards the equator as maximum approaches, following patterns summarised by Spörer’s law. During the transition between cycles, sunspots from both may coexist; they can be distinguished by their magnetic polarity and latitude.
The solar magnetic field reverses polarity approximately every eleven years, creating a full magnetic cycle of about twenty-two years. Most large solar flares and coronal mass ejections originate in magnetically active regions around sunspot groups, highlighting their significance for understanding space weather. Quantitative measures such as the Wolf number index track the number of sunspots and sunspot groups and remain a central tool in monitoring solar activity. Systematic numbering of cycles began with observations in the mid-eighteenth century, providing a long-term record of solar variability.