Sunspot

Sunspots are temporary, dark patches located on the Sun's surface, known as the photosphere. They appear darker than the surrounding areas because they are regions of reduced surface temperature. These spots are not actually black, but they look dark against the bright background of the photosphere. They are caused by intense concentrations of magnetic flux. This magnetic activity inhibits convection, which is the process that carries heat from the Sun's interior to its surface. Because the heat cannot flow upward easily, the temperature in these specific areas drops.

A sunspot is composed of two distinct morphological structures. The central, darkest region is called the umbra. In the umbra, the magnetic field is at its strongest and is oriented approximately vertically to the photosphere. Surrounding the umbra is a lighter, more extended region called the penumbra. The penumbra consists of radially elongated penumbral filaments and has a more inclined magnetic field than the umbra. The temperature of the umbra ranges from roughly 3000 to 4500 K. In contrast, the surrounding photospheric material is much hotter, at about 5780 K. Sometimes, bright material penetrates the umbra, forming features called light bridges.

The lifecycle of a sunspot begins when magnetic flux tubes in the Sun's convective zone project through the photosphere. Initially, these appear as small, dark spots called solar pores. As these pores increase in diameter, typically reaching about 1,000 kilometers, a penumbra begins to form. Sunspots can expand and contract as they move across the solar surface. Their diameters can range from 5,000 to 10,000 kilometers. Individual sunspots or groups may last from a few days to several months before they eventually decay. Observations from the Solar and Heliospheric Observatory (SOHO) suggest that a powerful downdraft and a rotating vortex beneath the spot help sustain the magnetic field.

Humanity has observed sunspots for millennia. The earliest record is found in the Chinese I Ching, completed before 800 BC, which mentions small obscurations. In 364 BC, the astronomer Gan De recorded observations in a star catalogue. By 28 BC, Chinese imperial records included regular sunspot observations. The Greek scholar Theophrastus provided the first clear mention in Western literature around 300 BC. In 1610, Thomas Harriot made the first telescopic observations. Shortly after, Johannes and David Fabricius reported their findings. Galileo Galilei also began famous telescopic records around 1612.

Sunspots follow a predictable pattern known as the solar cycle, which lasts approximately 11 years. This cycle is characterized by a rise in sunspot numbers toward a solar maximum and a decrease toward a solar minimum. During the cycle, sunspots follow Spörer's law, appearing at higher latitudes and moving toward the equator. George Ellery Hale discovered the link between magnetism and sunspots in 1908. He suggested the full magnetic cycle might actually be 22 years. This period includes the reversal of the Sun's magnetic dipole field. The variation in sunspot numbers also correlates with changes in solar radiation intensity.

Historical attempts were made to link sunspots to Earthly events. In the early 19th century, William Herschel hypothesized that sunspots might affect Earth's temperature. He noticed a lack of sunspots between July 1795 and January 1800 and suggested this coincided with high wheat prices in England. However, later scientists like Richard Carrington and John Henry Poynting could not find this connection. Modern analysis has confirmed there is no statistically significant correlation between sunspot numbers and wheat prices. Instead, the solar cycle primarily affects the Sun's magnetic activity and radiation output. This output varies by about 0.1% of the solar constant.

To study these phenomena, scientists use advanced technology. They employ land-based and Earth-orbiting telescopes. The Swedish 1-m Solar Telescope is one such example of specialized equipment.

Because direct viewing can cause permanent eye damage, observers must use protective filters. Methods include using dark filter glass, such as #14 welder's glass, or projecting images onto a white screen. Scientists also use tools like spectroscopes and spectrohelioscopes to examine the magnetic regions. Studying sunspots is vital because they are often the sites of solar flares and coronal mass ejections. These events are driven by the intense magnetic activity found in sunspot groups.