Sunspots

First viewed through a telescope by Galileo Galilei in 1610, sunspots are dark, active regions on the surface of the sun (the photosphere) that have a stronger magnetic field and cooler temperatures than the surrounding areas. The dynamo process (flow of solar plasma) within the exterior layers of the sun creates a magnetic field where convective motions transfer heat from the interior of the sun to its surface. As the magnetic field strengthens, the heat flow from the sun’s interior is impeded, resulting in solar phenomena such as sunspots, solar flares, or coronal mass ejections.

The regionally intense magnetic field inhibits convective heat flow, thereby reducing the temperature, energy emission, and brightness level, and giving sunspots their dark appearance. Fluctuations in solar irradiance and sunspot activity levels have been linked to global climate variability; however, much uncertainty exists regarding the mechanism and extent of their climate influence.

Sunspots are not uniformly dark (or cool), as most are characterized by two distinct regions, the umbra and penumbra. The umbra is the dark inner region of a sunspot with an average temperature around 6,700 degrees F (3,700 degrees C), about a third lower than the average photosphere temperature of 10,300 degrees F (5,700 degrees C). Based on Stefan-Boltzmann’s law, the temperature difference between the umbra and photosphere means that the energy emitted from an average umbra region is only 20 percent of that emitted from the solar surface. Surrounding the umbra, the penumbra is a much warmer, lighter region with a fibrous edge that has temperatures somewhere between the umbra and solar surface. The average sunspot’s size is comparable to the diameter of the Earth (about 8,000 mi. or 12,800 km) but some sunspots have been recorded with diameters 10 times larger.

The typical lifetime of a sunspot can be anywhere from a few days to a few months, but the average is about six days. The longest sunspot duration ever recorded was 134 days. The number of sunspots varies over a period of 11 years, going from a minimum to a maximum and back to a minimum in 22 years. This 11-year period is known as the solar cycle, or sunspot cycle, which is a periodic element of solar variation. Solar variation is the amount of change in radiation emitted by the sun and its spectral output over time. At the beginning of the solar cycle, sunspots begin to develop at midlatitude regions of the sun. However, they are transient (comprised of a leading spot and a trailing spot) and migrate toward the solar equator as the cycle continues. Sunspots typically emerge parallel to the solar equator (on either side) and appear in bipolar pairs with opposite directing magnetic fields.