Sunspots are temporary, darker, cooler regions that appear on the Sun’s visible surface, known as the photosphere. They represent areas of intense magnetic activity that interrupt the normal flow of energy from the solar interior. Sunspots are still extremely bright, but their presence indicates a localized reduction in temperature and energy output. They vary greatly in size, from small, short-lived features to massive structures that can span over 160,000 kilometers and persist for many weeks.
The Sunspot Phenomenon
The existence of sunspots is directly tied to the Sun’s magnetic field, which is generated by the movement of electrically charged plasma within the convection zone. The Sun’s differential rotation, where the equator spins faster than the poles, twists and tangles these magnetic field lines over time. This process concentrates the magnetic field into bundles called flux tubes, which eventually become buoyant and erupt through the photosphere.
These concentrated magnetic fields suppress the normal process of convection, which is the mechanism that transports heat from the Sun’s interior to the surface. This magnetic blockage means that the sunspot region does not receive the same energy supply as the surrounding photosphere. Because the area is cooler, it radiates less light, making it appear dark by comparison. The surrounding photosphere has a temperature of approximately 5,800 Kelvin, while the sunspot itself can be about 2,000 Kelvin cooler.
Distinct Zones: Umbra and Penumbra
A typical, large sunspot is composed of two distinct regions: the dark central core, known as the umbra, and the surrounding, less dark region, called the penumbra. The umbra is the coldest and darkest part of the sunspot, with a temperature often falling in the range of 4,000 to 4,500 Kelvin. In this region, the magnetic field lines are strongest, typically measuring between 2,500 and 3,500 Gauss, and are oriented nearly vertical relative to the Sun’s surface.
Surrounding the umbra is the penumbra, which is warmer and brighter than the core, but still cooler than the normal photosphere. Penumbral temperatures are intermediate, generally around 5,500 Kelvin, and its brightness is significantly enhanced compared to the umbra. The penumbral structure is characterized by distinct radial filaments, which are alternating bright and dark lanes that give the region a fibrous appearance.
The magnetic fields in the penumbra are weaker than in the umbra and are much more horizontal, or tilted, as they fan out from the core. This more horizontal orientation allows for some horizontal transport of energy, which explains the penumbra’s higher temperature and brightness compared to the umbra. The Evershed flow, a horizontal outflow of gas, is observed across this region.
Sunspot Evolution and Arrangement
Sunspots do not form in isolation, but emerge from the Sun’s interior as bundles of magnetic flux, often appearing first as small, temporary dark pores. They then grow over a period of days, with the umbra and penumbra developing as the magnetic flux increases and expands. The growth phase of a sunspot group is typically shorter than its decay phase, creating an asymmetry in its overall lifespan.
Sunspots almost always appear in groups, typically forming bipolar active regions with a leading spot and a trailing spot that have opposite magnetic polarities. As the group matures, the distance between the leading and trailing spots increases, a process that continues through much of the decay phase. The magnetic field lines loop through the Sun’s atmosphere, connecting the spots of opposite polarity.
The frequency of sunspot appearance is not constant but follows an approximately 11-year cycle, known as the solar cycle. Sunspots are rare during solar minimum, but become numerous and widespread during solar maximum, correlating with an increase in solar flares and other magnetic activity.