Sunspots

Sunspots appear as temporary dark spots on the photosphere of the Sun due to their lower temperature and stronger magnetic field than surrounding regions. Sunspots had been observed by ancient Chinese more than 2000 years ago. A silk painting discovered in a Han tomb of about 165 BC in Hunan province of China shows a bird on the solar disk, mystifying sunspots as a bird (see the figure below, which is from the top section of the silk painting). The first record of sunspots was discovered in the famous philosophical book "Huainanzi"《淮南子》, a thesis compiled by the king of Huainan about 140 BC. It says that there is a bird on the Sun [日中有踆鳥]. Although earlier mythological book of "Shanhaijing" 《山海经》 claims that the Sun is driven by the bird, it is not clear whether this is related to observations of sunspots. Since then it has been noticed that the appearance of sunspots varies with time and records of its location, size and shape have been seen for more than 270 times in ancient Chinese literatures. These records are consistent with the 11-year solar cycle. Sunspots were first discovered in Europe in 807 AD and were misinterpreted as transit of the Mercury.

Sunspots usually appear in groups (see the left panel of the figure below). Individual sunspots may last from a few days to a few months, but eventually decay. Sunspots expand and contract as they move across the surface of the Sun with sizes ranging from 16 kilometers to 160,000 kilometers in diameter. The larger varieties are visible from Earth without the aid of a telescope. They may travel at relative speeds or proper motions of a few hundred meters per second when they first emerge.

Observations using the Zeeman Effect show that prototypical sunspots come in pairs with opposite magnetic polarity (see the right panel of the above figure). Their number varies according to the approximately 11-year solar cycle. From cycle to cycle, the polarities of leading and following (with respect to the solar rotation) sunspots change from north/south to south/north and back. George Ellery Hale first linked magnetic fields and sunspots in 1908. Hale suggested that the sunspot cycle period is 22 years, covering two periods of increased and decreased sunspot numbers, accompanied by polar reversals of the solar magnetic dipole field.

The point of highest sunspot numbers during a cycle is known as solar maximum, and that of lowest sunspot numbers as solar minimum. This period is also observed in many other solar activities. Early in the cycle, sunspots appear in the higher latitudes and then move towards the equator as the cycle approaches maximum. Spots from two adjacent cycles can co-exist for some time. Spots from adjacent cycles can be distinguished by direction of their magnetic field. The butterfly pattern shows the variation of sunspot area with solar cycle (see the upper panel of the figure below). The Wolf number sunspot index counts the average number of sunspots and groups of sunspots during specific intervals. Sunspot populations rise quickly and fall more slowly (see the middle panel of the figure below). Sunspot number also changes over long periods (see the lower panel of the figure below).

Modern scientific studies show that energy released in the core of the Sun via nuclear fussion is tranferred into the photosphere via convection in the outer layers of the Sun as evidenced by granules on the photosphere (see the figure below). The temperature depressions of sunspots are caused by concentrations of magnetic fields that inhibit convection. Although the details of sunspot generation are still a matter of research, it appears that sunspots are the visible counterparts of magnetic flux tubes in the Sun's convective zone that are twisted by differential rotation. If the stress on these tubes reaches a certain limit, they curl up and puncture the Sun's surface. Convection is inhibited at the puncture points; the energy flux from the Sun's interior decreases; and with it the surface temperature. Although they are at temperatures of roughly 3,000–4,500 Kelvin, the contrast with the surrounding material at near 5,780 Kelvins leaves sunspots clearly visible as dark spots.

Morphologically, sunspots have two parts: the central umbra, which is the darkest part, where the magnetic field is approximately vertical (normal to the Sun's surface) and the surrounding penumbra, which is lighter, where the magnetic field is more inclined. Magnetic pressure tends to remove field concentrations, causing the sunspots to disperse, but sunspot lifetimes are measured in days to weeks. Observations from the Solar and Heliospheric Observatory (SOHO) show that a powerful downdraft underneath each sunspot forms a rotating vortex that sustains the concentrated magnetic field.

Generally, a group of sunspots with both polarities are called a solar active region. The magnetic field structures in the corona above solar active regions can be quite complex, driving a varieties of solar activities. Most solar flares and coronal mass ejections, two kinds of explosions that can have great impacts on life on the Earth in general and space weather in particular, originate in solar active regions.