By far the most noticeable blemishes on the surface of the Sun are sunspots. They have been directly observed since ancient times (from 350 B.C. by the naked eye, and in 1610 after Galileo invented the telescope) and can be inferred over a much longer period using records of auroral activity and carbon-14 and beryllium-10 isotope records from the Earth.
Figure 5: Sunspots on the surface of the Sun. This continuum intensity image was taken at Sacramento Peak Observatory, New Mexico, by the HAO Advanced Stokes Polarimeter and shows one spot (to the right) with the classical structure of a dark umbra and radially spoked penumbra. Nearby regions of more complicated magnetic fields are also evident, as well as the surround granulation pattern.
Figure 6: The annual mean sunspot number since 1749 to the present. The sunspot number counts the total number of sunspots and sunspot groups that come and go over a solar cycle. (Courtesy P.S. McIntosh, NOAA/SEL.)
Sunspots typically come in pairs or groups and are restricted in their position on the Sun between about 35N and 35S relative to the solar equator. Their size varies between 4,000 and 30,000 kilometers in diameter (2,500 to 11,000 miles, or up to 460 millionths of a hemisphere) and their shape can be anywhere from the classical circular pattern to a very complex pattern. The inner part of the sunspot, called the umbra, is darkest and coolest (some 4000 Kelvin), whereas the outer part, called the penumbra, is less dark and features radial structure, like spokes. Typically sunspots occur in pairs (called a sunspot group) and they exist for about a month (a solar rotation). For this reason, they can be considered to contribute to solar weather.
Perhaps the most distinctive feature of sunspots is that their number increases and decreases in a regular fashion over about a decade. This was first noticed by the German astronomer Samuel Heinrich Schwabe in 1843, and has become known as the solar magnetic activity cycle (or solar cycle). The number of sunspots in each cycle is by no means regular (see figure), and in fact, there have been periods where very few sunspots were observed. The most famous of these is known as the Maunder minimum, from 1645 to 1715. The variation in the number of sunspots over a decade or so suggests than they also contribute to solar climate. In conjunction with the sunspot cycle, the magnetic poles of the Sun reverse their polarity on the same time scale, which influences the interaction with the Earth's magnetic field.
As we will see, sunspots and the features that accompany them on the solar surface are an important component of variations in the radiation from the Sun. Sunspots directly contribute to the reduction of the solar irradiance.
Approved by Peter Fox
Last revised: Mon Apr 10 15:08:11 MDT 2000