Sunspots

The strong magnetic field in a sunspot, which is several thousand times stronger than that at the surface of the earth, accounts for the relative dimness of the spot. The hot atmosphere of the Sun contains a significant number of atoms having a net positive charge resulting from collisions between them (i.e., they are ionized, having each lost one or more electrons). Moving charged particles tend to spiral along magnetic field lines. The magnetic field lines passing vertically through a sunspot therefore tend to suppress the convection (heat-driven vertical circulation) that usually transports heat to the Sun's surface from its depths; convection requires horizontal motion so that material can reverse its direction, but the strong vertical magnetic field in the sunspot hampers cross-field (horizontal) motion of the electrically charged gas of the solar atmosphere. With less heat being supplied from below, a sunspot cools to about 3,000K (versus about 5,800K over the Sun's normal surface). Because of its lower temperature, the spot's interior is relatively dark—about 40% dimmer than the rest of the Sun's surface. Close inspection of a sunspot shows it to have a dark central region called the umbra (Latin for "shadow") surrounded by a lighter, radially structured region called the penumbra ("almost-shadow"). These regions are created by structural differences in the magnetic field responsible for the sunspot. Further, many sunspots are surrounded by bright rings. It is likely that the heat energy which cannot convect upward into the sunspot leaks up instead around its edges, superheating the material there.

A visible light image of the Sun, showing large sunspots. U.S. National Aeronautics and Space Administration (NASA).