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Close-up of a sunspot obtained by the Japanese-led Hinode spacecraft
sunspot group
Sunspot group. Image courtesy Richard Crisp
An area of the Sun's photosphere, typically 2,500 to 50,000 km across, that appears dark because it is cooler than its surroundings. Sunspots are concentrations of magnetic flux and usually occur in pairs of opposite polarity that move in unison across the face of the Sun as it rotates; these pairs are linked by loops of magnetic field that arch through the Sun's corona.

The magnetic flux of a spot (normally measuring about 0.4 tesla but occasionally reaching 1.0 tesla or more), inhibits the rising of convective heat from below and so keeps the spot at a lower temperature – 1,500 to 2,500 K cooler than the rest of the photosphere.

Moderate to large spots usually have a darker central region (umbra) surrounded by a lighter halo (penumbra) with many short fine fibrils. In the umbra, the magnetic field lines tend to be nearly vertical, while in the penumbra, they are almost horizontal.

Sunspots are most commonly found within about 30 either side of the Sun's equator, although they can occur at higher latitudes. The lifetime of individual sunspots varies, from as little as a few days, or even hours, to (in the case of the largest spots) several months.

As a whole, sunspot activity rises and falls regularly on an 11-year sunspot cycle. Sunspots are described using the McIntosh scheme, classified in terms of their field structure using the Mount Wilson magnetic classification, and counted using the relative sunspot number.

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