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sunspot areas [Q&A]

Vinay

Madhusoodan of Poornaprajna College asks, is there a particular size for a sunspot, and if so how does one find that out?

The answer to this can get very complicated very quickly, but the short answer is, no, there is no characteristic size to a sunspot. Sunspots are detected and measured in optical light and magnetogram images, and the spot areas on the Sun vary over a wide range. Typically, there are a lot more spots that are small in area than there are large spots. It appears though that the distribution is not entirely self-similar (that is, it is not free of a defining scale, and is not characterized by a pure power-law functional form), and can be approximated as a log-normal distribution. There is an upper limit to how big a spot can be, since it cannot exceed the size of the Sun. At the low end, our instruments do not have sufficient resolution to detect spots smaller than about 700 km.

Empirically, people have measured the most frequent area of spots, averaged over many spots at many phases of the solar cycle, as approximately one quarter of a millionth of the total surface area of the Sun, and this actually typifies a small spot. The width of the distribution is very large, and covers many orders of magnitude in size, so the largest spots can be larger than the size of the Earth.

On active low-mass stars, spot sizes can become so large that they cover as much as half the surface or more.