Wilson effect

In astronomy, the Wilson effect is the perceived depression of a sunspot's umbra, or center, in the Sun's photosphere. The magnitude of the depression is difficult to determine, but may be as large as 1,000 km.

This article is about the Wilson effect in astronomy. For the unrelated effect in psychometrics research, see Heritability of IQ.
Diagram of the Wilson effect. The umbra is the darker inner circle and the penumbra is the lighter ring around the umbra.

Sunspots result from the blockage of convective heat transport by intense magnetic fields. Sunspots are cooler than the rest of the photosphere, with effective temperatures of about 4,000°C (about 7,000°F). Sunspot occurrence follows an approximately 11-year period known as the solar cycle, discovered by Heinrich Schwabe in the 19th century.

History
Wilson's original sketch demonstrating the Wilson effect.

In 1769, during solar cycle 2, Scottish astronomer Alexander Wilson, working at the Macfarlane Observatory, noticed that the shape of sunspots noticeably flattened as they approached the Sun's limb due to solar rotation.[1] These observations showed that sunspots were features on the solar surface, as opposed to minor planets or objects above it. Moreover, he observed what is now termed the Wilson effect: the penumbra and umbra vary in the manner expected by perspective effects if the umbrae of the spots are in fact slight depressions in the surface of the photosphere.[2]

Alternate interpretations

While the surface-depression interpretation of the Wilson effect is widespread, Bray and Loughhead contended that "the true explanation of the Wilson effect lies in the higher transparency of the spot material compared to the photosphere".[1]: 93–99  A similar interpretation was expressed by C.H. Tong.[3]

See also

References
  1. R.J. Bray and R.E. Loughhead (1965) Sunspots, page 4 "Discovery of the Wilson Effect", John Wiley & Sons
  2. John H. Thomas and Nigel O. Weiss (1991) Sunspots:Theory and Observations, page 5: "Wilson depression", Kluwer Academic Publishers
  3. C.H. Tong (2005) "Imaging sunspots using helioseismic methods", Philosophical Transactions of the Royal Society A 363:2761–75
This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.