In astronomy, the Wilson effect is the perceived depression of a sunspot's

umbra The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast. ...

, or center, in the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared rad ...

photosphere The photosphere is a star's outer shell from which light is radiated. The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...

. The magnitude of the depression is difficult to determine, but may be as large as 1,000 km. Sunspots result from the blockage of

convective Convection is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoyancy). When the cause of the convec ...

heat transport by intense

magnetic field A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and t ...

s. 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 The solar cycle, also known as the solar magnetic activity cycle, sunspot cycle, or Schwabe cycle, is a nearly periodic 11-year change in the Sun's activity measured in terms of variations in the number of observed sunspots on the Sun's surfa ...

, discovered by Heinrich Schwabe in the 19th century.

History

In 1769, during solar cycle 2, Scottish astronomer Alexander Wilson, working at the

Macfarlane Observatory The Macfarlane Observatory was established at the University of Glasgow in 1757. It was the first purpose-built university observatory in Britain. History The Observatory was named after Alexander MacFarlane, a merchant and slave-owner in Kin ...

, noticed that the shape of sunspots noticeably flattened as they approached the Sun's

limb Limb may refer to: Science and technology * Limb (anatomy), an appendage of a human or animal *Limb, a large or main branch of a tree *Limb, in astronomy, the curved edge of the apparent disk of a celestial body, e.g. lunar limb *Limb, in botany, ...

due to

solar rotation Solar rotation varies with latitude. The Sun is not a solid body, but is composed of a gaseous plasma. Different latitudes rotate at different periods. The source of this differential rotation is an area of current research in solar astronomy. ...

. 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 The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast. ...

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.

Alternate interpretations

While the surface-depression interpretation of the Wilson effect is widespread, Bray and Loughhead (1965) contended that "the true explanation of the Wilson effect lies in the higher transparency of the spot material compared to the photosphere". They develop this idea on pages 93 to 99 of their book. A similar interpretation was expressed by C.H. Tong in 2005.

References

* R.J. Bray and R.E. Loughhead (1965) ''Sunspots'', page 4 "Discovery of the Wilson Effect",

John Wiley & Sons John Wiley & Sons, Inc., commonly known as Wiley (), is an American multinational publishing company founded in 1807 that focuses on academic publishing and instructional materials. The company produces books, journals, and encyclopedias, i ...

. * John H. Thomas and Nigel O. Weiss (1991) ''Sunspots:Theory and Observations'', page 5: "Wilson depression",

Kluwer Academic Publishers Springer Science+Business Media, commonly known as Springer, is a German multinational publishing company of books, e-books and peer-reviewed journals in science, humanities, technical and medical (STM) publishing. Originally founded in 1842 in ...

. * C.H. Tong (2005) "Imaging sunspots using helioseismic methods", ''Philosophical Transactions of the Royal Society A'' 363:2761–75. * C.A. Young (1882) ''The Sun'', page 126,

Kegan Paul Charles Kegan Paul (8 March 1828 – 19 July 1902) was an English clergyman, publisher and author. He began his adult life as a clergyman of the Church of England, and served the Church for more than 20 years. His religious orientation moved f ...

. {{The Sun Solar phenomena

History

Alternate interpretations

See also

References