In solar astronomy, a plage (pronounced plahzh) is the chromospheric Fraunhofer line emission region above a cluster of faculae. They are associated with sunspots and active regions on the Sun.

The reason for the existence of faculae and plages is not precisely known, but the best explanation is as follows. The strong local magnetic field within the faculae causes them to have a lower gas density, temperature, and pressure than their surroundings, with hydrostatic equilibrium maintained by the increased magnetic pressure. This makes the gas within the faculae less opaque, and places the point where the gas becomes optically thick at a much lower depth (and hence higher temperature) in the photosphere. So despite the fact that the gas within the faculae is actually cooler, it allows radiation from hotter layers around the faculae to escape more easily, and thus appears brighter. Plages would then come either from the hotter radiation field or magnetic energy from the faculae ionizing atoms in the chromosphere, generating Fraunhofer line emission.

Plage is also the french (and russian) word for "beach". These features on the Sun are reminiscent of sandy beaches surrouding a dark ocean of a sunspot.

Source: Foukal, P., Solar Physics, Wiley Interscience (1990)

Plage (?), n. [F., fr. L. plaga.]

A region; country.

[Obs.] "The plages of the north."

Chaucer.

 

© Webster 1913.

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