In solar astronomy, a plage (pronounced plahzh) is
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)