facula

Facula (plural faculae) is a bright region on the photosphere of the Sun. Faculae are linked to concentrations of magnetic field and are commonly found around sunspots or within the network of magnetic elements that cover the quiet Sun. They are visible as luminous patches in white light, and become most conspicuous toward the solar limb.

The brightness of faculae arises from the structure of magnetic flux tubes in the photosphere. The magnetic fields locally inhibit convection, creating relatively cooler interiors, but the surrounding walls are hotter and allow a view into deeper, brighter layers along oblique lines of sight. This effect makes faculae appear brighter than the surrounding granulation, especially when viewed near the limb. In addition to white light, faculae are prominent in chromospheric lines such as Ca II K and in magnetograms that map magnetic fields. Typical facular regions are of varying size, from small points to elongated patches, and often outline the outskirts of sunspot groups.

Observationally, faculae contribute to variations in solar irradiance over the 11-year solar cycle. While sunspots darken the solar surface, the accompanying faculae brighten it, and the net effect tends to increase solar output during higher solar activity. The term facula originates from Latin, meaning "little torch."