Bright segment

From Glossary of Meteorology

bright segment

(Also called twilight arch, crepuscular arch.) During clear twilights, a faintly glowing band that is visible above the solar point when the sun's elevation -7° < h0 < -18° (the lower limit may occur at h0 > -18°).

This band's azimuthal width is ∼20°–30°, but its vertical or elevation-angle width is only a few degrees. The bright segment follows the disappearance of the purple light and can persist until the end of astronomical twilight. During clear nautical and astronomical twilights, the bright and dark segments comprise the entire sky.
See also afterglow.

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