From AMS Glossary
A faint band, purple in color, seen over much of the solar sky during a clear twilight.
The purple light exists when sun elevation -2° < h0 < -6°. Its azimuthal width is ∼40°–80°, and its vertical or elevation-angle width is ∼10°–15°. Maximum luminance occurs at h0 ∼ -4°, and the area of purple light steadily descends toward the solar point during evening twilight. At h0 ∼ -7°, the bright segment replaces the purple light.