From AMS Glossary
A bright wedge of pink, orange, or purple light that extends around the antisolar horizon during clear twilights and that has the greatest vertical width (∼3°–6°) occurring above the antisolar point.
The antitwilight arch rises with the antisolar point at sunset and sets with the antisolar point at sunrise. The antitwilight arch is brighter than the bluish gray dark segment beneath it. The boundary between these features may be distinct shortly after sunset (sun elevation h0 ∼ -2°), but it fades as evening twilight progresses.