What is "subhelic arc"

n. A rare halo, formed by internal reflection through ice crystals, that curves upwards from the horizon and touches the tricker arc above the anthelic point.

A subhelic arc is a rare halo, formed by internal reflection through ice crystals, that curves upwards from the horizon and touches the tricker arc above the anthelic point. Subhelic arcs are a result of ray entrance and exit through prism end faces with two intermediate internal reflections.