From AMS Glossary
(Also called Bouguer's halo.) An infrequently observed, faint white, circular arc or complete ring of light that has a radius of 39° and is centered on the antisolar point.
When observed, it is usually in the form of a separate outer ring around an anticorona.
Tricker, R. A. R. 1970. An Introduction to Meteorological Optics. 192–193.