Subsuns are sometimes shockingly bright. This one was.
Plate crystals are responsible. Untold millions of them reflect the sun from their upper and lower near horizontal faces.
The reflection is enlarged when the crystals wobble slightly. Even larger wobbles generate a lower sun pillar. There are vestiges of a pillar in the photograph. Some clouds above and below the subsun are suspiciously bright.
Look carefully around a subsun for rare Bottlinger's rings.