“On the afternoon of 16th November I noticed a dark ring around the sun outside of an aureole. However, the effect could be catched better with the naked eye than the photographs assume.” (2)
“I took some picture in close succession, but none of them shows the ring. Conspicuously, the ring was distinguished by high regularity and evenness. Subsequent enhancing of the contrast let the dark ring become visible clearly on these pictures.”
Quite often, you can spot clouds moving in front of the sun. With a little luck and a cloud layer, that isn’t too thick, beautiful halation and aureoles can be seen at these conditions.
Since a cloud act as an obstacle, it causes a shadow. Apriori, this shadow is invisible to the observer. But if the shadow is projected onto a lower layer of haze, the shadow gets visible in the haze.
Hence, the example above generates a dark ring around the sun, induced by altocumulus cloud shadows on the hazes below.
Time : 16 November 2011
DSLR Camera : Nikon D 3100
Exposure : 1/500 sec, f/22#18mm, F/11, ISO 100