Bottlinger's Rings ~ Tim Stone (Tim's Pix) pictured this very rare sight from 32,000 ft over Northwest Illinois March 23, 2010.

.. I was (of course) watching for atoptics, and saw a brilliant subsun. That's actually not too unusual, but as I was taking pictures a ring developed around it. The ring appears in three of my frames [right]. As that particular cloud bank fell behind us, the subsun then resumed its usual appearance.” All images are without enhancement from the original camera RAW files.
©Tim Stone, shown with permission.

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A subsun is a familiar sight beneath an aircraft. It is a blindingly bright reflection sunlight from flat-plate ice crystals in altocumulus cloud.

This is different.  Bottlinger’s rings - delicate and shimmering ovals around the subsun - are an unusual sighting. They are ephemeral and might last for no more than a few seconds. Tim’s images show two main rings 3.1 and 5.4° across their narrow axes with less distinct glows.

Bottlinger’s rings are poorly understood. Pyramidal crystals with sides sloping only 1-3° might make them. The rings are probably related to the small elliptical halos sometimes seen around the sun.

The HaloSim ray tracing is for the sun elevation of ~31° and uses the crystal at top right. The second ring B is from direct mirroring by the pyramid facets. The inner ring A comes from internal reflections. Quite surprising ray routes generate bright patches above and below the subsun.

Unlike above horizon elliptical halos, subsun Bottlinger’s rings require precisely oriented crystals with no more than small wobbles from horizontal.

The simulation might better match the images after more adjustment of parameters like the tilts of the upper and lower pyramids, the face aspect ratios and the central prism thickness.  Maybe!   After a while, further iterations rarely lead to significant improvement leading to the suspicion that the basic model is deficient. The required pyramid slope angles are also quite unphysical. To get over the latter objection, the outer dendrite extensions of snowflakes have been invoked to account for the necessary gently tilted faces. But then it is hard to see how large dendritic crystals could be so well aligned in clouds.

Back to observation - If you see Bottlinger rings, search for subparhelia - their shapes might just be a surprise.