The halo belongs to a family of �Odd-radius column arcs� produced by oriented pyramidal crystals. Common pyramidal crystals have short central hexagonal prism sections and tend to take random orientations. We then see a series of circular �odd-radius� halos.
Long hexagonal columns with pyramidal ends can, in contrast, take precise orientations. The long axis is horizontal, often to better than a degree, while the crystal is free to take any rotational position about the axis. An individual crystal does not necessarily �spin�. Instead, millions of crystals happen to have all rotational positions to produce �column arcs�.
The 24° column arc of the image was produced by rays entering a pyramidal face and leaving through an ordinary prism side facet (path 15,3). The reverse path gives an equivalent arc mainly beneath the parhelic circle.
Other ray paths generate 20°and 9° column arcs. The 20° rays (13,16) pass through two pyramidal faces which, if small, would give only faint halos. The 9° column arc (13,6) is out of the camera field and may or may not have been visible.
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