Finland Halo Night ~ A collection of rare and not so rare sights seen all in a single day & night by halo expert Jari Luomanen (his site). " I drove some 400 kilometres chasing diamond dust, slept 4 hours in between and got these."     All images ©Jari Luomanen, shown with permission
 

Elliptical halos ~ "Really difficult to shoot, fleeting stratus fractus kept crossing the Moon and, of course, a corona was visible."

Elliptical halos, here ~1.5x2.5° (moon 45° high) remain enigmatic. They are rare and their size varies between occurrences and moon/sun elevation. They can be modeled - but not very well - by invoking very flat pyramidal crystals but these are not crystallographically realistic. Larger cluster or sterically hindered/strained dendritic crystals are other possibilities. The 6-pointed star is a camera aperture blade artifact, the lens had to be considerably stopped down.
 

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Light pillars from plate crystal diamond dust
Finland gets 'ordinary' halos too.
Moilanen Arc ~ Captured late afternoon and deliberately underexposed to darken the very bright sky.   The "V" shaped Moilanen arc above the sun and inside the 22° halo is magnificently displayed here.

Another enigma. The arc requires rays to pass through two ice prism faces inclined 34° to each other. That is also crystallographically unrealistic. Some kind of crystal twinning could be responsible but we have no evidence.