On Jan 8 WANG Jingxian from Genhe, China captured a very interesting diamond dust display in which the only prominent halo was a (almost) solitary parhelic circle.
According to WANG, the display had already passed its prime when the pictures were taken and the parhelic circle had faded somewhat. Except for a diffuse pillar and very weak parhelia, nothing else showed up throughout the display - no circumzenithal arc, no 120° parhelia, no upper tangent arc.
Solitary parhelic circle has been observed before in China, but not at such low solar altitude. Marko Riikonen later reminded us of a very nice photographically confirmed case by Jarmo Moilanen back in 2004. (hopefully Jarmo can visit this page and share his capture in the comments). Marko himself also witnessed a streetlight display in Oulu back in 2002 ( https://www.ursa.fi/ursa/viestinta/listat/halo-l/0543.html ) in which the parhelic circle and circumzenithal arc shined without parhelia.
Crystal samples from such displays are still much needed to pinpoint responsible crystals. Marko noted he saw stellar plates were falling on his sleeves during the Oulu display, and suggests such stellar shapes could theoretically be the cause of the absence of parhelia. In the Genhe display where even the circumzenithal arc is missing, the crystals probably need to be more optically subpar, allowing only external reflections on vertical faces. Besides, as both Marko and JI Yun suggest, optically imperfect columns could also play a part.
Jia Hao
