The arcs are unexplained. We need more observations to establish their full extent and dependence on solar altitude. The only two known observations were both at the same sun height of 1°.



It's tempting to ascribe the pillar echoes to wobbly, highly flattened pyramidal plate crystals similar to those invoked for elliptical halos. However, ray tracings using them are not convincing.



Sun pillars are thought to be sometimes made by large plates with imperfections and even snowflake like appendages. These could have tilted facets due to steric hindrance and drift downwards with peculiar gyratory motions. They would require different simulation approaches and such models have an uncomfortable degree of arbitrariness. However, a credible explanation is needed both for these pillars and to some extent also for elliptical halos themselves.



Look carefully at sun pillars - they might have surprises!