Sun Pillar Echoes ~ This tall sun pillar topped by an upper tangent arc has something more. On each side are faint "echo" arcs. These are exceeding rare having only been recorded once before. This is one of 13 images by Jon Inghram taken near sunset on 12th November 2010 east of Stockton, Kansas. A triumph of careful observation. All images except otherwise labelled ©Jon Inghram, shown with permission
December 23, 2001 Finland

The first and only previous known observation. The pillar echoes were seen at Espoo and Kotka near the southern coast of Finland by Mika Sillanp��, Eero Savolainen and Jukka Ruoskanen. Each photographed them.
Another image of the Kansas sighting. The echoes are quite diffuse and the sky appears darker between them and the pillar, reminiscent of the behaviour of the 22° halo.

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!
Atmospheric
Optics
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HaloSim ray tracings for flattened pyramidal plate crystals.

Left: Crystals with a pyramidal upper face and flat lower one.

Right: Hexagonal pyramids on upper and lower faces.

Neither simulation reproduces the two sightings at all well.
                   
A big thank you to Jari Luomanen, Marko Riikonen, Jukka Ruoskanen, Nico Lefaudeux and Mika Sillanp�� for their help in preparing this OPOD.
It pays dividends to take several images of an unusual event, preferably with a hand-held camera.

A single image of these echoes could easily have been disregarded as due to unusual patches of cloud.  Multiple pictures over several minutes help decide against cloud patches and streaks. Hand holding helps identify camera artefacts. Shooting through windows is a good source of artefacts.


At right - Five images stacked together, better to show the rare arcs.
Catch them quickly - 90 seconds after the previous image the echoes are no longer evident.

The bright upper tangent arc and height and narrowness of the pillar suggests that the latter was generated at least in part by horizontal column crystals.