Sub-Horizon Lowitz Arcs?

Erica Damen saw this rare sight looking down towards the ground somewhere over Spain.  

“I flew on September 20, 2011 from Rotterdam in the Netherlands to Faro in Portugal.   And of course I was looking outside all the time.   It was very clear most of the flight but at one time I saw some clouds appearing far away. When the plane flew nearer there was a part of the cloud slowly breaking and falling down, like as though it was faint smoke. My intuition told me that there was something extraordinary going to happen...but really...I don't know why!   Then the plane took a turn to the left and the sun was shining on these "faint fallen smoke clouds" for some time with this beautiful sighting as a result!"
The bright colourful halo is a subparhelion and tailing away from it to the right is a fragment of the subparhelic circle.    Sub horizon arcs are not reflections of those above the horizon nor generated from the usually intensely bright subsun.   They are produced from direct sun rays by an additional ray reflection inside each ice crystal.    The subparhelion is 22° or more from the subsun and the result of an odd number of reflections inside horizontal plate crystals.

The really great rarities are the two coloured arcs forming a cross through the subparhelion.    These are very likely subhorizon Lowitz arcs.    It is wonderful enough to see Lowitz arcs, their existence doubted until some 20 years ago – but to see subhorizon Lowitz!   The remainder of the trip would be seriously anticlimactic.

The sun altitude was roughly 30° and a HaloSim ray tracing for rather thick Lowitz oriented plates and a few horizontal plates is below. The match is reasonable, although a very high concentration of crystals was necessary. Lowitz arc production is very inefficient. The area above the subsun would have been interesting had the Lowitz crystals extended that far.

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