Lower 23 plate arc is a rare halo, that, as far as I am aware, has been observed and photographed only once. This was in Chile on 28. November 1997. Now the lower 23 plate arc seem to have made another appearace - and again in Chile.The accompanying picture is from halo display seen at Aucanquilcha volcano on 3. Debember 2005 by J. Ruoskanen and I. The image is stacked from 63 individual photos taken during 20 minutes. Unsharp masking has also been used. The simulation is made of populations of random and plate oriented pyramid crystals. Halos are 9°, 18°, 20° and 23° circular halos and lower 23 plate arc (23 lpa). Sun elevation change in the photos was from 71° to 75°, the simulation is for 73°.
Lower 23 plate arc is probably not that rare halo in itself. It's just that resident observers are lacking from the locations where sun regularly rises high enough for it to be seen.
Simulations are made with HaloSim and stacking with Registax.
More stacking technique halo images can be seen here.
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