Saturday, 19 February 2011

Diffuse arcs in the spotlight display on November 2007 in Rovaniemi

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These photos have been around and some are still in some blog probably, but I have not made simulations. This was seen in the beginning of November 2007 in Rovaniemi, my second experiment to make halos with portable spotlight. And it was good! Even in the middle of this all light pollution near the center of the city the display was strong.

Different views of the display are given above but the attention here is towards the anthelic arcs in the third and fourth image. There is a combination of diffuse and Tricker anthelic arcs, which together with the subhelic arc make some people see a ruminant skull in the third image. Now a good Tricker arc like this in spotlight displays is quite rare, usually it is the diffuse arcs that dominate and Tricker is almost non-existent. The simulations of this display were made with regular hexagons (with little variation allowed) that had on average aspect ratio 0.9, which is already more plate than column crystal (last image). This created the Tricker and diffuse arc combination as close to the photo as possible. It does not stand close scrutiny, but that's the best I got. Simulations are made with Jukka Ruoskanen's HaloPoint software.

The diffuse arc is mainly composed of two components, diffuse-A and B, which can be called also as Tränkle and Greenler, who found these halos in the computer simulations. But it is known that there is more diffuse arcs components, in sixth image I have dissected them into four components, A, B, C and D. Thusfar we have been only able to distinquish A and B in photos of the spotlight halo displays, the C and D are weak and get mixed with the main components. But perhaps here one can see the influence of diffuse C in the photo. The seventh image gives a comparison with simulations where are all four components and the C component removed. The difference between the simulations is subtle, but maybe there is a slight indication that diffuse C may play here a little part in strenghtening the diffuse A.

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