Free Halo Simulation Program
© Jukka Ruoskanen 2010
South Pole, 11th January 1999
Photo by Jarmo Moilanen *
On 11th January 1999 Jarmo Moilanen enjoyed a magnificient halo display at 90°S. The halo laboratory otherwise known as South Pole offers beautiful and interesting displays which were unique only ten years ago. Now the snow guns in Finland and Central Europe seem to have the capability to help in creation of South Pole -type of spectales. Still, there is no other display yet that would top this one.
Jarmo's two photos show part of the Sun side of the sky and part of the opposite side. Photos are taken with so little time in between, that one may assume the same parameter values have to explain halos in both images. Let's view the results then.
Four populations were used. The crystals in the random population were slightly columnar to avoid too strong 46° halo. I have no explanation to how such crystals would orient randomly - perhaps they are forming bullet rosettes or such? Plate crystal tilts were increased all the way up to 5°, which looks good at sun side but diffuses the parhelic circle outside the 46° halo. Also the column population tilt had to be increased to 1.2° on basis of the Sun side of the display. Again, this causes problems in the opposite side. The Wegener anthelic arcs as well as subhelic arc are not as compact as in the photograph.
The Parry population, on the other hand, does not bring any problems into either images when the tilt is set to 0.7° and rotation to 0.3°. With these settings the 46° Parry arcs are realistic in shape, although they always look too well defined in simulation compared to real life. The helic arc is narrow and clearly visible, although faint as in the photo. Hastings anthelic arc manifests itself in the simulation exactly as in the photograph: Wegener is split into two arcs when approaching supralateral arc. Sometimes you see two arcs there and sometimes just a diffuse light area. (In original slides the Hastings is a distinct, independent arc, though).
As was mentioned earlier, the tilt of plate crystals is OK if one looks at the appearence of parhelia and circumzenith arc. On the opposite side the story is different. Liljequist parhelia are almost lost - one can barely see them in the simulation. Also the blue spot is wider as in the photo. What would you do to correct these defects? can you find a balance between both photographs with a single set of parameter values? The parameter files of the simulation above can be downloaded: the Sun side and the opposite side.
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