Free Halo Simulation Program
© Jukka Ruoskanen 2010
Sievi, 17th December 2005
Photo by Marko Mikkilä *
This Moon display shows faint arcs that seem to be tangent to the Parry arc - the extremely rare Hastings anthelic arcs. Hastings resembles closely Wegener anthelic arcs, which arise from similar ray path, but from different crystal orientation (singly oriented columns). There are only a handful of photographic records of Hastings anthelic arcs, and this is the only case in Finland thus far.
However, the first feature that meets the eye is the peculiar intensity profile of the Parry arc. The brightening at the centre of the Parry arc immediately brings two possibilities in mind: shortness of the Parry oriented crystals (aspect ratio less than 1) and the presence of Lowitz arcs. Increasing the tilting angle would indeed gradually transform the Parry arc into more Lowitz -like halo, but at the same time the Hastings anthelic arcs would be lost. Therefore let's examine the aspect ratio issue.
Very thin platelike crystals in Parry orientation tend to accumulate light more into the central part of the Parry arc and leave the tips poorly defined. However, it is not easy to believe that thin plates would fall in Parry orientation. Plausibility of the hypothesis can be increased by thickening the plates into aspect ratio of 0.4 and allowing tilting angle of 0.5°. Furthermore, the crystal profile can be modified into triangular direction. Marko Riikonen has photographed such crystals many times during a Parry-display, so maybe putting thick plates into Parry orientation is not purely imagination.
With these parameters the simulation is in the ballpark but still far from perfect. Parry arc intensity does not match with the photo and helic arc is too weak. Perhaps one could try introducing yet another population, namely a Lowitz population. There seems to be a subtle decrease in the intensity of the 22° halo at the top, which could be explained with Lowitz activity. One could then turn down the amount of random crystals in the simulation and thus weaken the 46° halo as well which is too strong in the simulation (tuning the aspect ratio could do the trick as well). Also the Parry intensity profile, as was discussed earlier, would welcome some Lowitz -type of explanation. However, there is absolutely no Lowitz arc detaching from the 22° halo, which is a strong point against Lowitz theory.
What do you think? There are several aspects from which this case can be approached. The parameter file of the simulation above can be downloaded HERE.
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