Neklid Antisolar arcs: Case closed?

In my last post I outlined several possibilities to explain the great brightness of the antisolar arc (AA) compared to the heliac arc (HA) in the Neklid display from Jan 30th, 2014. All of them were a bit off the main road of traditional halo science, but traditional arguments did not help to clarify what was observed, hence I had to look for something else.

Both the concepts of plate Parry crystals and trigonal Parry columns should yield weak traces of unrealistic (or better to say non-traditional) halos that might appear in a deeper photo analysis. Claudia Hinz provided me with a set of pictures from the display to unleash any kind of filters that would seem appropriate. Indeed it was possible to pin down traces of the Kern arc in some of the pictures after the initial application of an unsharp mask (1, 2), followed by high-pass filtering (1, 2) or, alternatively, by Blue-Red subtraction (1, 2). Note that the Kern arc was weakly present in the simulations for hexagonal, Parry-oriented plates. This, of course, must not be confused with the recently proven Kern arc explanation relying on trigonal plates in plate orientation. Finally, trigonal columns in Parry orientation are a third non-traditional crystal configuration giving rise to new halos. However, these do not yield a Kern arc.

Obviously, the Kern arc fragments in the photos are very feeble and the whole procedure reminds a bit of the search for higher order rainbows. It is mere guesswork to detect how far the arc stretches around the zenith, but doubtlessly it extends up to 90° and more in azimuth, thus being clearly distinguishable form the circumzenith arc. Nonetheless, one would feel safer with further evidence. Comparing the simulations for Parry columns and Parry plates, three more differences are discernible (apart from the changed AA/HA ratio):

1) For Parry plates, the upper suncave Parry arc does not show an uniform brightness, but appears brighter directly above the sun and loses some intensity towards the points where it joins the upper tangent arc.

2) The upper loop of the Tricker anthelic arc is suppressed for columns, but shows up for plates.

3) Some extensions of the upper Tape arcs appear between the Wegener arc and the subhelic arc.

At least the first two points can be answered in favor of the Parry plates, being visible even without strong filtering. However, I failed to detect any extended Tape arcs as “ultimate proof” so far. This might not surprise since they are, according to the simulation, comparable to the Kern arc in intensity and appear in regions of the sky where the crystal homogeneity was not as well developed as in the vicinity of the zenith.

Piecing the parts together, it seems evident that at Neklid the AA intensity was due to Parry-oriented hexagonal plates. Their traces were detectable, whereas nothing appeared that would hint on trigonal Parry columns. In contrast to this, Parry trigonals were responsible in Rovaniemi 2008. This implies that in nature at least two different mechanisms occur for AA brightening.

Finally the question remains how plates may get into a Parry falling mode. But as long as no one understands how symmetric columns do this (though we have the empirical evidence), we should be prepared for surprises. There might also be a connection to recently discussed details of the Lowitz orientation (2013 Light and Color in Nature conference, talk 5.1).

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Posted on May 11, 2014, in ice phenomena, theory and tagged , , , , , , . Bookmark the permalink. 3 Comments.

  1. Marko Riikonen

    Sassen suggested in “Parry arc: a polarization lidar, ray-tracing, and aircraft case study” (2000) the possiblity of thick plates in Parry orientation. In Finland it is quite common to see Parry-orientation halos that are caused by trigonal plates in diamond dust displays. Trigonal crystals also occur in the crystal samples. It is not possible to make a link in this comment, but googling “Another subhorizon view” will show a photo with many triangular crystals from snow gun generated diamond dust displays. The trigonals are the property of the snow gun diamond dust, we seem to never need them in simulating the high cloud displays or “natural” diamond dust displays that occur in the Antarctic or high arctic.

    One reason for the additional halos in the simulations of the Neklid display is that they are made much more intense than the display was. Also, the spot light displays in diamond dust have teached us that daylight background prevents the weaker halos from being seen – we see some of them in spotlight displays because of the strong contrast between the space lit by the beam and the space outside it. But even then, you can always burn the simulation to the extent that it shows halos not seen in the spotlight displays. This is particularly true with Parry-orientation halos.

    Possibly plates tend to take Parry orientation when thick enough. Crystal size may also be a factor affecting this behaviour. There was this guy who let big artificial crystals sink in a liquid filled tank with Reynolds numbers similar to ice crystals falling in the air (he wrote a paper about it). I suggested him to experiment with Parry and Lowitz crystals. Don’t know if he carried on.

    As for the uneven distribution of the crystal cloud as a cause of intensity variations of the displays, this gets less of a problem as one moves from high cloud displays to daylight diamond dust displays to spotligh diamond dust displays. In the last case, the problem of uneven crystal cloud distribution is pretty much eliminated completely as crystals pass swiftly through the beam (providing, of course, that the exposure is longer than just a few seconds). In many respects, the spotlight diamond dust displays are the best material for research. The also look great – something that can be truly understood only with personal experience.

  2. Marko Riikonen

    Commenting on my comment, on the second thought, the spotlight display photos actually can show uneven distribution of crystals even with long exposures. But it is different kind of unevenness: the crystals that pass through the areal light by the beam can have fast changes in their properties and long exposure blends the resulting different stages in the display into one. The closer to the snow guns the displays occurs, the faster the changes are. Couple of kilometers away – if the display is that extensive – this should not be such a problem.

  3. Alexander haußmann

    Hi Marko,

    thank you for your comments. I’ll check the Sassen paper next week when I have access to the journal via university. Concerning the trigonals, are there any ideas why they form only in snow gun related diamond dust? During meetings in Germany and Switzerland we speculated about condensation nuclei that are emitted from these guns, but nothing clear is known yet. However I really appreciate that you make the effort to collect crystal samples from your displays!

    I agree that the sky background will prevent the weak “non-classic” halos from appearing in the real daylight display. Nonetheless the main focus of my small study is the unmistakingly bright antisolar arc, and I wanted to employ the weaker halos only to tip the scales between hypotheses about the crystal shapes. Of course it would be an unrealistic hope to detect all of them in the photos (although image processing seems to be quite advanced, e.g. in the case of higher-order rainbows).

    So far it seems that hexagonal plates in Parry orientation were responsible for the bright antisolar arc in Neklid. If this hypothesis is right, they cannot be too thick (c/a approaching 1), because this will diminish again the antisolar arc. Claudia and Wolfgang reported that the nearest snow guns were some kilometers off and may not have influenced the crystal formation. You mentioned that someone made orientation experiments with crystal models – can you please find out who it was?

    I don’t know how good the chances are for spotlight displays in Germany and the neighbouring countries. Maybe it is possible in places like Neklid, but it seems that nobody has tried yet.

    By the way, how can I order your book? It seems that it is not available via Amazon etc.

    Best regards,
    Alex

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