Trying to simulate arcs associated with parhelia in Rovaniemi solar display

Above is a display showing arcs extending outwards from parhelia and an attempt at simulating with Lowitz oriented plates using HaloPoint software. Sun elevation was 14.5 degrees.

In the lower right panel is a simulation with all rays. On the upper right is the same simulation (well, not exactly the same, ray numbers are a bit different) that shows from Lowitz orientation population only rays that experience 3 or 5 hits. This leaves the arcs from 325 and 315 type raypaths (the so called Schulthess arcs) visible while some others, such as Lowitz arcs are filtered out. The Lowitz arcs are seen in the lower left simulation, which has only rays with 2 and 4 hits from Lowitz oriented crystals.

The Lowitz arc scenario is clearly out of question to explain the arcs in the display, there is no match. Then how about the 3 and 5 hit scenario? Comparison is easiest with the flashing image below. The outer arcs fit together quite well but the inner ones don't. The inner arcs are angling too steeply inwards in the simulation.

Did I just do a botched job in matching the simulation to the photo? I admit it is not perfect work, but likely not so bad as to account for the large mismatch of the arcs.

Could it be because of the projection difference between the photo and simulation? I know that my 8 mm Sigma fisheye doesn't align perfectly with HaloPoint fisheye projections. For example, subanthelic arc is always narrower and taller in simulations than in photos. So maybe this too is just down to projection. Some day we might have a simulating tool that allows customization for individual lenses. That would settle the issue.

But even then we would still be long way off explaining these kind of displays. How do you get rid of Lowitz arcs in the simulation? I could not find a way. You have to really burn the Lowitz arcs to get 315 and 325 type arcs. This scorching inevitably gives also 46° contact arcs as is shown by the fuller version of the simulation below. In the fuller version of the photo next to it there are no signs of these arcs. Also, I was not able to simulate the correct length of the outer arcs. Lessening tilts seemed to make them disappear altogether. But I wasn't exhaustive in my attempts, it might have been possible to get them right.

What else? The display has no Kern arc even though cza was quite nice and diamond dust smooth. Clearly Kerns are not to be taken for granted. While observing, I thought there could be a chance here, but at the same time had a feeling that sun is probably too high, the plates too thin (you pretty much can say it without looking at a crystal sample) and tilting too much for the good old H.F.A.

Some words about the hunt itself  - the ryynäys as we say here. I was doing spotlight at the night and things got crappy as supersaturations increased. Diamond dust was everywhere but really poor halos. Expectedly it was no better in the morning. Close to the ski centre, diamond dust was so thick the sun did not shine through. At the edge of the cloud where the sun shone brightly there was quite some crystal glitter in the air, but, as it is with this glitter, it comes at the expense of halos: only pillar and subsun was visible. So I went shopping to a grocery store, hoping that in the meantime warming air brings a positive change.

It got better. However, the display was good only at the receding edge of diamond dust cloud. So I photographed in one place and when it weakened, hopped into car and followed the cloud to a new place to photograph again. I did six such manoeuvres, after which the swarm was soon done away completely by the heating sun. The photo shown here is the best set that I got at the third location.

Some technical info. The display was seen on 7 March 2017 in Rovaniemi. Temperature was probably around -16°C at the time I took the stack. It has 54 photos taken during 5m10s, starting at 10:13. Stacking was done with Registax 5.0. The artefacts seen in the image are a scourge of some series of Nikon cameras, they come in daytime photography and there is nothing you can do about them in post processing. 3000 series is one plagued with this issue (I have 3200 and 3300) and from what I have heard from other people's experiences 5000 series too. Definitely not cameras for halo photography.

Faint Kern arc, long Schulthess arcs and possible 46° contact arcs

On 12. November routine plate stuff prevailed over large areas around Rovaniemi. As I drove around, in one location my eye caught an uppervex Parry arc so I stopped to photograph. Visible was also circumzenith arc and even though it was far from dazzling, I aimed the lens at zenith with Kern in mind. After all, you have to account for the sun elevation upon judging how good the cza really is. Those bright at higher sun may lose much of their lustre at low sun. And the lower the sun, the better for Kern. In this case the sun was low: elevation at 1315 local time was 4.6 degrees.

As shown above, the attempt was not in vain: there really seems to be a full circle around zenith. True, the Kern is extremely faint and there are banded artefacts in the image, so one must be careful here. But the regular circle seems different from these artefacts.

The image above shows three versions of a stack of 21 photos taken during 1m 59s (sun has not been tracked in any part of the process). One is blue-minus-red image and two others are made with background removal technique with two different gaussian blur and median noise values. Nicolas Lefaudeux had instructed me on this method. It is also described on his site. The fourth image is a simulation made with HaloPoint.

Because of the Kern’s visibility display rather close to the noise threshold, it would probably be best to refrain from further analysis. But because I made the simulation anyway, let’s say that the Kern’s relatively uniform intensity seems to point at crystals towards triangular shape and I had to use quite extreme triangularity in simulation as depicted by the crystal figure below.



The display has also exceptionally long Schulthess arcs – they extend all the way to 46° halo. The Lowitz oriented population used to simulate them gives also 46° contact arcs. In the actual display 46° contact arcs may be visible too, but it is hard to say because the artefact bands are oriented in similar direction.

For reference, below is also a single image straight from camera. Enhancing it (not shown) seemed also give out Kern arc, but it was even closer to noise threshold than the stack. In any case, the diamond dust was so uniform stacking did not really bring that much benefit.