Halos on 6th March 2017 in Rovaniemi

This display came into existence suddenly, when in the morning sun came out from behind a clear cut edge of a receding cloud area. Some points:

- Ounasvaara arc in a solar display
- There is a kind of extension to the Ounasvaara arc which makes it drop shaped. The extension is responsible for the narrow end of the drop near the helic arc. It seems to be present also in the first Ounasvaara arc display. It's caused by the same raypaths as Ounasvaara arc, but with an added basal face reflection.
- 120° arcs. Simulations say they are formed in Lowitz oriented crystals. The halo is not centered on 120° parhelia: it lies along a circle of 60 degree radius centered on the subanthelic point. It is made by raypaths that in plate oriented crystals give 120° parhelia and sub-120° parhelia.
- An uppervex Hastings in solar display. All the others – the spotlight ones – are indirect detections, because the lamp has always been at an elevation where Wegener and Hastings overlap.
- Helic arc is brighter than subanthelic arc. I could simulate this by lengthening the Parry crystals, but then the simulation went wrong in other ways too much. If we can't get this right, it kinda tells me we don't understand this display. I was told that I probably regret of not taking a crystal sample. It had not occurred to me, but once I was reminded, it was obvious that I should. In the simulation no column shaped crystals were used. It would have been of interest to see whether the crystal sample concurred.

This version was worked by Nicolas Lefaudeux

I also regret of photographing this with Nikon D3200 which is why we have to suffer all the artefacts. Not that there would have been any choice at the time, but about a month earlier I was offered a chance to borrow D800 for my halo hunts. I declined in fear of breaking it. A third regret, that was more personal and could have been easily avoided, was that I didn't have mirror with me and thus did not see many things. Didn't see, for example, the 120° arcs, Hastings / Wegener separation, both of which would have been visible easily with mirror. Spotting the 120° arcs in situ – that would have been something. 

The display occurred at around -17° C. You don't expect a major display at such temps in Finland. But sometimes they do occur, like was the case with Mikkilä's Kern display in 2007. Low supersaturations were in my mind the key here. They were low because the night was cloudy and thus the diamond dust could not have spawned before the morning when it started clearing up. And at that point, with the sun above horizon at such late time in the season, the air was not getting so much colder as for supersaturations to increase to the point of crapping the display (which is the usual scenario).

This post was modified on 15 Oct 2017 to replace the simulation with a new one. It has labels added to mark some of the features that seem to appear in the photos (sorry for the degree numbers having placed at the end of halo names, there is some glitch in Photoshop which forces this). Also, the parameter file was replaced becase I adjusted the parameters of the column oriented population a bit, increasing crystal h/d from 0.5 to 0.8 and making it a little more triangular. This more burned more fine grained simulation has gained features that are not seen in the images. Possibly with a better camera some of these could have been dug out from the display. 

The post was further modified on 21. Oct by adding "originals" of the stacks.

 

A singe frame from the very beginning of the series (added 11 Oct. 2017)

"Originals" of the stacks.

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.

Diamond dust season opened in Finland

By Marko Riikonen

On the fifth of November the diamond dust season opened in earnest, when Esa Palmi photographed a major display at the Kittilä airport. While the sun side is always the attention gatherer, the main attraction of the display is really on the opposite part of the sky, where a strong subanthelic arc dominates the scene.

Of its loop visible are the segments on the sides, and we are used of having seen this kind of subanthelic arc more commonly in spotlight displays. Simulations tell such intensity distribution results from triangular crystals. Triangulars are regularly needed to model snow gun displays and here the source was the Levi ski resort some 10 km to the north. The triangle interpretation is also in accordance with the missing of uppercave Parry arc: the crystals are falling with their wedge pointed straight up, so only uppervex Parry can form.

Palmi is familiar with halos and did a good job of documenting the display. If something more could have been done, that would have been covering also the area around zenith, where a faint and rare halo called the Ounasvaara arc might have lurked. It forms in triangular Parry crystals and the few known observations are from spotlight displays. Granted, solar displays don’t have as good contrast as spotlight displays, and so if the halo was there, it might have been impossible to see with the eye. But camera would have still recorded it. Actually, no one has seen Ounasvaara arc in spotlight displays either, all finds have been made from photos.

All Palmi’s photos of the display

Solar diamond dust display with 87° arc

A stack of 40 photos. An average stack has been combined with maximum stack to show the crystal glitter of the 87° arc. The photos were taken during ~2 minutes. Sun movement has not been accounted for.

By Jarmo Moilanen and Marko Riikonen

The diamond dust season is soon to arrive in Finland and it is time to wipe the dust off the equipment. In a meanwhile, here is the last winter’s starter for Rovaniemi, on October 30 2015. The temperature during the display was -5° C, a guaranteed number for great stuff.

So, what do we have here? First of all, visually the upper tangent arc was a breathtaking sight. In addition to its brightness, the myriads of moving crystals made it “swarm”, as if it were alive. The subhelic arc was also intense.

Then take a look at the zenith. There is an intensity threshold very familiar from spotlight displays, which we have been calling the 87° arc and which is made by 357 raypaths in rotating columnar crystals of triangular habit. Perhaps surprisingly, this is only the first time it has been observed with traditional light source. Most likely it would have been observable in some earlier photographed solar displays, but usually the photos don’t offer the luxury of all sky views.

We spotted 87° arc first visually, which we were quite happy about, as often faint effects only turn out from photos. Sun was behind the hill and not shining where we were standing, but some way up the crystals were lit. We saw crystal glitter on the sun half of the sky, cutting off abruptly at the zenith, leaving no glitter whatsoever on the opposite side.

Simulation and two versions of the stack which has 50 photos taken during 2 m 31 s.

Another feature of interest is the dark band between the Parry arc and the 46° halo. A simulation with HaloPoint having four populations of columnar crystals reproduced it quite well. Its formation is contributed by the 87° arc, 46° halo, Parry and 22° tangent arc. Crucial was making the area between Parry and tangent arc to have plenty of light by giving the four populations a continuum of limited rotations. Also, to enhance the upper edge of the gap it was necessary to keep 87° arc from extending inside 46° halo. The triangular column population with 10 degrees rotation did the work (the uppermost active population in the parameter table – number 3). Fully spinning crystals would have extended the 87° arc all the way to the sun.

Assuming the live crystal sample of this display is representative, we see that most crystals have end cavities and yet the subhelic arc (that uses both basal ends) is striking. According to the traditional picture, hollows are bad for halos, but this is not the first display to shake that belief, and actually simulations with column oriented crystals by Nicolas Lefaudeux in 2011 using idealized hexagonal cavities boosted the subhelic and particularly the Tricker arc. But then there are column displays where cavities seem to do their expected work and all rare halos are absent (we will post an example later). So it looks like it might not be a question of whether there are hollows, but rather of what kind of hollows there are.

Or maybe in nature the cavities are always bad, but in this case there were enough optically high quality crystals to make the strong subhelic arc. After all, not all crystals in the sample have cavities or the cavities are so small that their effect is negligible. It is also possible that there was more high quality crystals in the display than the sample lets us know – if they were small, we don’t see them much in the collecting dish because larger crystals have faster falling velocities and will dominate the sample.