Monday, 1 June 2020

Odd radius display at low sun in Berkshire, UK

11-frame stack at sun elevation 2°: unsharp mask and colour enhancement (top), and background subtraction + blue-minus-red colour subtraction (bottom; the latter by Nicolas Lefaudeux).

In the evening of 24th May 2020, a notable display of odd-radius halos and their associated plate arcs occurred in Berkshire, UK. I first noticed faint traces of circular halos - 20° and 23° as I could later confirm from photos - at around 19:20 BST, when the sun was at 13° elevation. Less than 30 minutes later the sun had come down to 9° and first signs of the upper 20° plate arc appeared, making obvious the need to find a view down to the horizon. The display got weaker after some time but regained some intensity less than 30 minutes before the sunset. Unfortunately there were some lower-level cloud interfering with my view for most of the observing time.

Stacked and further processed images from the first stages of the display (below) reveal 18°, 20°, 23°, and 35° circular halos in addition to the aforementioned 20° plate arc. At the end of the display (above), the circular halos are less clear, but plate arcs at 20° and 35° show up better. Most interestingly, perhaps, there are suggestions of 28° circular halo and the associated plate arc at the upper left-hand side at solar elevations 7° and 2°. Previously 28° arcs have been reported in the Lascar display of 1997 and in a few more recent occasions in China, but possibly never before in Europe.

50-frame stacks at sun elevation 9° (top) and 7° (bottom): Background subtraction (left) and background subtraction + blue-minus-red colour subtraction (right).

Thursday, 14 May 2020

Map in the Sky - High Cloud Light Pillars in Xiamen, China

In the evening of May 10 2020, residents in Xiamen saw a strange, patterned cluster of light spots hovering over the city. Photographer YUAN Quan captured the phenomenon at its peak with his handphone.

© YUAN Quan, shown with permission. Equivalent focal length 39mm (from EXIF, unclear whether it's accurate).

Shortly after YUAN’s photo went public, LI Yahong and HUANG Tengyu from the China Sky Enthusiasts community found out that the light pattern, when flipped and rotated, perfectly matches downtown Xiamen’s map.

YUAN Quan's photo flipped and rotated 180°. Map from Apple Maps.

With the help of real night time images of Xiamen captured by Wuhan University’s Luojia-1A satellite (http://59.175.109.173:8888/index_en.html) and Chang Guang Satellite Technology's Jilin-1 satellite (https://mall.charmingglobe.com/Archive/), LI and HUANG’s finding was verified - we’re looking at a reflection of Xiamen’s city lights off the clouds.

*Special thanks to the Luojia-1A and Jilin-1 satellite teams for authorizing data usage.

Satellite images copyrighted to Jilin-1 and Luojia-1A satellite teams, shown with permission.

This phenomenon is most likely a high cloud light pillar event, similar to a previous case in Finland (http://www.thehalovault.org/2017/03/map-in-sky.html) but on a much larger scale. The reflection captured in YUAN’s photo corresponds to an area of 200 square kilometers on the map. What’s even more interesting, the ground temperature in Xiamen during the event was 23°C -  a fairly warm night.

Due to the lack of background stars in YUAN and other people’s photos, it’s hard to do accurate cloud height calculations. A rough estimate by ZHANG Jiajie places the clouds between 6 and 7 kilometers above sea level. Sounding data from the night indicates the existence of a moist and wind-free layer in the 6 to 7 kilometer range so the height estimates may not be too far off.

Such warm night high cloud light pillar events are becoming more frequent in China - another two weaker ones were observed in the past few months in different cities. We believe the ubiquitous usage of decorative LED strips (which are very bright) on tall buildings, as well as the vast improvements in low-light photography on newer smartphones are two of the key factors leading to the surge in new observations.

Saturday, 2 May 2020

Hidden Treasures in the Jutland Display

The Jutland display (http://www.thehalovault.org/2020/04/new-halo-triple-cza-jensen-arcs-in.html) introduces us to the Jensen arcs, a brand new halo form and so far unexplained. While the new halo attracts all the plaudits, there are some hidden treasures in the display which are equally interesting and worthy of more attention.


Full circle 'pillar CZA'


During the peak of the display before the Jensen arcs appeared, another elusive, sub-visual halo lurked near the CZA and Kern arc. Anders’ Kern arc photo (likely the first high cloud Kern arc visible in a single frame), when enhanced, reveals a faint, full-circle arc outside the CZA + Kern combo.

Photo by Anders. Sun elevation 19.8°

First of all, it’s not a weaker version of the Jensen arcs. When compared to the actual Jensen arcs which emerged 10 minutes later, this one dips lower to around 57° elevation.

Left: ‘Pillar CZA’ (arrowed) , sun elevation 19.8°; Right: Jensen arcs, sun elevation 21.5°. Photos by Anders.

The 57° elevation reminds us of the multi-scattered 'pillar CZA' in the Siziwang Qi display (http://www.thehalovault.org/2020/03/secondary-cza-from-sun-pillar.html). The overall appearance of the arc in Anders’ photo, albeit dimmer, greatly resembles the Siziwang Qi ‘pillar CZA’.  Chances of the two being the same halo are quite high.

Siziwang Qi photo by LI Tingfang. Jutland photo by Anders.

However, unlike Siziwang Qi where multi-scattering was strong, the absence of other multi-scattered halos (such as secondary UTA and PHC from the bright UTA) in Anders' photos clearly doesn't favor the conventional multi-scattered ‘pillar CZA’ solution for this display.

Nicolas Lefaudeux proposed an interesting theory which sounds highly viable in this case. He suggests that the sunlight scattered inside the cloud layer may have acted as an 'integrated glow', feeding light to each crystal from all azimuthal angles. In other words, the crystals are seeing from their viewpoint an infinite number of 'pillars' around the horizon, and each 'pillar' creates its own 'pillar CZA' via the crystals. The combination of these infinite numbers of 'pillar CZAs' will of course be a full circle with even intensity distribution.

This theory, when extended, may potentially explain the mismatch in the Siziwang Qi case too. In the Siziwang Qi display, available materials suggest the arc likely went full-ring as well (couldn’t be confirmed due to lack of DSLR photos). The sun pillar alone can't create a full-circle multi-scattered CZA without invoking unrealistically thick and triangular plates.

Simulations with ZHANG Jiajie’s program (https://github.com/LoveDaisy/ice_halo_sim/tree/dev/cpp). Sun elevation 20°.

Ji Yun from the Chinese halo watching community came up with the idea of facilitating the 360° glow from the snow covered terrain to increase the ‘pillar CZA’ azimuthal extent. His idea is in nature very similar to Nicolas’. If this ‘integrated glow’ theory holds, the name ‘pillar CZA’ should probably be changed to ‘integrated CZA’ instead. But for now, it’s still too early to make that move. More observations are needed to see if the arc always goes full-circle.

Skywatchers world-wide should start keeping an eye out for this elusive feature in future displays. Whenever there’s a bright CZA in diamond dust or high clouds, it’s highly recommended to always photograph the entire zenith area in RAW for in-depth analysis. It may also be worthwhile to go through past images and see if this arc shows up via heavy post processing.


Mismatching Lowitz arcs


Now let’s turn our attention to those arcs above the parhelia, which are seen in many photos of the Jutland display. Below is Anders’ photo for solar elevation of 14.8 degrees, and shown are four simulations of arcs, two from Lowitz oriented crystals (simulations 1 and 2) and two from alternate Lowitz oriented crystals (simulations 3 and 4). The alternate Lowitz orientation was included because recently it was realized that alternate Lowitz arcs actually exist. Ji Yun identified them in a display that Lasse Nurminen photographed in Raisio, Finland on 28 June 2019.

None of these scenarios really work. Only simulation 2 gives a match with the photo, and that’s just the outer arc – the inner arc is not right. And even the outer arc is on a rather short side for comparison. More length would have been preferred to see whether the apparent similarity in geometry really holds.

Photo by Anders. Sun elevation 14.8°. Simulations with HaloPoint 2

Below is another Anders’ photo, the sun elevation is seven degrees higher. Here no outer arcs are visible but we think the inner arcs are the same as in the earlier stage. This time comparison is made only with two Lowitz orientation scenarios. The circular Lowitz arc seems a better match here, but the arc in the photo doesn’t look quite as curved as in the simulation.

Photo by Anders. Sun elevation 20.4°. Simulation with HaloPoint 2.

Possibly these arcs in the Jutland display are like the subparhelic arcs (Schulthess arcs) of scenario 2, that is, arcs characterized by an added basal face reflection to the Lowitz arc raypath. The shape is not exactly what the simulations predict, but this is a known issue: several people have pointed out the disagreement of the observed subparhelic arc geometry with the theory for various displays. One such illuminating display was seen on 7 March 2017 in Rovaniemi, which happens to have solar elevation comparable to the first photo above.

And that’s just the geometry. Another contradiction, a glaring one, is that subparhelic arcs are in simulations accompanied by much stronger Lowitz arcs, yet usually Lowitz arcs are nowhere to be seen. Actually, the subparhelic arc and alternate subparhelic arc simulations above were tweaked for clarity: Lowitz arcs were removed in order to not let subparhelic arcs overwhelm them. Moreover, only the above parhelia parts of the subparhelic arc were plotted.

Until recently the subparhelic arcs have been exclusively seen in diamond dust, in which they are a common feature. The first high cloud case to show them appears to be the display that occurred on 7 February 2020 display in Finland.


2 and 10 o’clock spots


Many photos that Anders and others took of the 14 April display show an intensification in the subparhelic arcs at 2 or 10 o’clock positions, or both. Also one video has captured this feature nicely.

In some photos an outright spot is visible. Just to be sure, we tested with simulations for one of Anders' photos whether this could be a 24 plate arc. After all, the cloud contained pyramid crystals, as betrayed by the odd radius column arcs in some photos.

The position indeed corresponded to the 24 plate arc position, but the orientation isn’t quite right, as shown below. So, considering also the lack of other odd radius plate arcs in the display, these spots are probably better understood as parts of the subparhelic arcs.

Could they be just chalked up to irregularities in the high cloud? The fact that we haven’t seen such brightenings on subparhelic arcs in diamond dust displays – which tend to be more uniform – would seem to give appeal for such an explanation. However, considering that several photos from various people show more or less well defined spots in the Jutland display, it might be the case that there is something about the subparhelic arc crystals themselves that makes the spots.

The spot (on the right) in a photo by Richard Østerballe

The simulation has a 24 plate arc to compare with the spot on the left in Anders' photo. Simulation with HaloPoint2.

Another photo from Anders showing the spot. The 24 column arc is visible here, as well as in the photo above.


Odd radius column arcs


Anders' father, Ole Jensen also photographed the display and he took note of 9 column arcs on the sides of the sun. His photo is shown below. A selection of Anders’ photos, too, have 9 column arcs and additionally a very faint 24 column arc, which is visible in the two photos above.

9 column arcs. Photo by Ole Jensen. Sun elevation 13°


Seeing red in the blue spot?


There are some reports of people claiming to have seen red in the blue spot visually. But red has never been caught on photos to substantiate these claims and the theory is not forthcoming either.

Interestingly, some heavily enhanced Anders’ photos would seem to suggest that there is indeed red in the blue spot. However, Nicolas Lefaudeux demonstrated that it is an illusion that disappears when the blue color and its cyan end is covered. 

From the optical point of view, the colors in the blue spot are supposed to add to each other starting from the violet. It should be:

violet only = violet
violet + blue = blue
violet + blue + green = cyan
violet + blue + green + yellow = light cyan
violet + blue + green + red = white

So from this, any red should be an illusion (as there is never more red than any other color), and indeed, it seems to be the case when we look at the blinking grey patch on the blue and cyan of the blue spot (shown below).

Photo by Anders

The red on the blue spot more or less vanishes when blue-cyan is covered. Photo by Anders.

- Co-authored by Marko Riikonen, Jia Hao and Nicolas Lefaudeux

Wednesday, 22 April 2020

New Halo - Triple CZA (Jensen arcs) in Denmark

* All photos in the post are copyrighted to Anders Falk Jensen

* * For ease of reading, in the following texts: CZA = circumzenithal arc, CNA = circumnadir arc.

On April 14, a layer of high quality crystal cloud blanketed Denmark and treated sky watchers there with a feast of rare halos. Among those lucky ones, Anders Falk Jensen was probably the most prepared and dedicated, and what he has captured this day will go down in halo history books as a legend.

Let's first enjoy Anders' halo chasing story and beautiful images.

The Jutland Display

When I got up that morning I was looking out the window and noticed the bright parhelia and colorful superlateral arc. I quickly put on clothes, grabbed a camera and ran outside our house after telling my son and wife to have a look. I found a whole-sky display was surrounding me and I was completely blown away!

Solar elevation: 15.6°

At first I thought I was looking at a diamond dust display because of the brightness of the haloes and temp below freezing, but this turned out to be high clouds. I reached out and tried to touch the halos, it felt like they were so close! I knew this was an important display, so I took a series of images between 8:10 and 8:30 at our place in Them near Silkeborg, trying to cover as much of the sky as possible with 24 mm on a Nikon D700. I noticed arcs going upwards from the parhelia, odd shapes of the 120 degree parhelia, sub-helic and Wegener arcs crossing above the Liljequist parhelia, among the more familiar halos.


Parhelia at this stage sport at least two 'tails', indicating the presence of Lowitz arcs. Solar elevation: 15.6°

120° parhelia, subhelic arc, Wegener arc, blue spot and (?)Liljequist parhelia. Solar elevation: 16°

Unfortunately I had to go to work, so I drove off. I stopped to take more pictures minutes later in the nearby village Salten, since the halos still were bright and I had a good view from there. The anthelion with tricker arcs was making a nice appearance here.


 120° parhelia, subhelic arc, Wegener arc, blue spot, (?)Liljequist parhelia and Tricker arc. Solar elevation: 17.4°

 Tricker arc. Solar elevation: 17.7°

After a few minutes I decided to drive further south and kept one eye on the sky while driving on road 52. After a while I noticed the halos were getting brighter again, so I made a stop near the little place called Addit. The CZA was at this point extremely bright, so I also made some shots of the zenith region, looking for the Kern.


The Kern arc can be seen on the left. Solar elevation: 19.8°

Then I got on the 52 again. Southeast of the town of Brædstrup I noticed something strange was going on with the CZA, so I pulled over to make photos at the village of Bredstenbro at 8:58 local time. When I got out of my car I was seeing a double CZA and a I felt like I was on another planet, what an amazing sight! I could not tell what was happening here.


Double CZA. Solar Elevation: 21.6°

After this exciting photo session, I continued further south towards my workplace, rather uplifted. The display was rapidly fading from 9:10. The last halo I saw that day, was an orange lower tangent arc near eastern horizon at 9:26 local time.

Anders Falk Jensen "

The display is obviously an extremely high quality one with loads of great stuff. However, the mysterious 'double CZA' in the last stage completely stole the show. As far as we know, such phenomenon has never been photographed in the past. We're looking at a NEW halo here!

As if a 2nd CZA isn't enough, when we treat Anders' photos with aggressive processing, a 3rd CZA vaguely appears below the 2nd one!

Processed by Marko Riikonen. Sun elevation: 21.6°

So how do we explain this stack of CZAs? So far the experts have come up with three hypotheses, but each has its flaws:

Theory 1 (proposed by the Chinese halo community) - Concave pyramidal plate crystal

Similar to the ultra-flat pyramidal plate theory used to explain the elliptical halos, but with the pyramidal sections concave instead of convex. This will enable a reduction in the wedge angle between the top basal face and prism faces. CZAs coming out of such reduced wedge angles will be closer to the sun.

However, with the addition of extra pyramidal faces, other unwanted arcs will inevitably show up. To get rid of the unwanted stuff, we'll likely need plenty of triangular crystals. Simulation with such crystals is possible with Zhang Jiajie's program. Once he's got the time for it we'll update this post with the results.

Update:

Zhang spent some time making regular hex-shape concave pyramid crystals possible in his program and the simulation results are as follows. Apex angle used is -177°.


Simulations with ZHANG Jiajie’s program (https://github.com/LoveDaisy/ice_halo_sim/tree/dev/cpp). Sun elevation 21.5°.

Ignoring the weird-looking parhelic circle and 120° parhelia, it appears that the CZAs coming out of such concave pyramid shapes are very sensitive to pyramidal cavity depth and prism height. Thick crystals seem to create better results but there're the Kern elements which didn't show up in the display at all. There's also a x-shape arc sandwiched between the two CZAs which arises from 13-4 ray path. Besides, the two CZAs look perfectly parallel, while in the actual display, as pointed out by Nicolas earlier, the bottom one seems to exhibit less curvature. 

Overall, such concave crystal model seems to offer more cons than pros thus less likely to be the solution in this case.

Theory 2 (proposed by Nicolas Lefaudeux) - Plate crystal with slanted prism faces

By making the plate crystal's prism faces a bit slanted, the basal-to-prism wedge angle gets reduced too. Nicolas managed to closely replicate the triple CZA scene in HaloPoint, by introducing two crystal populations with prism faces tilted 3° and 6° respectively. 


Simulation by Nicolas Lefaudeux, with HaloPoint 2.0.

Simulation by Nicolas Lefaudeux, with HaloPoint 2.0.

However, the slanted prism faces shift the parhelia and parhelic circle down too. Triangular shapes in this case won't help much in eliminating them. 

Update:

As requested by Marko, simulations with regular hex-shape stack pyramids - crystals with one pyramidal segment with bigger apex stacked on another segment with smaller apex, were also carried out with Zhang's program. Crystals with height 0.0012, apex 3° and height 0.0002, apex 6° pyramidal segments stacked below a height 0.1 prism give us the following result:


Simulations with ZHANG Jiajie’s program (https://github.com/LoveDaisy/ice_halo_sim/tree/dev/cpp). Sun elevation 21.5°.

Though the parameters look bizarre, the result looks quite all right and almost the same as Nicolas' simulations with two separate populations. Parhelic circle and 120° parhelia still look weird and there seems to be no ideal way to make them look normal with this crystal model.


Theory 3 (proposed by Nicolas Lefaudeux) - Reflected CNA

The sea shore is located at some 30km east of Anders' viewpoint. Theoretically it's possible that the sun's reflection on the sea was sliced by the sea shore and strait into at least two patches of light blobs, acting as two sub-horizontal light sources giving rise to two reflected CNAs below the main CZA.

However, the CNA ray path is less efficient than the CZA path, making it difficult for a less-than-ideal light source to create a bright, sharp and colorful CNA. Even if the reflection gets as bright as the sun, the resulting reflected CNA will still be a lot dimmer than the CZA.

Nicolas later managed to find satellite images of April 14 revealing the cirrus cloud's evolution over Denmark. Video is copyrighted to sat24.com. Original link: https://www.youtube.com/watch?v=gMKsN4APgrk



It appears that at around 7:00 UT, which is the time when the triple CZA peaked, the cirrus was in the middle of a fast sublimation process. Anders' photos between 6:51 and 6:59 UT (8:51 to 8:59 local time) seem to corroborate Nicolas' findings.

Processed by Marko Riikonen. Photos taken behind windshield thus the extra reflections. Sun elevation: 20.4° ~ 21.6°

As can be seen from the sequence above, other halos in the display degraded as the cloud sublimated while the extra CZAs gradually appeared and gained strength. If erosion of crystal faces constitutes part of the cirrus sublimation process, theories 1 and 2 mentioned above could then become the more plausible solutions. Hopefully a repeat event occurs soon at different solar elevations so that these theories could receive deeper scrutiny.

Whatever the solution is, these extra CZAs, which should rightfully be called Jensen arcs now, will keep the experts busy for some time.

- Co-authored by Jia Hao, Nicolas Lefaudeux and Marko Riikonen

* There're more hidden treasures in this display. Please stay tuned for another post covering them. *

Thursday, 12 March 2020

A divergent 46 contact arc

Shown is a display on the night of 10/11 December 2016 in Rovaniemi. I observed and photographed it on an open bog called Rikanaapa, a dark location 7 km east of the ski center, which is around where you most often get diamond dust in Rovaniemi. I think it was one of those colder nights, with temps on the lower side of -25 C.



As I stood there, watching the display and letting the camera snap away photos, I was wondering why the glitter that was the circumnadir arc looked so broad. Photos confirmed this impression. A comparison of long series of shots taken in fixed configuration showed that in the beginning there was briefly a normal looking circumnadir arc (the first photo in the collage above) but then it thickened by extending towards the lamp. Actually, photos reveal it quickly stretches all the way to the lamp in a manner similar to divergent 22 tangent arc.

I was flummoxed and asked Nicolas to take a look when he was visiting in Rovaniemi March next year. He told what should have been obvious: it's 46 contact arc. At the same time also some kind of subparhelic arcs (Schulthess arcs) appear, so it all fits.

But at least one question still hovers in the air: why the 46 contact arc is divergent, with not much at all the classical form to talk of? Only in the second image of the sequence there is what appears to be a classical instance of the 46 contact arc, with some shape and red inner edge. After that it turns into divergent form.

It is also noteworthy that the occurrence of the 46 contact arc coincides with the occurrence of divergent subparhelia. Something in the crystal properties seems to sometimes favour picking up the divergent rays of the spotlight beam. I have seen some really strong divergent subparhelia come and go in the  beam and think they appear when plate crystals get thick. But even then you have also strong classical subparhelia. However, having just divergent 46 contact arc without its classical instance is puzzling.

CORRECTION (13 March 2020): in the second image it is not the classical instance of 46 contact arc. It is way too high up. Anyway, the 46 contact arc hypotheses still probably holds up for the explanation of the stretching of circumnadir arc towards the lamp.

Thursday, 5 March 2020

Halos on lake shore ice hoar frost

by Marko Riikonen and Petri Martikainen

Some fine surface halo displays were seen on lakes’ ice on January 9 and 10 in Finland. Photos were also taken on the night of 10/11 January when the moon was high in the sky. Halos appeared on two different surface-types, both of them showing completely different displays. Halos from what must have been azimuthally oriented crystals were seen on shiny, mosaic patterned ice surfaces. Near the shore in places hoar frost had developed, and on this less reflective surface more traditional type of displays appeared that were not from azimuthally regimented crystals.

In this post we will present our observations in Juva and Tampere concerning the hoar frost surface. The displays were quite similar in both locations. In Juva observations were done on both days, in Tampere mainly on the second day and the following night.

The halos were as follows:

46 halo/arc. A tightly confined and quite distinct segment of 46 halo appeared near the horizon on both locations. This might rather be defined as a kind of 46 arc because of the relatively restricted crystal orientations.

Helic arcs. In Tampere three surface helic arcs were visible. In Juva, where this feature was less well developed, two surface helic arcs could be discerned. In the night, with moon high in the sky, in Tampere a large, detached "Bottlinger's ring" helic arc was seen.

Subanthelion. In Juva, a prominent vertically elongated subanthelion was visible. In Tampere it was weak.

An arc at 45 degrees from antisolar point. Both in Tampere and Juva an arc that appeared to have a concave shape with respect to the subanthelic point was seen and photographed. It consisted of rather dense and faint glints. Its distance from the subanthelic point was measured from a stacked image at 45 degrees. There is likely a margin of error of one degree. Even though the arc appeared concave, it did not seem circular.





The display on 10 January in Tampere is shown above with a simulation. The composition of the display and the actual crystals on the surface (more about them below) guided our choice of parameters. Our initial simulation gave a strong 22 arc below the sun, while in the display there is no such feature. To solve this contradiction, we assumed that crystal attachment to the base might incapacitate two lowest faces which are needed to make this arc. To test, we curated the HaloPoint simulation software output file by removing all instances using these faces, and expectedly got rid of the 22 arc. Additionally, this also removed the extra 46 arc that is neither seen in the display. Comparison of scenarios with all ray paths considered and those having instances with two lowest faces removed, are below.







Crystals in simulations are oriented with their c-axis on average horizontally and rotated 17 degrees from Parry orientation. This rotation gives three upper prism faces (A, B, C) in different orientations to make three helic arcs to roughly match with those in the display (image above). Also, this gives the 46 arc near the horizon, the prism face responsible for it denoted with D. The precise parameters for simulation are below.





Below is a photo of the display towards the sun in Juva on 9 January. The 46 arc was prominent. No sign of 22 halo was seen on the hoar frost surface, but bright subparhelia were present in the shiny, mosaic-like ice surface further away. There are also two helic arcs visible on the hoar frost surface. These were detected only afterwards from the photographs, which is why they are only superficially documented. As their exact shape remained unclear, we used the parameters obtained from Tampere also for the Juva display.







Strong vertically elongated subanthelion, as shown by the two photos above, was seen in Juva on 9 January (sun elevation of 6.3 degrees). Below is a simulation using the same parameters as for the Tampere display above, with instances involving two lowest prism faces removed. No X-shape is seen in the photos, but glints in single photos may not be abundant enough to draw out the actual arc.

Subanthelion in similar looking conditions was first photographed by Jari Piikki in the display on 22 December 2007 in Juva (link to the photo is in the text).




The arc at about 45 degrees from the subanthelic point we were not able to replicate with simulations. Below is an image of this arc in Tampere on 10 January. The arc occurs on the same surface as the helic arc display above. The image is an average stack of 57 frames, the bottom version has been blurred.





We note that in Tampere on 10 January, on another hoar frost surface in a different part of the lake, a bottom part of the 22 halo was actually seen. This would rather seem to support the non-curated simulation scenario. Unfortunately no photos were taken. The observation was made at midday when the sun was at over 6 degree elevation, whereas in the place where photos were taken, and where nothing below the sun at 22 distance was visible, the sun elevation was at the time of the photography 2.7-2.9 degrees. In future observations this should be something to pay attention to.

The stacked image of 10 January in Tampere shows, however, some 22 stuff near the horizon. This may be the subparhelia from the mosaic patterned surface that had become partially mixed in the stack with the hoar frost surface. Or maybe there actually was also some near horizon 22 stuff in the hoar frost surface that wasn’t noticed visually.

In Juva close up photos were taken of the hoar frost surface near the spot where the vertically elongated subanthelion was seen. Below is one of these photos, showing what appears to be plate crystals in nearly vertical orientation. This suggested, together with the halo composition, that the displays might be simulated using plate crystals in relatively fixed upright position and with two lower faces disabled due to attachment to the base.





It seems that surface helic arcs can be produced by many kinds of crystals. A close look at the hoar frost crystals that lighted up a surface helic arc in Oulu on 30 and 31 January 2014 revealed that they were kind of blades with a step like structure. The light that made the helic arc passed through the crystals instead of returning back from the entry face as might have been expected. The step structure probably redirected the transmitted light. In Oulu displays the only halo was the helic arc and indeed that appears to be the case with most of the other surface helic arc displays that we know of, exceptions being these latest displays presented here, and a display observed by Jari Luomanen in Tampere on 27 December 2012 with two helic arcs.

Finally, in Tampere a trip to the lake was made also on the midnight between 10 and 11 January when the moon stood at 50 degrees above horizon (image below). Beautiful crystal glitter was seen with a distinct empty area at the subhelic point, where only a pale and diffuse reflection of the moon was visible. At this elevation two of the three helic arcs are located below horizon, the inner and the middle one (A and B). The inner one is detached from the light source and has become a huge, partially eaten doughnut encircling the subhelic point (essentially it is an oversized Bottlinger’s ring). Visually this was quite striking but photos are, expectedly, a disappointment. Additive stacking would have been needed to bring it to the level of visual impression.


Sunday, 1 March 2020

Secondary CZA from Sun Pillar

* For ease of reading, in the following texts: CZA = circumzenithal arc, CNA = circumnadir arc.

The Siziwang Qi display (http://www.thehalovault.org/2020/02/intense-kern-arc-from-china.html) on Feb 14 2020 has almost certainly secured itself a spot in the halo history book, featuring record-high 44° parhelia, full circle Kern arc and off-the-chart overall intensity.

The story doesn't just end there.

In a discussion with Marko Riikonen, he pointed us to a similar Finnish moonlight display in 2013 (https://www.taivaanvahti.fi/observations/show/19422), during which a novel arc was discovered below the moon CZA. Seven years have passed and a repeat event is long overdue. Now the wait is officially over.

As more Siziwang Qi material emerged, one of them caught our attention. In the following iPhone photo by LI Tingfang, there seems to be hint of a long, sun-vex arc below the dazzling CZA.

© LI Tingfang, shown with permission

With some minor processing, the arc stands out and appears surprisingly well defined. It looks exactly like the novel arc in the 2013 Finnish display.

© LI Tingfang, shown with permission

© LI Tingfang, shown with permission

Back then, Nicolas Lefaudeux managed to replicate the Finnish display scene with simulation and identified the novel arc to be a multi-scattered secondary CZA created by the moon pillar. In the Siziwang Qi display, strong sun pillar appeared in most material and multi-scattering has been proven intense, so it's very likely Nicolas' theory applies here too. After some experiments, we managed to reproduce the secondary CZA in simulation by introducing a large amount of pillar-making wobbly plates.

Simulation by ZHANG Jiajie

At first the appearance of the secondary CZA in both displays baffled us. It's hard to imagine how the long and diffuse pillars create such a sharp-looking arc. With the help of ZHANG Jiajie's simulation program we were able to dissect the arc and fully grasp the underlying mechanism.

First let's review the CZA mechanism:

  • Light source altitude 0° ~ 33°: CZA ray path 1-3 works. When light source drops to 0°, CZA reaches minimum altitude of around 57°. 
  • Light source altitude -33° ~ 0°: CZA ray path 1-3 no longer works. Instead, the 'flipped' CNA ray path 2-1-3 kicks in and produces a flipped CNA overhead. This flipped CNA doesn't get lower than 57° either. 

Simulation by ZHANG Jiajie, sun altitude ranges from -35° to 35° at 2.5° increment.

When the pillar is treated as the light source in multi-scattering scenarios, it can effectively be viewed as an infinite number of light sources with altitude covering the 33° to -33° range. The 0° to 33° portion creates an infinite number of CZAs, while the -33° to 0° portion creates an infinite number of flipped CNAs. These two light clusters turn out in simulation as two broad, sun-vex, zenith-hugging arcs, both capping sharply at 57° altitude.

Simulation by ZHANG Jiajie, sun altitude 20.8°

These two secondary arcs together get us the sharp-looking novel arc in the Finnish and Siziwang Qi displays. As long as the pillar covers the horizon, the arc's overall appearance hardly changes as light source rises. The intensity of the arc peaks when light source sits low, which makes sense since pillar also peaks under the same condition.

Simulation by ZHANG Jiajie, sun altitude ranges from 0° to 25° at 5° increment.

When light source altitude drops below 15°, the arc's close proximity to the original CZA could severely hinder detection. According to the animation below, altitude 15° to 25° may be the most ideal observing window.

Simulation by ZHANG Jiajie, sun altitude ranges from 0° to 25° at 5° increment.

There's one thing in the actual display that doesn't go well with simulations. The azimuthal extent of the arc is very long in the actual photo, as if it'll go full circle. The simulated arc, however, hardly goes beyond the 46° halo. To make it longer in simulation, very thick triangular plates need to be employed, which doesn't sound very realistic. There're probably other light sources responsible for the arc's azimuthal extension and we'll need your help to figure it out : )

Jia Hao