Mysterious sunvex 28° arcs in Yunnan, China

On August 26 2020, Zhang Yibing from Pu'er, Yunnan captured a sunset odd-radius display, in which a bright 28° arc dominates the show. 

Stacked and sharpened. © ZHANG Yibing, shown with permission.

Evolvement of the display. No sharpening. © ZHANG Yibing, shown with permission.

The arc’s brightness is almost on par with the 20° plate arc, while other plate arcs such as the 24° and 35° ones are exceptionally weak and barely show up in stacked photos. What’s more interesting about the 28° arc is its shape. What we are seeing in this display is a gently sun-vex arc which doesn’t quite follow the 28° sun-centric circle.

Stacked and background subtracted photos reveal a left 28° sun-vex arc as well. Note how both arcs curve away from, instead of follow, the 28° circle.

Such appearance contradicts current theories. In the 30-32 pyramid and cubic ice models ( http://www.thehalovault.blogspot.com/2018/09/28-plate-arc-captured-in-haikou-china.html ), the predicted 28° arcs do not appear sun-vex like the 24° plate arcs do. Quite the opposite, when the crystals are wobbly, the arcs should look more sun-cave and follow the 28° sun-centric ring.

The puzzling shape can still be achieved by tweaking the available theories though. JI Yun and I did some experiments in ZHANG Jiajie’s simulation program and came up with a bizarre solution.

First is to tilt a triangular 30-32 upper pyramid by 109.5° until one of its pyramidal faces goes horizontal. Then, apply a loose ( 15° wobble or more) sun-ward azimuthal lock on both tilted 30-32 crystal and cubic ice.

The revised models produce identical sun-vex 28° arcs which closely match the ones seen in the display.

The idea of az-locking crystals in the air is certainly outlandish and should hardly be taken seriously unless concrete evidence is found in real world. That being said, there’s only so much we can do with available theories. New ideas/experiments are much needed.

This display marks the 11th record of 28° arcs in China since 2016, and more importantly, the 1st record revealing the arcs' real shape. Is it possible that what we're seeing in this display are actually new halos, different from the previous 10 Chinese records and the legendary Lascar arcs? Or is it that all these Chinese 28° arcs are a same new breed different from the Lascar ones? We'll need many more similar displays to draw a conclusion. Looking forward to the next summer.

Update:

Nicolas Lefaudeux proposed another possibility for the arc's peculiar appearance:

"On my side, the sunvex shape is still not so obvious to me. I see more a vertical elongation of the 28° arc + some patch of 28° halo. 

Effects like diffraction by "vertically narrow" crystal faces would create such vertical elongation (like a plate crystal with exotic faces instead of regular prism faces). Unfortunately, such effects cannot be simulated with our current softwares. The appearance of the halos (smooth and rather undefined) make me think of diffraction-affected halos (small crystal faces), like most of the odd radius halos. We would need cases with sharper arcs in order to be more conclusive."

28° arcs from Yunnan, China

Two rounds of week-long odd-radii outbreaks swept across southern China in July and August. The second outbreak turned out to be the more noteworthy one, involving predominantly Chengdu-like low sun plate displays ( http://www.thehalovault.blogspot.com/2018/10/time-machine-chengdu-display-from-july.html ).

Zhong Zhenyu, a member of the Chinese skywatcher community chat group, went halo hunting with his DSLR on August 21 after being informed of the on-going outbreak in his area. The community's collective effort paid off and Zhong was treated with some great celestial rarities.

© Zhong Zhenyu, shown with permission.

Upon first glance, the scene immediately reminds us of the Chengdu display, with 3 colored arcs piling up to the right side of the sun. The arc in the middle possesses the same color separation as the other two, especially when USM is applied.

© Zhong Zhenyu, shown with permission. Unsharp mask applied.

B-R analysis later carried out by Nicolas Lefaudeux further confirms the arc's authenticity. The left-side component also shows up in the processed image. At this point it's quite clear that we've got the third confirmed sighting of 28° arcs in China.

Processed by Nicolas Lefaudeux.

Over the past few years, this type of weak, low sun odd radius plate displays occur rather frequently during summer monsoon over southern China. Now with 3 confirmed and 1 possible cases of 28° arcs within 3 years, chances of these plate displays involving exotic arcs may not be as slim as we expected.

Jia Hao

Possible 28° (plate) arc from Changsha, China

On July 1st, skywatcher Luo Wuping captured a decent odd radius plate display during sunset hours from Changsha, Hunan Province, China.

© Luo Wuping, shown with permission. 3 images stacked.

Upon first glance at his photos, we immediately noticed the display's striking similarities to the previous two Chinese displays involving 28° arcs ( http://www.thehalovault.blogspot.com/2018/10/time-machine-chengdu-display-from-july.html and http://www.thehalovault.blogspot.com/2018/09/28-plate-arc-captured-in-haikou-china.html ). The brightening between the left 35° and 24° plate arcs looks very much like the 28° arcs confirmed in previous cases and its position matches simulations.

© Luo Wuping, shown with permission. 3 images stacked with minor USM applied.

Unlike the Chengdu display, the brightening only appeared on one side of the sun. Uneven distribution of clouds/crystals might be at play here but the absence of the right side component definitely complicates the situation.

Typically B-R analysis on raw files resolves difficult cases like this (which worked well for the Hainan display). Unfortunately, only handphone photos are available at the time of writing, and the jpeg files turned out insufficient for serious image processing.

After discussions with Nicolas Lefaudeux and other halo experts, we reached the conclusion that it's safer to consider the case a possible one for now given the lack of solid evidence. Hopefully DSLR records of the event will surface some day in future.

Jia Hao

Possible observation of exotic 28d upper plate arc

Upper left and upper right are image processing by N. Lefaudeux. Lower left is my own processing, and lower right the simulation. All versions of processing are based on the background subtraction method, but use different ways of enhancing contrast. Sun elevation is about 36 deg.

Some time after the publication of the 31 May 2018 display (see previous post), I returned to analyze the photos in order to take a closer look at one suspicious feature. After strong contrast enhancement, one stack showed a brightening in the area just above the upper 23d plate arc as though a halo may be present. At first, we assumed it was an artefact caused by strong enhancement of the 23d plate arc. However, after fine-tuning of the image processing, we have managed to better reveal this feature, and we can say with the enough confidence that this feature is an another exotic halo — an upper 28d plate arc.

For simulating this halo, I have used a poor plate oriented crystals with a triangular habit, which consist only of a lower exotic (2 0 2 3) pyramid. These crystals are also responsible for the 28d halo, the very faint parts of which you can try to see diagonally upwards from the sun on colored photos above. I have used a triangular habit instead a hexagonal one to remove a 13d halo, which is absent in the observation. In the table below you can find all the crystal populations used in the simulation, and also their some parameters. For ease of comparison, the simulation is placed in the photo scene. It moves into alignment with the photo, as far as field distortion (and other geometric imperfections of the resulting image) allows. As always, the simulation was made with the aid of Jukka Ruoskanen's HaloPoint 2.0. According to the photo, it seems there is faint 18d plate arcs just above 19d plate arcs. If that is the case, this feature makes the display even more similar to the Lascar one (on correspond sun elevations, of course). In order to display this feature in the simulation, regular pyramid crystals in a poor plate orientation were used. But mostly these crystals contribute to 18 and 23d halos. The crystals do not have any prismatic faces because 9 and 24d halos are not present in the display. Based on appearance of the 28d plate arc, the responsible crystals were rather small. Unfortunately, HaloPoint does not allow to change size of crystals, therefore, it is not possible to display the fuzziness of the arc.

Time machine: the Chengdu display from July 20, 2016

The 1997 Lascar display ( http://www.thehalovault.org/2008/12/lascar-display.html ) opened the door to a world of exotic halos. Halo researchers and enthusiasts alike have all been eagerly waiting for a repeat event. Twenty years have passed and not a single reappearance was reported, until recently.

On July 20 2016, photographer Jin Hui captured an odd-radius halo display from Chengdu, China and later shared his photos with the Chinese sky-watcher community. The significance of the display wasn't immediately recognized and the halos involved were mistakenly identified as ordinary pyramidal plate arcs. Fortunately, the photos were brought back up on the table for better scrutiny earlier this year when members from the community performed housekeeping on past digital archives.

 

© Jin Hui, shown with permission. Taken from Chengdu at around 22:00UT, July 19, 2016.

 In the reprocessed images, we noticed that the two colored arcs sitting below the 35° plate arcs seem too far out to be 24° plate arcs. The observation was quickly verified by simulations - the arcs are actually positioned at an angular distance of around 28° from the sun. The overall appearance greatly resembles the 28° plate arcs in the Lascar display at low solar elevations ( http://www.thehalovault.org/2008/12/lascar-display-v.html ). 

 

Dr. Nicolas Lefaudeux, who carried out in-depth research[1] on the Lascar display, confirmed our findings with his outstanding post-processing techniques. In the stacked B-R image, the arcs exhibit excellent color separation. At this point the presence of the arcs is unmistakable - we now have the world's second known record of the 28° plate arcs.

Post-processing by Nicolas Lefaudeux

 

Compared to the Lascar display, what happened in Chengdu is different in several ways:

• no other exotic arcs/circular halos
• 9° and 24° plate arcs are present
• 28° circular halo is weaker, if present at all

Unfortunately, the lack of other exotic arcs makes it impossible to pin point what produced the display. At least two types of crystals, pyramidal crystals with 30-32 pyramidal faces and octahedral cubic ice crystals, possess the interfacial angles suitable for 28° plate arcs ( more discussions can be found at: http://www.thehalovault.org/2018/09/28-plate-arc-captured-in-haikou-china.html ).

Facing a dead end with the Chengdu case, we took a deeper dive into the archive hoping to find more sightings of the same event. The effort paid off with three photographic records recovered. Though these records contain no additional exotic halos either, they do help us paint a better overall picture of what happened geographically on July 20.

100km southwest of Chengdu, photographer Lin Yong recorded an almost identical scene from the summit of Mt. Emei, except that the 28° arcs are much weaker. Further southwest in Yuexi, crystal quality in the clouds plummeted. Founder of the Chinese sky-watcher community Ji Yun saw only a poor, traditional odd-radius plate display. These reports combined suggest that crystals responsible for the 28° arcs only appeared regionally that morning and probably require more demanding conditions to form.

 

© Lin Yong, shown with permission. Taken from Mt. Emei at around 22:00UT, July 19, 2016.

© Ji Yun, shown with permission. Taken from Yuexi at around 23:40UT, July 19, 2016.

 

 

Satellite image sequence from http://www.sat.dundee.ac.uk/geobrowse/geobrowse.php?sat=12

According to the photographers, the halos over Chengdu and Mt. Emei quickly weakened and disappeared after sunrise. However, four hours later on Mt. Emei, Yang Jialu captured a display with 18° and 23° plate arcs with her handphone. Unfortunately the 28° area above the 23° plate arc was left out of the frame, making it impossible to know whether the 28° plate arc showed up or not. 

© Yang Jialu, shown with permission. Taken from Mt. Emei at around 2:00UT, July 20, 2016.

 

It's a real bummer that the display didn't last longer after sunrise in Chengdu and Emei. Studying how the 28° arcs changes with solar elevations could be another approach to closing the case. Anyways, what we have here is undoubtedly a milestone on our way to fully working out the Lascar puzzle. Till then, let's enjoy the era we're living in where there're still puzzles to be solved.

Jia Hao

[1] Nicolas A. Lefaudeux, "Crystals of hexagonal ice with (2 0 -2 3) Miller index faces explain exotic arcs in the Lascar halo display," Appl. Opt. 50, F121-F128 (2011)

 

 

28° plate arc captured in Haikou, China

On the evening of Sep 5, 2018, an odd-radius plate display of great significance was captured in Haikou, China, by photographer Zhan Guorong. The photos, when enhanced, reveal an elusive coloured arc between 24° and 35° plate arcs, which doesn't fit into any ordinary odd-radius halo families.

© Zhan Guorong, shown with permission

The arc was later confirmed by Dr. Nicolas Lefaudeux to be the exceedingly rare 28° plate arcs, which previously had only two known records world-wide. They were first observed in the 1997 Lascar display in Chile (http://thehalovault.blogspot.com/2008/12/lascar-display.html), and spotted for the second time in Chengdu, China by photographer Jin Hui on July 20, 2016. We've got permission from Jin Hui to share his great capture to the world.

© Jin Hui, shown with permission

Unlike the Lascar display which lasted for almost a full day with many new arcs/halos discovered, displays in Chengdu and Haikou were short-lived with no other new arcs/halos apart from the 28° plate arcs. The lack of associated arcs and restricted solar elevation make it difficult to fully understand what really happened up in the clouds. Isolated 28° plate arcs can be reproduced in simulations by either triangular pyramidal crystals with 30-32 pyramidal faces [1] or octahedral cubic ice crystals with an octahedral face horizontal [2]. Both models require rather restricted shape/orientation conditions.

photo enhancements by Nicolas Lefaudeux, simulations with home-made program by Zhang Jiajie

Dr. Lefaudeux brought up another interesting point. The 9° and 24° plate arcs were totally missing in Lascar, implicating the absence of middle column sections in the pyramidal crystals. In Haikou and Chengdu though, they were present and quite strong.

Are these displays simply variants of the Lascar display with different crystal combinations? Or are we looking at a totally new breed? We'll need more photos at different solar elevations to unravel the mystery. Good news is that now we know such displays can probably occur anywhere. Before the Haikou case, we thought that the responsible crystal clouds are high mountain related since Lascar and Chengdu sit beside the Andes and the Himalayas respectively. The clouds responsible for what happened in Haikou, however, had their origin in the middle of South China sea.

We encourage skywatchers world-wide to keep an eye out for these elusive arcs. They might just pop up in the next odd-radius display over your backyard.

Jia Hao

[1] Nicolas A. Lefaudeux, "Crystals of hexagonal ice with (2 0 -2 3) Miller index faces explain exotic arcs in the Lascar halo display," Appl. Opt. 50, F121-F128 (2011)

[2] M. Riikonen, M. Sillanpää, L. Virta, D. Sullivan, J. Moilanen, and I. Luukkonen, “Halo observations provide evidence of airborne cubic ice in the Earth’s atmosphere,” Appl. Opt. 39, 6080–6085 (2000)