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."

High Quality 28° Arcs in Ji'an, China

Moments before sunset on June 17 2020, a high quality odd-radius plate display with bright and vivid 28° arcs was documented by multiple observers in Ji'an, Jiangxi Province, China.

© HUANG Qian, shown with permission. Single exposure.

© ZHOU Ling, shown with permission. Single exposure.

Annotated version as follows:

The intensity of the display rivals the 2016 Chengdu display as the 28° arcs stand out even in smartphone photos above. 20°, 24° and 35° plate arcs in the photos are also quite well defined.

Unfortunately, like previous displays, no other exotic arcs are found in the photos we received from the community.

Now that we have a great and early start of the season, let's hope for more great stuff to come.

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).

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

Odd radius plate display from the Czech Republic

On 4th March Jitka R. from the Czech republic captured this magnificent odd radius display on the Moon. After stacking multiple images, many halo forms showed up - particulary 9°, 18°, 20°, 24° and 35° halos, along with 9° and 24° plate arcs.

It was a case of sublimating cirrus, which could be easily seen on the satellite images.

And I have a question for the halo experts - is it possible that the 9° column arcs are captured, too? Or could it just be a cirrus of a higher quality?

Odd radius plate display in Czechia

On June 4th a cold front was advancing towards Czechia with thick Ci/Cs cloud cover ahead of the front.
Zbyněk Černoch was keeping an eye for any potential halos that might show up. As soon as the cloud cover started to sublimate (it was nicely visible on satellite imagery), bright pyramidal plate arcs showed up. The 23° parhelion is very easily distinguishable with its characteristic shape.

A few moments later, the 23° parhelion lost its initial shape, but became a little more coloured.

The same photos, with B-R processing:

Odd radii in Southern Finland

A horrendously quiet April passed by without any noteworthy action (except Jarmo's fantastic bolide halo). Halo people were awakened from oblivion on 5th May, when a smooth cirrostratus veil crept in from west and offered plate crystals and odd radii halos. The display was seen and photographed by Ismo Luukkonen in Turku, Marko Riikonen in Helsinki (photo on the right by Marko) and Jukka Ruoskanen in Riihimäki.

Most likely all odd radius halos were on the sky, of which 9, 18, 20 and 35 degree halos were easily discernible. The 22-24 region may have consisted of all three possible rings. Also few of the odd radius plate arcs were present, although rather poorly.