A fresh 2 o'clock spot display

On 6th February 2021 I noticed a nice 2 o'clock spot outside the 22° halo in the sky. I succeeded to take a stack of 40 photos in 4 minutes, during which the spot vanished and was replaced by a thick arc. Sun elevation was 12.8°.

 

Similar halos were observed in Jutland display 22 April 2020, but the explanation for the spots and arcs remained somewhat unclear then. So I did some simulations to see what could be behind the current display. Here is a video of the first 2 minutes showing how the spot disappears: 

 

The same photos are used in the stacks below. The first 30 seconds (5 frames) are on the top, the first 2 minutes (20 frames) in the middle and the last 2 minutes (20 frames) are undermost.

The possibility of 24° plate arcs was discussed in the Jutland-case, but it was concluded that although the position was correct, the orientation of the spots was not. The spots were understood as intensified parts of subparhelic arcs (Schulthess arcs) - which did not match the simulations either.

The spot was most intense in the very beginning of the current observation. I tried to simulate 24° plate arcs to correspond to the first stack of 5 frames, and included subparhelic arcs to the simulation, too. When the 24° plate arc is weak enough, there is a good match with the actual spot (HaloPoint 2.0, comparison below). So at least in this case the simulation seems to give support to 24° plate arc. A weakness is that the 9° plate arc produced by the simulation was missing from the display. And the subparhelic arcs are not even close.

When the spot started to disappear, the arc became stronger and thicker. It was possible to get rather similar thick arcs by simulating upper and middle Lowitz arcs with more tilt in the crystals (std 30 and 40). So, at least in this case, Lowitz arcs seem more plausible ones than subparhelic arcs.

It is hard to say how well these results correspond to reality or to Jutland display, but hopefully they give some ideas for the interpretation of 2 and 10 o'clock spots and associated arcs.

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.

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

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)

Pyramids returned and came with multiple plate arcs

The halos I got earlier today ended at midday but while up mowing the ATV and foot trails I saw halos reappearing! I immediately saw a nice bright lower 9d plate arc my second one since 2002 and both lower 24d plate arcs my best to date! As the clouds moved in closer the halos got better the 18d plate arcs got bright and rather colorful and actually showed a slight curve. At 7:00PM when the sun got low enough for it to happen I got upper 9 and 24d plate arcs and those were two new halo forms for me.

Odd radius display in spotlight beam

Here is a photo of a diamond dust odd radius display in the spotlight beam. Of the less commonly seen halos visible are lower 20° and upper 35° plate arcs.

The image which is a stack of several photos, was taken in Rovaniemi on the night of 6/7 January. The odd radius stuff seemed confined to this particular location, Rikanaapa bog, to which we paid several visits during the night. From our observations and the photos taken it looks like the display remained there pretty much unchanged through the whole long night. The crystal swarm originated from snow guns 6 km away.

The version above, which shows the halos best, is done with the “blue-minus-red” method. Below is also a version with minimal intervention and a one with an unsharp mask.

The display has lower 24° plate arcs and it seems like there may be the lower 9° too. As the lamp is about 5 degrees below the camera, that makes these plate arcs of the 23-5 and 23-6 type respectively. In other words, the B-components for these arcs, if you will.

Nicolas Lefaudeux / Marko Mikkilä / Jarmo Moilanen / Marko Riikonen

Giving better looks for the stacked images

There is no denying it - stacked and unsharp masked halo image can be rather ugly. Ground objects are blurred and horizon is coated with blue glow.

But the looks can be improved by taking the ground from single untreated frame. That's what is done in the upper right image. It is the same image as the upper left except for the ground.

Near the horizon the clouds from the single frame dominate the sky, but in this case it has no significance as the halos are anyway blurred at the horizon. This technique works best with high sun displays where halos are far from ground objects.

The display was seen in Lahti on June 9. The upper image pair is stacked (using Registax) from 72 frames taken during 15 minutes. The lower is 60 frames /12 minutes stack. It shows lower 24 plate arc on the right and perhaps some 24 halo on the left. Jukka Ruoskanen also photographed the display some 40 kilometres from Lahti. Another example of the ground adding is here.

Odd radius plate arcs

35° plate arc in lower center, 24° plate arc above it and slightly to the right, and faint infralateral arc (?) at far left. The angular distances from the sun still need to be measured, however. Dalton Highway, northern Alaska, April 17, 2006. The photo has not been manipulated. Copyright Walt Tape.

A wider angle photo of the same display is here. A photo of another display, about 20 minutes earlier and ten miles further south on the highway is here; (then at "on assignment: arctic Alaska").