Odd Radius Display, Southern Ohio, USA, 17th August 2019

On the evening of Saturday, 17th August 2019, I observed this complex pyramidal halo display. In total, I saw a faint 9d halo, lower 9d plate arc, bright 18d plate arc, 18d halo, upper and lower 24d plate arcs and a weak upper 23d plate arc. This display is very similar to the one I captured in 2017, the only difference being the one in 2017 the lower 9d plate arc was bright and colorful and in the present display it was weak.

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)

Pyramidal Crystals Attacked Hungary

Photo by: Károly Tuszinger - Budapest, Hungary (single frame)

Despite the government’s efforts to stop “invaders” at the border, a lot of pyramidal ice crystals could penetrate our homeland on the 3rd of April. At least 3 legions of them rushed across the country arriving from West and reaching the Eastern borders late in the afternoon. The attack had left a lot of people with severely high adrenaline levels – and a lot of excellent photographs! 

 

Photo by: László Dombai - Vép, Hungary (single frame)

The first signs of the display appeared at early dawn when a vivid circumscribed lunar halo formed with some moondogs, faint CZA and easily visible, still faint supralateral arc. In some pics an extremely faint 9d halo was noticed later (I could not see it while being outside). Then after sunrise in the early morning, parts of 18d arcs were visible, probably they were 18d parhelia, but this only lasted a very few minutes and they were also faint. For some hours nothing happened, then at noon another pyramidal crystal containing cirrus arrived and caused a fine display at my place which lasted for about 20 minutes. 9-18d halos were well visible and also the „22d” halo’s arc was too broad to be a simple 22d halo. I took some pics around, but 35d was not present. In the meanwhile the clouds drifted towards East, they were on their way to Budapest and the surrounding area where a lot of people could observe and capture the best of the show in the afternoon. This region had received the elite commando of the pyramidal legions, with an extremely vivid display; the arcs were easily seen even on the worst quality weather webcams’ images! Webcam videos of idokep.hu: Fót and Hajdúszoboszló, Hungary. 

A decent full circle 9d halo with colourful 9d parhelia appeared on most pics, the full circle 18d halo with vivid 18d parhelia was also present, 23d full circles with some cases of 23d upper parhelion (or at least the arc’s colour was much more vivid at the location of the 23d upper parhelion). One wide angle pic shows signs of a very faint 35d arc too. The show lasted about an hour (at least the N parts of the capital). Then the clouds drifted on towards the East and caused a less spectacular but still fine display late afternoon. The northern half of the country was affected by the display or at least parts of it during the day. 

35d halo on the single frame image of Péter Lenkei from Ötschergraben, Austria

Of course this is the case when stacking would lure out more detail, but most people just rushed out from their workplaces to capture the rarity with their cellphone cameras, still the beauty of the show was that even without stacking many details were visible.

I guess it was very lucky that the clouds had done their best in the Budapest area where most of the population concentrates, so many observers could capture the show. I can’t remember any similar case of pyramidal display in Hungary in the past 20 years, it was so vivid, so widespread and so complex!

By: Mónika Landy-Gyebnár

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

Halos from oriented pyramids in the spotlight beam

On the night of 20/21 January we photographed oriented pyramid crystal halos in the spotlight beam on two occasions. The first occasion, shown above, got us completely unawares and the odd radii stuff was all gone in the next shot which would have had the exposure right. Colored and solid upper and lower 9° plate arcs were seen, but if they were still present when the camera was placed in the beam, they are nevertheless washed out by the overexposure. In the photo one still sees the 35° halo and lower 20° plate arc. The lower 9° plate arc was of the 23-6 type as the lamp was about 5 degrees below below horizon.

We drove around and switched on the lamp here and there, stubbornly trying to get a rerun, but to no avail. Diamond dust was already on the retreat, the air was drying up even though it was around -33° C. Yet we managed to get something just before it all evaporated, as shown by the image here:

A little simulation study as was necessary to see whether it was column (left) or plate (middle and right) pyramid stuff, but that did not give definitive answer. The plate scenario simulations are identical except that in one is 22° halo and in the other 23°. The 35° and 18/20° stuff above the lamp were initially noticed visually which prompted us take the photo series for this stack.

These last photos were taken at 3:30 am. The two photos below are the first photos of the night at 6 pm. We photographed these lunar and beam displays simultaneously. No odd radii yet at this early hour.

Marko Riikonen / Jarmo Moilanen

originally published in Halo Phenomena