Thursday, 18 October 2018

New case of exotic 19d plate arcs

When I first noticed the display at 08-10 local time on 31 May 2018, it was already well-developed. The high cloud layer was very thin and it was visible only in the area close to the sun. At first the display consisted of an upper 23d plate arc and an upper quarter of a 23d halo. A little later I noticed arcs as light spots in the side area of the sun, that reminded me lower 24d plate arcs.


When I processed images, I noticed that these arcs are more like 18d plate arcs than lower 24d plate arcs. But during the observation I distinctly saw that the arcs were located at an elevation lower than the sun, while 18d plate arcs are always located at the same elevation as the sun. I applied stronger processing and revealed a gap between the arcs and a 18d halo. As a result, it became clear that the arcs are exotic 19d plate arcs that were first observed during the legendary Lascar display. In addition, a trace of exotic 28d halo was also revealed.

Sun elevation is about 36 degrees

Some analysis

The halos, known as Lascar halos, are caused by exotic pyramidal crystals with pyramidal faces of (2 0 2 3) Miller index. These exotic pyramids have a 39.1 apex angle while pyramids from regular pyramidal crystals have a 56.1 angle. To simulate the display, I used four different crystal populations. Not one of them have basal crystal faces. The first population is plate oriented pyramidal crystals with upper exotic and lower regular pyramidal faces. This population makes most visible features of the display (19d and 23d plate arcs). The second population consists of crystals with regular upper and lower pyramids, and it contributes to 18d and 23d halos. The population is poorly oriented, in order to  reproduce some features of 18d and 23d halos. The third population contains plate oriented regular pyramidal crystals consisting only of lower pyramidal faces. It needs only to enhance the upper 23d plate arc. Finally, the fourth population is added to reproduce the 28d halo. Its crystals is randomly oriented and consists of upper exotic pyramidal faces in triangular habit. That is, the crystals are almost regular tetrahedrons.

My attempt to simulate the display.
Software: HaloPoint 2.0 by Jukka Ruoskanen
The result shows quite good agreement with the observation, except for an exotic lower 3d plate arc. There are two possible reasons for it. The first is that a glow around the sun has much more intensity than halos presented here. It does not allow to reveal a 3d arc, unlike the Lascar display, whose observing place was located at an altitude more than of 4000 m above sea level. At this altitude the atmosphere has a low level of aerosols, and therefore the glow around the sun is very small, and the sky background is dark. My observation point was in Pskov Oblast, which has a flat topography with usual atmospheric conditions. The second reason is that exotic crystals may have triangular, but not hexagonal habit. The 3d arc disappears when triangular exotic crystals are applied.



References

 - Nicolas A. Lefaudeux, "Crystals of hexagonal ice with (2 0 2 3) Miller index faces explain exotic arcs in the Lascar halo display"
- Nicolas A. Lefaudeux (personal communication, 2018)

Sunday, 7 October 2018

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.
 
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)

 
 

Tuesday, 18 September 2018

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://www.thehalovault.org/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)

Thursday, 31 May 2018

Odd radii halos outbreak in the Czech Republic



There was an odd radii halos outbreak in the Czech Republic and Hungary in the past few days.
Multiple rare halo forms were captured.
Let me show you some of the photos:

These were taken by Jiří Kaňovský from Černotín, Czech Republic:




He managed to capture 9° contact arcs, 9° parhelion and 24° parhelion among others. Uppercave Parry was captured, too.

These were taken by me:





I am very confused about the halo forms captured here. First I thought I captured 18° parhelions, but now it turns out those are probabaly 20° and also 35° column arcs. There is also a faint lower 23° parhelion. 9° contact arcs are possibly captured, too.
The sun was 62° high at the point of the photoshoot.
I managed to stack multiple photos and edit those in Photoshop to bring out the halo forms.
Can anyone confirm that those are indeed column arcs?

Here are some .NEFs (Nikon's RAWs) if you're interested:
https://1drv.ms/u/s!ArJV0E31n7WMg0jl3Lh02eZjZy0p


Saturday, 14 April 2018

Odd Radius Display, Bolton, United Kingdom

On the 5th April 2018, there was a short lived but rather nice odd radius display in Bolton, a town situated in the north west of the United Kingdom. As I was preparing to go to work, I noticed the top of what I initially took to be a 22d halo. However, it rapidly developed and became more complex and very soon an 18d became easily visible to the eye. The display lasted about thirty minutes in total and I was only able to take a few single shots and a couple of stacks before it began to fade and I had to leave. Upon processing the images, 9, 18, 23, 24 and 35d halos with attendant plate arcs were identified as being present.

Processing courtesy Nicolas Lefaudeux.


Thursday, 5 April 2018

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

Sunday, 1 April 2018

In Memoriam Tibor Ádám Tátrai


Tibor's photo taken on 14 April 2015.
Tibor Tátrai, one of the most prominent and active figures of the Hungarian halo scene died at the age of 22 this week. He had been watching the sky since his teenage years, and made many excellent observations. He was our diamond dust king who had more diamond dust observations in his short life than most of the senior members.
 
Tibor's observation two weeks ago
His last post in our forum this Tuesday was about a dream he had on halos: "I was dreaming about diamond dust and a 22 degree lunar halo with paraselenae on both sides. (...) In the morning the diamond dust fall continued, and in the sunshine the lower tangent arc became visible. In the afternoon there were showers and May weather, and in the clear Western sky a rainbow appeared, while the sky was cloudy above me. The rain was falling, and I was walking in the garden. In my dream I was thinking about how the afternoon Sun could be in the Eastern sky, and how there could be a rainbow if clouds were covering the Sun (...). Yet, I continued photographing happily."

Last summer he received an award from his village for his photos which became the part of the village's "treasure trove".

We'll miss him.