Secondary CZA from Sun Pillar

* For ease of reading, in the following texts: CZA = circumzenithal arc, CNA = circumnadir arc.

The Siziwang Qi display (http://www.thehalovault.blogspot.com/2020/02/intense-kern-arc-from-china.html) on Feb 14 2020 has almost certainly secured itself a spot in the halo history book, featuring record-high 44° parhelia, full circle Kern arc and off-the-chart overall intensity.

The story doesn't just end there.

In a discussion with Marko Riikonen, he pointed us to a similar Finnish moonlight display in 2013 (https://www.taivaanvahti.fi/observations/show/19422), during which a novel arc was discovered below the moon CZA. Seven years have passed and a repeat event is long overdue. Now the wait is officially over.

As more Siziwang Qi material emerged, one of them caught our attention. In the following iPhone photo by LI Tingfang, there seems to be hint of a long, sun-vex arc below the dazzling CZA.

© LI Tingfang, shown with permission

With some minor processing, the arc stands out and appears surprisingly well defined. It looks exactly like the novel arc in the 2013 Finnish display.

© LI Tingfang, shown with permission

© LI Tingfang, shown with permission

Back then, Nicolas Lefaudeux managed to replicate the Finnish display scene with simulation and identified the novel arc to be a multi-scattered secondary CZA created by the moon pillar. In the Siziwang Qi display, strong sun pillar appeared in most material and multi-scattering has been proven intense, so it's very likely Nicolas' theory applies here too. After some experiments, we managed to reproduce the secondary CZA in simulation by introducing a large amount of pillar-making wobbly plates.

Simulation by ZHANG Jiajie

At first the appearance of the secondary CZA in both displays baffled us. It's hard to imagine how the long and diffuse pillars create such a sharp-looking arc. With the help of ZHANG Jiajie's simulation program we were able to dissect the arc and fully grasp the underlying mechanism.

First let's review the CZA mechanism:

• Light source altitude 0° ~ 33°: CZA ray path 1-3 works. When light source drops to 0°, CZA reaches minimum altitude of around 57°. 
• Light source altitude -33° ~ 0°: CZA ray path 1-3 no longer works. Instead, the 'flipped' CNA ray path 2-1-3 kicks in and produces a flipped CNA overhead. This flipped CNA doesn't get lower than 57° either. 

Simulation by ZHANG Jiajie, sun altitude ranges from -35° to 35° at 2.5° increment.

When the pillar is treated as the light source in multi-scattering scenarios, it can effectively be viewed as an infinite number of light sources with altitude covering the 33° to -33° range. The 0° to 33° portion creates an infinite number of CZAs, while the -33° to 0° portion creates an infinite number of flipped CNAs. These two light clusters turn out in simulation as two broad, sun-vex, zenith-hugging arcs, both capping sharply at 57° altitude.

Simulation by ZHANG Jiajie, sun altitude 20.8°

These two secondary arcs together get us the sharp-looking novel arc in the Finnish and Siziwang Qi displays. As long as the pillar covers the horizon, the arc's overall appearance hardly changes as light source rises. The intensity of the arc peaks when light source sits low, which makes sense since pillar also peaks under the same condition.

Simulation by ZHANG Jiajie, sun altitude ranges from 0° to 25° at 5° increment.

When light source altitude drops below 15°, the arc's close proximity to the original CZA could severely hinder detection. According to the animation below, altitude 15° to 25° may be the most ideal observing window.

Simulation by ZHANG Jiajie, sun altitude ranges from 0° to 25° at 5° increment.

There's one thing in the actual display that doesn't go well with simulations. The azimuthal extent of the arc is very long in the actual photo, as if it'll go full circle. The simulated arc, however, hardly goes beyond the 46° halo. To make it longer in simulation, very thick triangular plates need to be employed, which doesn't sound very realistic. There're probably other light sources responsible for the arc's azimuthal extension and we'll need your help to figure it out : )

Jia Hao

Intense Kern arc from China

After years of waiting, we finally have the very first Chinese Kern display, and it's a big one.

On the morning of Feb 14 2020, a blanket of natural, high quality diamond dust lingered above Siziwang Qi (Dorbod Banner), Inner Mongolia for about two hours (later reports suggest the display lasted the whole day), treating the locals with a jaw-dropping plate display.

© TIAN Xiangyang, shown with permission

Crystal density and quality were so high that parhelia, circumzenithal arc, parhelic circle, 120° parhelia and even Liljequist parhelia all look insanely bright in photos and videos. Such intensity undoubtedly made multi-scattering possible. 44° parhelia showed up very well in most locations despite the relatively high sun elevation. In the following photo, the sun had risen to 20° and the 44° parhelia were still there.

© YANG Yongqiang, shown with permission

The true highlight of the display, however, lurked near the zenith. The circumzenithtal arc appeared not only bright, but also as a full circle, even to unaided eyes. The Kern arc, finally!

While most observers’ attention were drawn to the low hanging gems near the horizon, some did bother to look up and documented Kern arc’s grand debut in China. These two untouched handphone photos below speak volumes about the arc’s top rate quality.

© ZHENG Dan, shown with permission

© TIAN Xiangyang, shown with permission

The following videos will give you an idea of how crazy the scene was:

https://m.weibo.cn/5471509356/4471997029347233 

https://m.weibo.cn/5893589762/4471826476341170 

https://m.weibo.cn/5299394320/4471803969617073

https://m.weibo.cn/5703217900/4473449294440181

https://m.weibo.cn/6624168505/4472005455908200

Once the initial excitements died down, we began to wonder about the Kern arc’s true origin in this display. The arc appeared rather smooth and uniform all around and somewhat broader than the circumzenithal arc. Could this broad, diffuse appearance be attributed to multi-scattering?

With the help of Zhang Jiajie’s simulation program (https://github.com/LoveDaisy/ice_halo_sim/tree/master/cpp), we found out that multi-scattering is capable of noticeably enhancing the Kern arc for both regular and triangular plate crystals. Also note how the gaps in the regular plate Kern arc get filled and smoothened out by multi-scattering.

Simulation by ZHANG Jiajie, sun elevation at 13°

Simulation by ZHANG Jiajie, sun elevation at 13°

The multi-scattering enhancements above have at least two components:

• A secondary circumzenithal arc created by parhelia, parhelic circle and 120° parhelia

• A secondary parhelic circle created by the original, single-scattered circumzenithal arc

Below is a comparison between the Kern arc and the above two secondary rings. They do appear broad and diffuse as expected.

Simulation by ZHANG Jiajie, sun elevation at 13°, semi-triangular plate crystals with c/a = 0.3 are used

These rings, when integrated, can get brighter than the Kern arc in simulations, especially when crystals are thin. So theoretically it’s possible for them to overwhelm the Kern and become the main player. In reality though, co-existence might be the more reasonable answer.

Back to the display itself, Marko Riikonen commented in our email exchange that this display is almost a clone of the legendary 1970 Saskatoon display (http://www.thehalovault.blogspot.com/2011/01/the-saskatoon-halo-display.html), in which the 44° parhelia were first photographed. According to Marko, visual sightings of the Kern arc were reported by the photographers but veracity of these reports has been much debated until recent years. Now that we have a repeat event with undeniable Kern arc presence, the Saskatoon chapter could probably be closed.

Best regards,

Jia Hao

Complex Halo Display, Borlänge, Sweden

On the 14th December 2018 at 12.30 UTC, Magnus Edbäck photographed an amazingly complex display in his home village of Utendal near Borlänge, Sweden comprising many extremely rare halos with at least one completely new halo form, a multiple scattering halo. Highlights include 46° contact arc, Hastings arc, extremely long Schulthess arcs and the rarely seen in daylight Ounasvaara arc. Like Marko Riikonen's 6th March 2017 Rovaniemi display, this one will go down in the annals of halo history and no doubt will be discussed and analysed for a long time to come. The two faint patches of light on either side of the 22° tangent arc are the new multiple scattering halo which at the time of writing has not been given a name. It is quite possible that the display was caused by snow guns in operation at the Romme Alpin ski centre situated about 12km away from Utendal.

Magnus has very kindly agreed to give an account of how he photographed the display.

"This is the story about my halo picture.

I was having lunch at my parents home when my mother asked me to look out to see how nice the sun was shining. At once I saw that this was not like any halo I had ever seen before. The sight of the sky was amazing. I then went to my home and grabbed the camera. My parents and I live next door to one another in Utendal, a small village outside Borlänge. I quickly checked that I had the appropriate lens, it was Samyang 14mm F2.8 that was on. The camera I used is a Canon 6D which I have modified with a Baader filter, mainly for use when I shoot the starry sky. From what I understand, the filter has no significance when shooting halo.

I went to a place on my parents courtyard where I could see as much of the display as possible. I quickly checked the settings on the camera, adjusted it to ISO200 and set the aperture to F8 to get a good depth of field. The day was quite cold (about 7 degrees below zero) and I was not wearing a jacket so I only took a few quick pictures (4pcs).

I then went back home to look at the pictures. On my way home, I also saw arcs to the north and I'm very sad that I did not take any photos of them. I quickly examined the images in Adobe Lightroom and picked an image that I uploaded to the Swedish astroforum www.astronet.se. I then went back to my parents to finish lunch.

Quite soon afterwards, I received comments on www.astronet.se by both Hans Bengtsson and Timo Karhula, who thought the display appeared to be something out of the ordinary. Hans thought I should send the image to Les Cowley and Timo thought I should publish it on www.taivaanvahti.fi. I received answers fairly quickly from both Les and www.taivaanvahti.fi. At 15:35 UTC 2018-12-14 I received mail from Marko Pekkola where he wrote:
"Congratulations Magnus of finding a new halo form in the sky in solar display! Several experts analysed this photo and Marko Riikonen identified one of the forms as the first multiple scattering halo of its kind.”  - Magnus Edbäck

Image processed by Nicolas Lefaudeux.

Image processed by Nicolas Lefaudeux.

Nicolas Lefaudeux has analysed and stacked the four raw files with background subtraction and produced these breathtakingly complex and beautiful processed images. The new halo is a multiple scattering [MS] halo, ie a "halo of a halo". These are extremely rare and can only appear in the brightest displays with very bright halos to forms.

This new halo is the uta of parhelion / parhelion of uta and it is the 5th MS halo form (after par of par, uta of uta, uta of pc/pc of uta, and cza of pillar). It requires both a very bright parhelion and a very bright uta to form. 

This uta of par/par of uta was the most likely MS halo form expected to be caught, because of its relative ease to appear in simulation and because it is not overlapped with other bright regular halos.

All images copyright Magnus Edbäck