Kern and Hastings arcs make appearance in the UK

Only three days before the low-sun odd radius display ( http://thehalovault.blogspot.com/2020/06/odd-radius-display-at-low-sun-in.html ), I observed a brief but intense display that included well-defined suncave Parry arc as the visual highlight. Shown above are the two 50-frame stacks that I managed to extract out of the display. Both cover five minutes and they are separated by another five-minute interval during which I was too busy to collect data. Solar elevation is 26° in the first (top panels) and 25° in the second (bottom).

Applying the usual background-subtraction on the average stacks (on the left) makes helic, Tape, and Lowitz arcs all stand out reasonably well in addition to the circumzenithal, supralateral, upper tangent, and Parry arcs. As the simulation (HaloPoint 2.0) in the top right-hand-side corner demonstrates, the anthelic arc close to the far left edge is Hastings rather than Wegener. Kern arc only appears in the second stack, coming out the clearest in the middle panel. Here, blue-minus-red colour subtraction is applied on top of the background subtraction. The bottom right corner is the average 50-frame stack without further processing.

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

Kern with subhelic arc in the UK

Berkshire (UK), 28 April 2019: Top left corner shows what's left after applying the gradient subtraction and stretching of histograms on a 38-frame average stack. Top right and bottom left, respectively, are with additional blue-minus-red subtraction and colour-channel enhancement. A HaloPoint simulation is included for reference at the bottom right corner.

While relaxing at home in the evening of a mostly cloudy day, a ray of light caught my eye and I checked the halo situation. In fragmented cirrus there was a mediocre circumzenithal arc (CZA) but not much more than that (my view towards the setting Sun is not that great). I took notice of the Sun being low in the sky so odds for Kern arc were on the rise: in the next moment I was setting up my DSLR to get some photos from my backyard.

CZA disappeared soon after I started shooting the first set of images, but it came back a few minutes later so I chose to do a re-run. Fortunately so, as the latter set turned out fruitful indeed in the post-processing.

The processed stack indicates presence of a faint Kern arc as an extension of CZA at the left. Slightly further to the left, there is a white arc that best matches the subhelic arc in the simulation. I think there is also a subtle suggestion of helic arc alongside of the subhelic, but this is less convincing. Furthermore, there are colored patches below the CZA approximately where 46° contact arcs would appear if there were Lowitz-oriented crystals in the mix. The simulation shown above is a synthesis of three distinct populations, including singly-oriented plates, singly-oriented columns, and Lowitz-oriented plate crystals.

Project Kern Update

Just under a year ago, we launched Project Kern, the aim of the project being to “try to photograph as many Kern arcs as possible [in a twelve month period]…. to better understand their frequency and to ascertain whether they really are the rarest of the rare”. I am very pleased to announce that almost at the end of the allotted twelve month period of the project we have received our first Kern.

On 20th January 2018, Pasi Vormisto observed an excellent halo display whilst driving just outside the town of Nokia close to the city of Tampere in Finland. Upon finding a suitable parking spot, he managed to document the whole of the display and in so doing photographed several rare halo forms including Tape arcs and 44° parhelia. However, of great interest to us is the Kern arc which appeared in a couple of the frames. One of the characteristics of this particular example is how bright it appears to be. It is readily visible without any additional processing and one wonders whether it would have been visible to the naked eye or in a mirror. One might also speculate as to what kind of monster this would have become if a sequence of images had been taken on a tripod for later stacking. However, every credit must been given to Pasi for even remembering to photograph the area around the circumzenithal arc. All too often, observers become transfixed by all the ‘action’ occurring in the immediate vicinity of the sun.

So then, can we draw any tentative conclusions? I think the jury is still out, but on recent evidence it seems as though they are appearing once or twice per year. For example, in 2015, we had Eresmaa’s two UK Kerns and in 2016 we had Riikonen’s daylight Rovaniemi Kern. Of course, we don’t know how many actual Kerns there were that went unobserved/un-photographed. Although Project Kern is now drawing to a close, there is still time to add to our grand total of one. If it has taught us anything, it is the need to encourage as many observers as possible to routinely photograph the area around the zenith and better still to stack their images. In this way we may better understand their frequency and distribution.

All images copyright Pasi Vormisto.

Project Kern

I would like to announce an exciting new initiative aimed at the whole halo community. 

For a long time, the Kern arc has been considered the “Holy Grail” of halos with only a handful of observations having been made and even fewer images captured. This year, I would like to invite all committed halo observers to really concentrate their energies on the area around the circumzenithal arc where the Kern arc appears. The aim is to try to photograph as many Kern arcs as possible this year, to better understand their frequency and to ascertain whether they really are the rarest of the rare. This initiative will go by the name of Project Kern.

I think that many more Kern arcs may potentially be photographed if observers routinely focus on this area of the sky. In the past, many Kern arcs may have been missed by observers concentrating their efforts too much on the activity occurring below the circumzenithal arc. In this endeavour, always try to use a tripod and take a series of images for later stacking. In this way, I hope that you will be able to capture several more of these elusive halos in 2017. I am sure that once more observers start actively hunting the Kern, more examples will follow as a matter of course.

I would very much like to encourage everyone to participate in this project and Halo Vault will be pleased to publish any Kern observation you might have the good fortune to make.

Alec Jones

Two observations of the Kern arc in cirrus

As was mentioned in the comment section of Riikonen's report of the recent case of Kern arc in arctic diamond-dust, I was lucky enough to catch the mythical arc twice in 2015 at my location in Berkshire, UK. In both cases the Kern arc was extremely faint and could only be uncovered after applying the colour subtraction technique on stacked photos, each based on 20-40 DSLR frames and spanning approximately five minutes in time. Remarkably, in both displays the Kern arc appears in two stacks separated by 10-15 minutes. The display of 20th April is shown above, while the collage below represents the display of 16th June. Each collection contains the colour-subtracted versions on the right-hand-side (processing in the bottom right panel for the April case is by Nicholas Lefaudeux). Left-hand-side panels are with enhanced colour saturation. More photos, including a couple of single frames and other stages of the displays, are available for viewing at the Finnish site at http://www.taivaanvahti.fi/observations/show/37300 for the April case and at
http://www.taivaanvahti.fi/observations/show/38958 for the June case.

In overall terms, the two displays are almost identical. Apart from the Kern arc, there is little to suggest particularly strong presence of oriented plate crystals. Of course, parhelia and circumzenith arc (CZA) stand out in the processed stacks, but in single frames they don't appear too extraordinary. Furthermore, part of the CZA intensity obviously comes from Parry-oriented crystals, which are not thought to contribute to the Kern arc. Even without the Kern arc, the displays would be rather extraordinary in my opinion, thanks to the presence of Tape and helic arcs, neither one often seen in cirrus displays. The June display takes this aspect even further by additionally containing the Hastings arc. Other halos present in the displays include the common ones produced by random, column, and Lowitz orientations.

There appears to be some common thinking that the Kern arc benefits from very low solar elevation. Against that background, the solar elevations in question here seem high (16°-19° in the April case, 18°-21° in June). Had the Sun been a few degrees lower in the sky, the distance of the Kern arc from other halos of interest and from the Sun had been too much for me to catch it in the first place, as I was not specifically searching for it and my personal toolkit does not contain an all-sky lens. But after all, I am not so convinced that the solar elevation makes such a big difference, except possibly when it comes to judging the likelihood of Kern from the intensity of CZA. To illustrate the effect, I produced a set of simulations using the Halopoint software and assuming oriented plate crystals with varying aspect ratios and base shapes. The results are combined in the figure below (each panel contains solar elevations 5°, 15°, 20°, and 25°). With regular hexagons (panels on the top) I get no Kern at all. With regular triangles (bottom panels), I get a decent Kern regardless of the solar elevation, unless the crystals are very thin. Intermediate crystals in the middle row make a decent Kern only if they are thick. In none of the cases does the lowest elevation show the most intense Kern - I'd rather say the opposite is true.

Lowitz - the new normal?

In 2017, halo activity in my part of the United Kingdom has so far been very quiet with not much to get excited about apart from the occasional odd radius lunar halo. However, on the 6th February things improved a little with a nice little display which included 22°, both parhelia (sporting parhelic tails), uta, possible Parry, cza, supralateral and all three Lowitz variants. The upper and lower Lowitz were vestigial, but the middle Lowitz illustrated here was quite clearly defined.

This burst of activity got me ruminating about the frequency of certain types of halos and the nature of “rarity”. Compared to many of you 'old hands', I am a comparative newcomer to halo observation. When I first started to observe, many halos were still considered to be extreme rarities or at least exceptionally infrequent. Others, such as the Kern, were so rare that they were considered to be the Holy Grail of halos, so few were their sightings. What I have noticed over the last few years is that many of these rarities are seemingly becoming far more frequent and in my own mind at least, I have downgraded them from ‘rare’ to ‘infrequent’.

Take the Lowitz arc for example. It is only a few short years ago that its very existence was being doubted, controversial to say the least. First, there were a few sightings, then a trickle of photographic images emerged which confirmed it as a reality. Over the past ten years, their frequency has seemingly increased to the point where I will see several per year and look out for them as a matter of course. Whilst not commonplace, they are certainly hovering somewhere in that vague, nebulous no-man’s land between infrequent and relatively frequent. Likewise, the Kern. At first a mythical creature, but now we are seeing the first drips which will eventually become a trickle….

So what has caused this apparent change in frequency, this downgrading of status? I would like to posit two possible causes: the ubiquity of digital imaging and advances in image processing. It is not that these halos are actually becoming more frequent in reality but that the effect of digital imaging and the development of image processing techniques are revealing them to be present in many more displays than were hitherto thought. In the past, one would take a roll of traditional film, but prior to image processing what did one do with them? Answer, nothing in most cases. You either saw the halo on a particular frame or you didn’t. This resulted in some of these rare halos only showing themselves under exceptional conditions. With the advent of inexpensive digital cameras and image processing software we saw far more phenomena being recorded. However, it is only with the introduction of the B-R processing technique pioneered by Nicolas Rossetto and perfected by Nicolas Lefaudeux and to a lesser extent image stacking, do we really notice an apparent increase in all types of rare halos. These techniques alone have been responsible for the massive increase of rare halo frequency and the identification of several new halo species. We have now reached a point where what was new and exciting yesterday, is passé today.

I think this trend is going to continue as more and more observers embrace these new methods of working. We have already seen it occur in the wider field of atmospheric optics where photographic identification of higher order rainbows, especially the third and fourth orders, are now becoming a reality. One might argue where is the hard evidence to prove this is anything but a theory or personal opinion? This evidence is indeed hard to come by, however projects such as Lefaudeux's 'HaloCam' has demonstrated without a shadow of a doubt that certain halos are far more frequent than previously thought. For example, he has very clearly demonstrated that the 46° halo is present almost as often as the 22°, whereas at one time it was considered to be a very infrequent visitor to the skies. So not only will this trend continue, I think it will also accelerate, especially in the field of spotlight displays where new halos are being discovered on a very regular basis. Even in daylight displays, new halos are still being captured from time to time using these techniques.

A few years ago, it seemed like everything that could be seen had been seen. We are now on the cusp of a new era of halo research. Today may indeed be considered passé by some, but tomorrow is definitely going to be tremendously exciting! I want to be a part of that new wave of discovery.

Alec Jones

Diamond dust halos on the night of 12/13 January, part II

More photos from the 12/13 January night. The image above is a view opposite to the spotlight. Seen is blue circle, diffuse, Wegener and subhelic arcs. Below are two more images, the blue-minus-red image shows the “column 351/361”, which is the Kern arc equivalent of 46° supralateral arc. The lamp is about 5 degrees below the camera.

Marko Riikonen