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.

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

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.

A pure breed uppervex Hastings

In snow gun diamond dust displays Parry orientation is often strongly emphasized in relation to column orientation. There may be no signs of column orientation at all, except for perhaps a slight tanget arc brightening on top of 22° halo.

Such displays have made observers to ask themselves whether the uppervex Hastings arc – the Hastings arc component that is touching the uppervex Parry arc – could be sometimes be obseved in addition to the usual Wegener, or even without it. So far displays where light source elevation allows separation of uppervex Hastings and Wegener have not resulted in any candidates.

However, in spotlight displays where lamp is at or below the horizon, we have managed to photograph during the last and this winter a several of cases that are suggestive of an uppervex Hastings. And not even suggestive, but outright assertive.

The uppervex Hastings intensifies with lowering light source elevation, only complication is that it then starts to overlap with Wegener and identification must be done on the basis of other halos in the display. If Parry orientation halos are prominent and column orientation halos such as 46° lateral arcs, diffuse arcs and subhelic and Tricker arc are very weak or absent, then we might say that the “Hastgener” in the display is indeed Hastings arc.

As an example of a display that leaves little doubt of its Hastings nature, here is shown a one that Marko Mikkilä observed this year, on the 2nd January in Sievi. The lamp was resting on the ground on a rather level field and is according to Mikkilä about 3 degrees below the camera.

We see no evidence of column orientation – no 46° lateral arcs, no Tricker, no subhelic arc. Instead, in the image there is an overwhelming helic arc and Tape arcs, which both are solely Parry orientation born. Thus the faint “Hastgener” must be an uppervex Hastings arc, rather than Wegener.

Above is a attempt at simulating with HaloPoint. It supports the Parry scenario untarnished by columns, even though the details may not be quite right. Perhaps the most striking mismatch is with the subanthelic arc, which is bright in simulation but seems to be missing from the photo. However, there is a darker area which looks as if shaped by subanthelic arc. This seems to be a real effect, we have seen similar looking dark voids inside anthelic arcs before and in an upcoming post I will be showing a good example of such darkness associated with this halo. Perhaps the more intense parts of subanthelic arc are outside left outside of Mikkilä’s photo, although in that case it should have been possible to make a matching simulation. I actually did find an option which was better in that respect, but other parts got too wrong to take it seriously.

An earlier display in the same location by Mikkilä seems to also contain a pure uppervex Hastings.

Diamond dust halos in spotlight beam in the evening of December 2, 2015

By Marko Riikonen and Olli Sälevä

Here are shown the rest of the photos from the night that yielded the second capture of the anomalous Wegener/Hastings. From the golf course parking lot, where we took those photos, we walked into the golf course, and were able to place the lamp even lower down.

The display was no more as good, but in the photo above and two below there is nevertheless again visible a short patch of Wegener/Hastings on top of the 22° halo. Whether this one has an anomalous curvature, is hard to say. Judging from the 46° stuff that is seen against the forest, the crystal orientation would rather be column than Parry, because it has the looks of a 46° infralateral arc, not Tape arc, which would be more spotlike.

The swarm shifted and we followed it to a field over the river. There we continued taking photos, as shown below, but that was cut short when the spotlight started flashing at 10 pm. The 60Ah car battery had run out of power and it was game over for the night.

It was painful because the conditions seemed to continue at least for the next five hours and probably even amping up as the temperature rose from -11 to -5 °C. Under clear skies such a warming would have killed the diamond dust, but now it was overcast and the clouds kept hanging low, meaning they were likely nucleated to ice by the snow guns.

From this on the winter’s halo hunt continued with two batteries, but on one cold night in January even that was not enough. That time, however, a helping hand was extended by the Ounasvaara ski center folks, who borrowed one of their batteries.

An occurrence of anomalous Hastings/Wegener

By Jarmo Moilanen, Marko Mikkilä and Marko Riikonen

The image above shows what looks like a patch of Wegener or Hastings on top of the 22° halo. But instead of having the usual horizontal curvature, it is bending slightly downwards. Because of the view angle, though, the effect is not as evident as it could be. Anyway, if it were standard Hastings or Wegener, it would curve steeper up in the photo.

We have no idea how it formed, our attempts at simulating have come up empty-handed. The display was seen in Rovaniemi on 23 November, 2015, and the arc appeared at a stage when the display was still progressing to reach its peak.

Nine days later, in the beginning of December, we got another, better sighting, suggesting it is not exceedingly rare. In a similar manner, it did not occur when the display was at its best, but when the display was undergoing a momentary low. We will post about this later.

Wegener or Hastings?

This spotlight display that was photographed in Rovaniemi on the night of 5/6 January rises the question of whether it is Wegener or Hastings arc or both together that is seen here. From the outset, as there is helic arc but no subhelic arc one could argue that it is rather the uppervex Hastings arc.

When comparing with simulations (below) it does not look that obvious anymore. The curvature and extent of the arc looks more akin to Wegener than Hastings (the side view image). And it is possible to simulate Wegener without subhelic arc by using thin plates in column orientation. Indeed, to have best agreement with the images, in all simulations thin plates were used both in Parry and column orientation.

In the image on the left the crystal h/d is 0.1 for all simulations. In the one on the right it is h/d 0.03 for rotations of 0.5 and 5 degrees, for full rotation simulation h/d 0.1 was used. Crystals are triangular, oriented in Parry orientation so that the tip is pointing up.

Well, none of these simulations come out satisfying. The light source elevation for simulations is -5 degrees.

Marko Riikonen / Nicolas Lefaudeux

– added one more image with simulation containing both Wegener and Hastings

The Hastings display of Max Emerson

Max Emerson kindly informed that photos of the display he had seen in Swizerland are indeed available and that there is also short video.

Somewhat elusive in the video, the Hastings arc shows up better in this unsharp masked screenshot. Simulation (using HaloSim) by Patrik Trncak indicates many other rare halos as well. And of course, the display itself is outstanding in its brightness and clarity.

Twenty years from first observation of Hastings arcs

Hastings arc is rare halo from Parry oriented crystals. It was predicted by C.S Hastings in 1920, but it wasn't until 1986 that it was first time observed, by Walter Tape at South Pole. 

The photo on the left is the latest documentation of Hastings arc. This moon display was seen by Marko Mikkilä at Sievi last December. Like so many of the good diamond dust displays in Finland, it was also made by snow machines. 

The image is stacked from 6 individual unsharp masked images. White arrow marks the Hastings arc, red heliac arc. The display resists succesful simulating. The problem is the oddly short Parry arc. Mikkilä's simulations indicate towards plate shaped Parry crystals - a crystal falling mode that has been thought of as unrealistic.

Hastings arc looks much like the Wegener arc, except that it touches Parry arc instead of 22° upper tangent arc. Simulations series from Parry oriented crystals are here.

A list of known observations:

21. Jan 1986, Walter Tape, South Pole (in Atmospheric halos)

11. Jan 1999, Marko Pekkola, J. Moilanen and M. Riikonen, South Pole.

Dec 2004, Max Emerson, Swizerland (photos and video) 

17. Dec 2005, Marko Mikkilä, Sievi, Finland.