Monday, 28 November 2016

On some more aspects of a display observed in Rovaniemi on the night of 9/10 November, 2016

By Marko Riikonen

In the previous post of this display I discussed two photos taken towards the end of the hunt, just before twilight. Now it is time to look at the photos taken earlier, from midnight onwards at another location. Please mouse over or click the photos to remove the milky veil that the systems adds as default to them.

Of the several stacks that were photographed, I made simulations of two that are shown below. Unlike the morning photos, now only one stricly oriented Parry population was needed to the explain the display’s halos from c-axis horizontally oriented crystals. So here we have a pure case of uppervex Hastings and nothing reminescent of Wegener.

Except that a little detail strikes a dissonant chord. In the lamp side photo above an arc is touching the bottom of the 22° halo whereas the simulation gives an arc that is separated – the lowervex Parry. Thus the lower arc in the photo is suggestive of tangent arc and column orientation. But everything else – the presence of Tape arcs, the bright helic arc, lack of subhelic or 46° lateral arcs – shouts out loud this is pure Parry. So what is going on?

Well, even though spotlight displays have a highly 2D character, there is divergentness involved and halos may extend more towards the lamp than true 2D halos would. Look for example at the upper Tape arcs in the photo. They are deeply embedded into the 46° halo, although in simulation they are separated. In the photo they even seem to extend slightly inside the 46° halo. And in other displays upper tangent arc always pushes through the 22° halo towards the lamp (it may be a different thing with that halo, though, than with Parry). Even in solar displays various arcs can be glued to their circular rings despite simulations telling they should be separated.

So I would not quite yet scrap the pure Parry character of this display because of that little anomaly at the bottom of 22° halo. It is not the first time to be faced with this matter: we were wondering about it last winter in another potential Hastings case.

As for this lamp side view in general, it seemed not possible to get a satisfying match. For example, with all parameters that I tried, a secondary upper Tape arc (if letters are used to denote it, maybe it should be upper Tape arc B, instead of the Tape arc D that I used in an earlier post) was produced outside the primary. As for the Hastings, in the photo it is brightest near the lamp, then has an intensity drop just inside 46° halo before getting again a bit brighter outside. I could not reproduce this variation. I think there is going on something that is beyond the reach of the simulation software. In the simulation shown was used just one Parry population optimized to make a Hastings that is brightest near the lamp and vanishes outwards. The crystals were thin h/d 0.05 plates. Such a solution did not quite produce matching intensity distribution for the helic arc, and I am certainly not proposing this as final solution. The parameter table for the simulation is not given as I happened not to save it. Because in the photo there is Moilanen arc, I added it to the simulation too. Simulation light source elevation is -5 degrees.

Opposite to the lamp the view is very much Parryish, as shown below. In addition to simulation I have provided also a br version of the image because it makes it easier to distinquish the spikes of lamp artefacts from halos. It seems to show that inside the subanthelic arc there is no diffuse arcs, just artefacts. Another image further below also has the hallmarks of pure Parry.

In the sideview image below noteworthy is the very thin core of the sub-120° parhelion. It is as thin as the sun pillar above the Parry arc. In br version this core is better separated from the glow surrounding it. Further below are four other photos from the first part of the night.


Finally, I make a return to the anthelic region photo from second part of the night. Earlier I had shown only br version of the blue spots in Liljequist parhelia, but they come out also in “visible wavelengths”, as demonstrated by the extremely saturated version below. Noteworthy is also the dark area that seems to be confined by the shape of subanthelic arc even though the top of the subanthelic arc is not seen. This was something I raised up in an earlier post that discussed a display photographed by Marko Mikkilä.

Saturday, 26 November 2016

Thin plates in rotating Parry orientation as an explanation for thedisplay on the night of 8/9 November 2016, in Rovaniemi

In an earlier post I told simulation attempts were not successful for this display. Well, I really did not put that much effort into it. Now I have given it a fresh look and managed to get some success.

The problem was the subhelic arc and anthelic arcs that could not be get rid of. In new simulations made with HaloPoint the subhelic arc issue is pretty much resolved and the anthelic arcs also play it low key.

As usual, the simulations do not stand scrutiny of details, but that does not matter regarding main message: that it was necessary to use thin plates in rotating Parry orientation to keep the subhelic arc and anthelic arc stuff in check.

Two simulations are shown above together with the photo. They are identical except that in the other the thin Parry crystals are rotating 15 degrees and in the other 5 degrees. The subhelic arc is actually in there, but it is masked by background noise from the random population. Well, maybe its curve can be detected in the 5 degree simulation, but it is shadowy. It becomes clearer with more burn and finer dot.

The simulation with 5 degree rotation replicates also the diffuse area of light seen above the Wegener arc in the image. It is a spread out helic arc. So maybe we could regard the Wegener rather as an intermediate form between Wegener and Hastings, the “Wegstings”.

If thin plates indeed are the culprit, how can they fall with their basal faces vertical? If we maintain that singular plates can not knife through the air in such orientation, then perhaps there were copious 90 degree crosssed plates in the air, built so that one was glued to the center of the other one’s basal face to provide the right balance for the required orientation. I did not take crystal samples, with one camera it is too much a hassle, particularly as we have again and again come to learn that samples rarely give answers.

Parameters for the other simulation in HaloPoint software. Shown is the thin plate in rotating Parry orientation.
Finally, a technical factor could have influenced the subhelic arc visibility in the photo. I had placed the camera slightly outside the center of the beam sideways to enhance the sides of the display and this may have had a dampening effect on the visiblity of the higher located subhelic arc. The effect is probably not significant, the camera offset was just a little, but in the future this practise must be dropped in order to get best comparability with simulations (of course, as the spikes at the anthelic region attest, my beam is not regular anyway, so to some extent this issue is there aways). Off the record, I don’t really think that this could have made subhelic arc disappear, rather the opposite, because when the camera is right in the center of the beam, you have more masking bright glitter than when the camera is offset and any threshold intensity halos should be then lost easier. But it is good to let this issue have known.

Friday, 25 November 2016

On some aspects of a display observed in Rovaniemi on the night of 9/10November 2016

By Marko Riikonen

This was a good no-hassle night of diamond dust hunt. The swarm was stationary and I didn’t have to pack up every 20 minutes to follow its whims. During the 6 hours of observing it was necessary to move only once. Also, both two locations were quite good concerning the light pollution. Especially the second place, where I wrapped it up in the morning ours, had a really dark segment which I used to light up the anthelic region.

As for the halos, the start of the night at around midnight was not so inspiring. As I arrived to the snow deposit area near the river, a sneak peek in beam revealed a run-of-the-mill plate display and I though it will just get worse because the temperatures were in the bad range, around -15 C. So I decided I might as well give some minutes for the half-moon display that had a smudge of Moilanen arc. In photographs it was transformed into a nice V-shape.

Then I switched on the sacred light, and to my delight, things improved soon. First to be noticed, when standing a little outside the beam, was an odd intensity threshold which materialized into a helic arc. “Hastgener” followed the suit, it was a beautiful colored arc of crystal glitter. And as I turned facing opposite to the lamp, higher up in the sky were glittering the two vertical parts of the subanthelic arc loop. Nothing much suggestive of column stuff was evident, neither visually nor from the camera display, so I was pretty sure the “Hastgener” must be a Hastings arc.

But was it? This post is about the two photos that I took towards the end of the night in the second location (which is another snow deposit site), where I moved after it got crappy at the first location. I have attempted to make a simulations of those two photos to test the issue, as shown below. While working on them, I also realized from the br image something of moderate interest: there are blue spots on both Liljequist and sub-Liljequist parhelia.

                            It was -20° C at the location where these photos were taken.

I got the best match for the display using two Parry populations in the HaloPoint software: one with strictly oriented crystals and other with 4 degree rotation. The contribution of these two populations are dissected in the image below, as well as of the other two populations used in the simulation. The 4 degree rotational population has an orientation that makes halos look like an intermediate between Parry and column. These two populations were necessary to make the subanthelic arc look right. It is actually quite typical that you have use about 4-5 degree rotating Parry crystals to simulate diamond dust displays. Earlier I have analysed one case from 2010 in Tampere.

Because of the need for rotating population, I would play is safe and say we don’t have a pure uppervex Hastings here, even though the display clearly has Parry domination over columns. The photos taken earlier in the night at the first location may have a cleaner Hastings, but I need to take a closer look at them.

Then to the colors of Liljequist parhelia. In the br version above of the image that points opposite to the camera there are visible blue spots of both Liljequist and sub-Liljequist parhelia. The flashing image below shows that the br image brightenings do not overlap with brightest parts of the Liljequist parhelia in visual image, but are a little further out towards the 120 parhelia. This is the location where the blue spots are seen also in the simulation.

The simulation shows also a narrow slice of reddish color further out from the anthelic point, at the very edge of Liljequist parhelia. The red is there because the fainter outer half of Liljequist parhelia away from anthelic point is made by parhelia which is rotated by 120 degrees (raypath 3567). In br the red color would make a dark spot, but it is not seen in the photos.

Everything said in the two paragraphs above applies also to the sub-stuff. The raypaths are the same, except for and added basal reflection.

So this observation of Liljequist parhelia blue spots adds to the growing list of various blue effects. The sub-Lilje blue spot we seem to have photographed already last winter on the night of 5/6 January, but normal Lilje blue spot is a new catch as far as I know. Other blue effects that have been photographed, in addition to the traditional parhelic circle blue spot, are blue circle, blue subsun and subanthelic arc blue spot. The latter has not been talked about, it is visible in my photo of a spotlight display from the night of 7/8 December 2008.

Correction: the subanthelic arc blue spot has been talked about in the comments section of the link above.

Wednesday, 23 November 2016

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.

Sunday, 20 November 2016

A distinct Wegener but other reflection halos from column orientation lacking

By Marko Riikonen

Spotlight displays are great in that almost every time you photograph them, you realize you understand halos less and less. This time the puzzle is: Why Wegener in the image above is so strong in comparison to other reflection halos? No subhelic arc is visible and neither there seems to be diffuse arc – I think the spikes at the subanthelic point are lamp artefacts. Of course I can’t not say that for sure, but around the subanthelic point even weak stuff shows up easily to the eye, so had there been diffuse arcs, I should have noticed it. If we accept this, then, in addition to the Wegener, the only suggestion of column reflection halos is what looks like a short patch of Tricker arc cutting across the sub-Kern arc (see the simulation below for comparison).

As for the posed question, I don’t have an answer. In simulations, using plate like column oriented crystals weakens subhelic and diffuse arcs in relation to Wegener, but they are still well visible, as shown by the simulation in the image below, which was done, I think, with h/d 0.3 plates.

Of the other stuff worth a mention in the image – which is a total of 27 min 20 sec exposure – is the sub-120° parhelion. It is quite common in spotlight displays, but to see it one typically has to run alonside the beam. This time it was clearly visible while standing still. There is also a blue spot. At 9 degree elevation it shares exactly the same location with 120° parhelion, which may be there too.

Another photo taken a bit later and at another location, shows a dark circular void at the zenith (or nadir, if you look from the point of view of halos), as shown below. A much more striking instance of this effect was captured by Jari Luomanen in 2013.

This night of halo hunt started on 8 November at 10 pm and lasted until 5 am. The development of the diamond dust quality was a slow and hopeless decline, which happens quite often when temperatures drop towards -15 °C. That is considered by halo hunters the worst temperature, giving crappy displays. Unfortunately the temperature notes were lost as my halo hunting diary got corrupted and shows now page after page ####s instead of letters. But I think it was between -11 and -14 °C through the night.

Wednesday, 16 November 2016

A possible new halo above Moilanen arc

By Marko Riikonen

A simple diamond dust display that I photographed on the 6th of this month in Rovaniemi, shows above the Moilanen arc another, weaker V-shape. As I uploaded the photo on Taivaanvahti, I was not conscious of the effect, it caught the sharp eye of Panu Lahtinen and Reima Eresmaa who commented on it. Then some photo processing made it stand out more clearly. The version above was worked by Nicolas Lefaudeux. It is a stack of 13 images taken during 125 seconds.

If the effect is indeed real, it can not be accounted for by any known halos. At 10:26-10:27 am when the photos were taken, sun was too high (5.6-5.7 degrees) for reflected Parry to explain it and there is no normal Parry arc or tangent arc in any case.

Soon after I posted the photo on Taivaanvahti, it turned out this has been recognized already years earlier. Marko Mikkilä and Jari Luomanen photographed displays with a brightening above the Moilanen arc, respectively in 2007 and 2014. Both had informed other enthusiasts about the effect in their images, but these failed to create excitement at the time. Now, with the latest display’s somewhat better defined V-shaped arc, these older observations certainly deserve a new look.

One more case of interest is that by Timo Martola in the township of Janakkala last winter. His display contains a V-shaped arc above Moilanen arc, however, because the sun elevation is just right for reflected Parry (4 degrees) and there is also normal Parry and tangent arc, everyone has been content with reflected Parry explanation. Probably that’s what it really is, the display looks much like an earlier one that has a definitive reflected Parry. But with the new observation, we can now entertain the possibility of another explanation for the arc in Martola’s images.

A search of all Moilanen arc photos would be in place to see if more candidates turn up.

The original version from which Lahtinen and Eresmaa noticed the effect.

Tuesday, 8 November 2016

A plate spotlight display on 5th November 2016

By Marko Riikonen

Showcasing the last winter’s spotlight displays is still under way, but fresh produce is already coming in. Here is the new crop that I harvested on the evening of 5th November in Rovaniemi. In the image above the lamp is around -6 degrees below the horizon and both parhelic and subparhelic circle are visible. Slight intensity enhancements in them on the side of the sky opposite to the lamp are suggestive of Liljequist parhelia. Included are also Sub-Kern and sub-120° parhelion. I did not spot sub-Kern this time, but the latter was quite discernible when running alongside the beam. As usual, it was a pale pillar of light in which no individual crystals were detectable – very different from the intense subparhelic circle patch towards the subanthelic point, which is always made of pure glitter.

The display disappeared immediately when it got cloudy and there was nothing to be had for the rest of the night. What is not visible in any of the photos, but what was there many times during the display was the subanthelic diffraction pillar. Discovered by Marko Mikkilä in 2012, it is a quite basic feature in plate displays, but tends to come and go, never lasting long. So it is not necessarily captured by the camera unless you take it as your target.

The temperature was around -10 and -11 °C at the location. It is in a typical range of good plate stuff, not yet too cold.

Monday, 7 November 2016

Diamond dust season opened in Finland

By Marko Riikonen

On the fifth of November the diamond dust season opened in earnest, when Esa Palmi photographed a major display at the Kittilä airport. While the sun side is always the attention gatherer, the main attraction of the display is really on the opposite part of the sky, where a strong subanthelic arc dominates the scene.

Of its loop visible are the segments on the sides, and we are used of having seen this kind of subanthelic arc more commonly in spotlight displays. Simulations tell such intensity distribution results from triangular crystals. Triangulars are regularly needed to model snow gun displays and here the source was the Levi ski resort some 10 km to the north. The triangle interpretation is also in accordance with the missing of uppercave Parry arc: the crystals are falling with their wedge pointed straight up, so only uppervex Parry can form.

Palmi is familiar with halos and did a good job of documenting the display. If something more could have been done, that would have been covering also the area around zenith, where a faint and rare halo called the Ounasvaara arc might have lurked. It forms in triangular Parry crystals and the few known observations are from spotlight displays. Granted, solar displays don’t have as good contrast as spotlight displays, and so if the halo was there, it might have been impossible to see with the eye. But camera would have still recorded it. Actually, no one has seen Ounasvaara arc in spotlight displays either, all finds have been made from photos.