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

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.

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

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

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.

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