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.

A distinct Wegener but other reflection halos from column orientation lacking

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.

Unsharp masked and br versions of the photo and simulations with column and Parry scenarios. Simulation light elevation -9 degrees.

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.

A plate spotlight display on 5th November 2016

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.

The hiding sub-120° parhelion

Nothing out of the ordinary here. Just a plate display and its crystals. Visible are the usual folks: subcza, sub-Kern, sub-120° parhelion. The behaviour of the last one in spotlight displays is a little curious, though: while it comes out well in photos, visually it is cryptic. One has to run along the beam to see that ghostly spot of sub-120°. It is not made of big glitter like the sub-Liljequist parhelia – it does not seem to be made of much glitter at all, just faint diffuse spot of light.

Rovaniemi, in the morning hours of 19th January.

Jarmo Moilanen / Marko Riikonen

Catching a divergent light halo effect predicted by simulations

Sometimes it is possible to make a deliberate attempt to photograph something predicted by simulations. On the night of 6/7 January we made such an attempt on the diffuse spots of light that in simulations are seen next to the divergent light subparhelion.

The effect is formed by a mixture of subparhelic circle raypaths, including 3157 raypath and sub-120° parhelion raypath. Its exact shape and position depends significantly on the crystal shape, like for the Liljequist parhelia.

To obtain an omnidirectional secondary light source that was bright enough we pointed the lamp directly to the snow surface. We took photos, looked at them more closely the next day, and there it was – those smudges of light predicted by simulations.

The photo above is actually from a slightly better case on the night of 18/19 January. Next to it is a simulation. Below is the one on the 6/7th.

Nicolas Lefaudeux / Marko Riikonen / Jarmo Moilanen / Marko Mikkilä