Another night of halos in Rovaniemi – with puzzles

It is more common than not to have something interesting to chew on in diamond dust displays, especially if they are stacked and of the spotlight kind. Such was the case with the displays that I photographed on the night of 24/25 November this year.

The forecast was good and I had come to the field early, poised to leap when the action gets going. It didn't take long. From west came low stratus clouds, and diamond dust appeared inevitably as the tiny ice particles from snow gunning nucleated it.

Yet it wasn't smooth sailing and it took some time before I got to actually photograph halos, first on a small field not far from the hospital. The image above is from this location, a stack of 40 photos, each with 30s exposures.

In the below BR version is seen the first one of the night's two puzzles: there is a weak sub-Kern arc, but no sub-CZA or subparhelia. It would in principle be OK to have sub-Kern without noticeable sub-CZA in this display if the plates were unusually thick, h/d > 0.7, but you still expect at least subparhelia to keep company for sub-Kern. The simulation below demonstrates the issue. 

The sub-Kern appears to be not all alone here, though. It seems there is – just like in the simulation – a faint subparhelic circle through the subanthelic point. And even faint sub-Liljequist parhelia, at least the other one, against the trees in the 8.30 o'clock position in the curve of the sagging wire. These features are made by plate oriented crystals just like the sub-Kern. Could it then be that the subparhelia are there, too, but are dissolved in the background? To my eye the background doesn't seem that restless or lit up to make subparhelia disappear. Neither does it seem that they would be outside the image field either. So it is a bit of mystery here.

 

In this HaloPoint simulation the attempt was to get a very weak sub-Kern (arrows), just like in the photo, to see whether subparhelia would not be made. This didn't pan out, the subparhelia come inevitably. I happened not to save the parameter file. Light source elevation was 7 or 8 degrees from the horizon. Plates were thick h/d 1.0 to not make at all sub-CZA. The blue spot on Tricker arc seems to be visible also in the image. I don't recall seeing it in earlier displays.

 

The relative brightness of sub-Kern / sub-CZA allows knowing how thick the plates are. In past observations the plate thickness inferred this way has capped at around h/d 0.5-0-6. Simulations with HaloPoint. Regular hexagons, 0.2 variation of prism face lengths, light source elevation 7 degrees from the horizon.

The session was finished abruptly by the swarm turning into snowfall. To my surprise the crystals melted on the lens, I would have thought the lens cooled enough by then, the temperature at the railway station official measuring point was -8 C. 

As I left the small field to see where the diamond dust had gone, I was wondering why in the part of the windscreen that was cleared of snow by wipers a quite nice 22 halo was visible under streetlights. I briefly considered taking a photo, but kept on driving. 

It didn't take long to locate the swarm. Streetlights had awesome parhelia, but the action was always in places where I couldn't do neither spotlight nor lunar. (And although it would be awesome to do videos of these 3D halos, I don't have video in my old Canon 40D.) After a good while I finally got a lunar display on a bog called Matkajänkä. The image below is a 10 frame stack of 30s exposures, the photos are rotated to account for the moon's azimuthal movement in the sky, which is why the trees and streetlight pillars around are blurred. The display was better before I was ready to photograph. It was a beautiful thing to look at.

 

Once the lunar display was gone I again struggled to find a place to photograph. After more than two hours I finally ended up on a little light polluted clearing near the Jätkänkynttilä bridge, where a plate display was visible. Only three 30s frames are stacked here, but the display is nevertheless quite smooth in the image because the wind had picked up, sweeping great masses of clumpy diamond dust fast across the beam.

And this gives us the second puzzle of the night: the 120 subparhelia look tilted. No matter how much I try, I can't get myself to say they point towards zenith, which is what would be expected. I don't know what's going on

 

 This half-image is a flip stacked version.

 

The night's final display I got in the Saarenkylä crop field. Just like in the previous location, I was late, and got only a couple of photos before it started fading. Nothing special is seen, the BR reveals blue spots on both Lilje and sub-Lilje. 

Note to self is in place here. After I had taken two photos with zenith centered view, I decided to change to side view. That's no good. Never change horses in midstream. You want to have as many photos in the stack as possible to smooth out the display. Always stick to the view you have chosen for as long as the display lasts. This allows you to do also a time lapse movie which may turn out useful.

 

 

Let's do a little digression for the end. The simulation I made for the display has this extra arc crossing the subanthelic point, fleshed out due to fine dot in the simulation. This is a "rotated Wegener arc" from raypaths 32567 and 32457. And as the filtered simulation I made below shows, it has also its own "rotated 22° tangent arc" which arises from similar raypaths, but without the basal face reflection. These theoretical halos are 120 degrees rotated versions of their ordinary cousins: their common point of contact is 120 degrees from the 22° tangent arc and Wegener as measured along the great circle passing through sun and zenith.

 

 

A closer look reveals there is also another rotated tangent arc and rotated Wegener pair. It comes about by triangular crystals, raypaths 3573 and 32573 respectively. The sharp red edge of these arcs is pointing away from the sun because they are rotated by 240 degrees. All these rotated arcs are shown in the side view simulation below. I have called them as 142°/262° tangent/Wegener arcs according to how much they are rotated from the sun. However, because the 262° rotated arcs are in reality found 98 degrees from the sun on the great circle, that number probably makes for a better name. Greenler, who considered the 3573 raypath to look for possible explanations for Hevel's halo and 90° parhelia in his book, talked of a 98 halo concerning the version of this effect in randomly oriented crystals. Indeed, to call it a 262° halo would sound rather quixotic. 

 

Whatever they will be called in the future, the rotated Wegener arcs seem distinct enough to become reality once a quality swarm in a dark location gets deep stacked. Parry orientation version of these rotated arcs, of which there are much more (including sub versions), may be discovered in monster Parry displays.  

 

As for those dark places, they have become more scarce, unfortunately. Four years has passed since I last chased in Rovaniemi, and now the light pollution is on a whole another level thanks to leds having come to replace the faithful old sodium vapour and metal halide lamps. Also some new roads – with streelights of course – have been built in some places that used to be good.

 

 

I also see now why it is good to preserve degree symbols in halo names. I have recently tended to drop them, but with these three digit names it started feeling like I am looking at raypath sequences.