Full circle 'pillar CZA'
|Photo by Anders. Sun elevation 19.8°|
First of all, it’s not a weaker version of the Jensen arcs. When compared to the actual Jensen arcs which emerged 10 minutes later, this one dips lower to around 57° elevation.
|Left: ‘Pillar CZA’ (arrowed) , sun elevation 19.8°; Right: Jensen arcs, sun elevation 21.5°. Photos by Anders.|
The 57° elevation reminds us of the multi-scattered 'pillar CZA' in the Siziwang Qi display (http://www.thehalovault.org/2020/03/secondary-cza-from-sun-pillar.html). The overall appearance of the arc in Anders’ photo, albeit dimmer, greatly resembles the Siziwang Qi ‘pillar CZA’. Chances of the two being the same halo are quite high.
|Siziwang Qi photo by LI Tingfang. Jutland photo by Anders.|
However, unlike Siziwang Qi where multi-scattering was strong, the absence of other multi-scattered halos (such as secondary UTA and PHC from the bright UTA) in Anders' photos clearly doesn't favor the conventional multi-scattered ‘pillar CZA’ solution for this display.
Nicolas Lefaudeux proposed an interesting theory which sounds highly viable in this case. He suggests that the sunlight scattered inside the cloud layer may have acted as an 'integrated glow', feeding light to each crystal from all azimuthal angles. In other words, the crystals are seeing from their viewpoint an infinite number of 'pillars' around the horizon, and each 'pillar' creates its own 'pillar CZA' via the crystals. The combination of these infinite numbers of 'pillar CZAs' will of course be a full circle with even intensity distribution.
This theory, when extended, may potentially explain the mismatch in the Siziwang Qi case too. In the Siziwang Qi display, available materials suggest the arc likely went full-ring as well (couldn’t be confirmed due to lack of DSLR photos). The sun pillar alone can't create a full-circle multi-scattered CZA without invoking unrealistically thick and triangular plates.
|Simulations with ZHANG Jiajie’s program (https://github.com/LoveDaisy/ice_halo_sim/tree/dev/cpp). Sun elevation 20°.|
Ji Yun from the Chinese halo watching community came up with the idea of facilitating the 360° glow from the snow covered terrain to increase the ‘pillar CZA’ azimuthal extent. His idea is in nature very similar to Nicolas’. If this ‘integrated glow’ theory holds, the name ‘pillar CZA’ should probably be changed to ‘integrated CZA’ instead. But for now, it’s still too early to make that move. More observations are needed to see if the arc always goes full-circle.
Skywatchers world-wide should start keeping an eye out for this elusive feature in future displays. Whenever there’s a bright CZA in diamond dust or high clouds, it’s highly recommended to always photograph the entire zenith area in RAW for in-depth analysis. It may also be worthwhile to go through past images and see if this arc shows up via heavy post processing.
Mismatching Lowitz arcs
None of these scenarios really work. Only simulation 2 gives a match with the photo, and that’s just the outer arc – the inner arc is not right. And even the outer arc is on a rather short side for comparison. More length would have been preferred to see whether the apparent similarity in geometry really holds.
|Photo by Anders. Sun elevation 14.8°. Simulations with HaloPoint 2|
Below is another Anders’ photo, the sun elevation is seven degrees higher. Here no outer arcs are visible but we think the inner arcs are the same as in the earlier stage. This time comparison is made only with two Lowitz orientation scenarios. The circular Lowitz arc seems a better match here, but the arc in the photo doesn’t look quite as curved as in the simulation.
|Photo by Anders. Sun elevation 20.4°. Simulation with HaloPoint 2.|
Possibly these arcs in the Jutland display are like the subparhelic arcs (Schulthess arcs) of scenario 2, that is, arcs characterized by an added basal face reflection to the Lowitz arc raypath. The shape is not exactly what the simulations predict, but this is a known issue: several people have pointed out the disagreement of the observed subparhelic arc geometry with the theory for various displays. One such illuminating display was seen on 7 March 2017 in Rovaniemi, which happens to have solar elevation comparable to the first photo above.
And that’s just the geometry. Another contradiction, a glaring one, is that subparhelic arcs are in simulations accompanied by much stronger Lowitz arcs, yet usually Lowitz arcs are nowhere to be seen. Actually, the subparhelic arc and alternate subparhelic arc simulations above were tweaked for clarity: Lowitz arcs were removed in order to not let subparhelic arcs overwhelm them. Moreover, only the above parhelia parts of the subparhelic arc were plotted.
Until recently the subparhelic arcs have been exclusively seen in diamond dust, in which they are a common feature. The first high cloud case to show them appears to be the display that occurred on 7 February 2020 display in Finland.
2 and 10 o’clock spots
In some photos an outright spot is visible. Just to be sure, we tested with simulations for one of Anders' photos whether this could be a 24 plate arc. After all, the cloud contained pyramid crystals, as betrayed by the odd radius column arcs in some photos.
The position indeed corresponded to the 24 plate arc position, but the orientation isn’t quite right, as shown below. So, considering also the lack of other odd radius plate arcs in the display, these spots are probably better understood as parts of the subparhelic arcs.
Could they be just chalked up to irregularities in the high cloud? The fact that we haven’t seen such brightenings on subparhelic arcs in diamond dust displays – which tend to be more uniform – would seem to give appeal for such an explanation. However, considering that several photos from various people show more or less well defined spots in the Jutland display, it might be the case that there is something about the subparhelic arc crystals themselves that makes the spots.
|The spot (on the right) in a photo by Richard Østerballe|
|The simulation has a 24 plate arc to compare with the spot on the left in Anders' photo. Simulation with HaloPoint2.|
|Another photo from Anders showing the spot. The 24 column arc is visible here, as well as in the photo above.|
Odd radius column arcs
Anders' father, Ole Jensen also photographed the display and he took note of 9 column arcs on the sides of the sun. His photo is shown below. A selection of Anders’ photos, too, have 9 column arcs and additionally a very faint 24 column arc, which is visible in the two photos above.
|9 column arcs. Photo by Ole Jensen. Sun elevation 13°|
Seeing red in the blue spot?
There are some reports of people claiming to have seen red in the blue spot visually. But red has never been caught on photos to substantiate these claims and the theory is not forthcoming either.
Interestingly, some heavily enhanced Anders’ photos would seem to suggest that there is indeed red in the blue spot. However, Nicolas Lefaudeux demonstrated that it is an illusion that disappears when the blue color and its cyan end is covered.
From the optical point of view, the colors in the blue spot are supposed to add to each other starting from the violet. It should be:
violet only = violet
violet + blue = blue
violet + blue + green = cyan
violet + blue + green + yellow = light cyan
violet + blue + green + red = white
So from this, any red should be an illusion (as there is never more red than any other color), and indeed, it seems to be the case when we look at the blinking grey patch on the blue and cyan of the blue spot (shown below).
|Photo by Anders|
|The red on the blue spot more or less vanishes when blue-cyan is covered. Photo by Anders.|
- Co-authored by Marko Riikonen, Jia Hao and Nicolas Lefaudeux