Crystal samples should be englightening, but all too often they just make you confused. The observation I made on 22 November 2015 in Rovaniemi is a case on point, although an observation recently published by Alexander Haußmann may now provide a solution.
ice crystals
All rare halos are missing in this spotlight display, but why?
The mystery deepens. In two previous posts we wondered why some displays are great in their column orientation halos even though the crystals have well caved ends. Here we show a case that appeared on November 22, 2015 in Rovaniemi, where crystals seem not much different, yet rare halos requiring basal faces are completely absent. Even the 46° supralateral arc gives just a whiff. Poor crystal orientations can’t explain the absence of rare halos as the tanget arc is quite sharp. Had we known only about this display, we would be quite happy to explain with cavities, but knowing about the other displays, it is quite puzzling.
Marko Mikkilä, Jarmo Moilanen, Marko Riikonen
Subhelic arc crossing at subsun (and some other stuff)
The four caleidoscopic arcs carry in their name the location on the celestial sphere where their loops’ cross. For three of these halos – helic arc, Tricker anthelic arc and subanthelic arc – there exists photos showing the crossing.
Solar diamond dust display with 87° arc
A stack of 40 photos. An average stack has been combined with maximum stack to show the crystal glitter of the 87° arc. The photos were taken during ~2 minutes. Sun movement has not been accounted for.
The diamond dust season is soon to arrive in Finland and it is time to wipe the dust off the equipment. In a meanwhile, here is the last winter’s starter for Rovaniemi, on October 30. The temperature during the display was -5° C, a quaranteed number for great stuff.
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
One display – three mysteries
It is difficult to get a matching simulation of pretty much any spotlight display. Some details tend to be always wrong. But one can usually obtain what could be called an acceptable approximation of the real thing.
The shown display is a true rebel in this respect, for it comes with three anomalies too blatant to be swept under the rug. First, the subparhelia were brighter than parhelia (this we noticed also visually). Second, of the Schulthess arcs (the arcs from Lowitz orientation) only the concave component was visible. And third, there is no subparhelic circle opposite to the lamp.
We can not simulate any of these anomalies. The solitary presence of the Schulthess arc concave component is not a new thing, there exists a handful of such displays. The missing of subparhelic circle opposite to the lamp in this level of display is something unheard of, as is the inverted relative brightness of parhelia and subparhelia. In the simulation above (light source elevation -5 degrees) only plate oriented crystals were used. Below is a sample of the simulation crystal shape variation, the “mother shape” shown in the upper corner.
The display had also a weak segment of parhelic circle between subparhelia. The crystal shape shown above struck the right balance between the sub-Kern and the parhelic circle segment inside subparhelia.
The night was 18/19 January, the location the Sieriaapa bog in Rovaniemi. The temperature was – 29° C.
Jarmo Moilanen / Marko Riikonen
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- Minimum stack of 5 images.
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- Single image of 30 s exposure. There was light snow falling through the diamond dust, drawing streaks in the photo
Odd radius display and its crystals
This odd radius display appeared on the night of 17/18 January in Rovaniemi at -29° C. Visible is the usual duo of 9° and 35° halos, and also what seems like 18° halo.
Crystals were collected. It is hard to make sense of most of the crystals. Many seem to have pyramid faces, but obvious pyramids were very few in the sample.
Jarmo Moilanen / Marko Riikonen
Blue subsun
On the night of 6/7 January we gave a try also on another halo effect that simulations predict: the blue subsun. It is an effect in the blue spot and blue circle family and it colors the core of the subsun blue at 58.5 degree light source elevation.
We though we failed to photograph the effect and then forgot about it and were surprised when it turned out in other photos where we were actually aiming at subparhelic circle.
The lamp-camera configuration did not correspond with that optimal elevation of 58.5 degrees, but the blue subsun effect is nevertheless seen in outer areas of subsun in the two images we got. In the image above it is visible on the upper edge of the subsun and a matching simulation was found at 60.5 degree light source elevation. The grayscale br image highlights the blue of the subsun particularly well (bright white).
In another photo (below) the lamp was, according to simulation, at 57 degree elevation and the blue color is on the other side of the subsun. Shown are also crystals collected during the display and uncropped images.
Jarmo Moilanen / Marko Mikkilä / Marko Riikonen / Nicolas Lefaudeux
Pyramid crystals and odd radius halos, part II
More halo and crystal photos from the 20/21 January night. While this second crystal sampling and photographing session was under way, the display in the beam was photographed constantly, but again odd radius halos are not visible in the photos, expect perhaps for a suggestion of 35° halo at one stage (not shown). Yet again the sample was littered with classical pyramids as shown by the images in the gallery.
The picture above is an appearance of odd radius halos after the crystal business was over with. It is a “blue minus red” (br) of stack with total of 9m30s exposure and contains 9°, 18° and 35° halos. Three more halo images are in the gallery, first a single frame from the stack above, then two that are versions of another stack, the br displaying full 46° halo.
Jarmo Moilanen / Marko Riikonen
Pyramid crystals and odd radius halos, part I
The night of January 20/21 in Rovaniemi was a night of odd radius halos. Here are shown mostly crystals collected during a half hour period. At the same time photos were taken continuously in the spotlight beam, below is one of those stacks. The sample contained plenty of traditional type pyramids – something we have not yet seen this winter. So it is curious that odd radius halos are not conspicuous in the beam photos and at times even completely absent. Maybe the abundant cavities in the crystals caused too much disturbance.
The stack is from a stage where the odd radius stuff was at its best, taken towards the end of the crystal collecting period. Visible are 9°, 18° and 35° halos. The temperature during the night was around -33° C.
Jarmo Moilanen / Marko Riikonen
