Friday, 12 June 2020

Surface halos from uniformly oriented crystals


I was searching for surface halos on ice plates for two years without luck because of unfavorable weather in my area (Romania and Hungary). On 18 January 2020 I finally observed my first subparhelia and 120 degree subparhelia complete with the colored nadir spot on a small icy patch near a lake in Romania. In February I continued my search for these kind of halos in northeastern Hungary. Clear weather after a rainy period with minimum temperatures between -5 and -10 degrees ÂșC promised me a good opportunity, so I went out to a nearby field with plenty of frozen puddles, many of which had air below the ice. Since most of them formed in shady hollows I broke off pieces of ice and placed them in the sunlight to produce the halos. To my surprise these halos appeared more vividly when I held the pieces upside-down (relative to their original position on the ground) indicating more and/or better quality ice prisms on the underside, so I used them in this way. On the plates there were large patches of uniformly oriented prisms, this feature made itself noticable as a break in the observed halos. I tried to take pictures of the crystals, but it was difficult because of their very small size (to the naked eye the surface seemed completely smooth). The only usable image I could get is shown above depicting crystals pointing away from the plate surface in the same orientation rather than being parallel with it.
Below I present the most interesting halo elements with screenshots grabbed from my videos made on 8 and 13 February.  Some of them were previously observed (in Hungary and Finland), others are new. The surface is defocused because I held the camera lens only a few centimeters from the ice plates. Since pictures are not showing the nature of these phenomena well, I recommend watching  my videos  available on Youtube (links can be found at the end of the post).
1. Subparhelia and other spotlike features on the pieces: bright subparhelia always accompanied the subsun on the ice plates. Sometimes a ,,duplication’’of the subparhelia appeared when I held the plate in an almost horizontal position and watched it in a flat angle. When holding the plates in the opposite direction of the Sun, white and colored spots could be observed.kukk
From left to right: colored spot (looking away from the Sun), white spot (probably related to the white arcs), a ,,duplication’’ of the subparhelion on its right side
2. Spots made by the underside crystals of the ice sheet left in its original place: one of the puddles had crystals only on the underside of its ice. The situation was the same on both days, despite of the melting and re-freezing of the ice in the meantime. From here I could not break off pieces for further investigation because there was water only a few centimetres below the ice that instantly destroyed the crystals if I tried to do anything.
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From left to right: orange spot (looking in the direction of the Sun), orange spot (looking away from the Sun), colored spots (the Sun is at the left), white spot in the place of the 120 deg. subparhelion.


3.  Arcs crossing at the subsun with a white spot on them: these arcs change their configuration as one rotates the ice plate. Sometimes a faint subparhelic circle is also visible. The white spot is usually located at the intersection of the subparhelic circle and a white arc, but with changing the angle of the plate it can appear elsewhere.

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White arcs with a spot visible on the right one at the intersection (top) and above the intersection (bottom) with the subparhelic circle.
4. Faint parhelia-like spots near the bright spots of the white arcs, in the direction opposite to the Sun: if one follows the white arcs pointing away from the Sun, the white spots can be found there as well, accompanied by two faint spots on both sides which look as if they were parhelia of the white spot. Also there is a subtle brightening on the arcs above the white spot. Clipboard01d
Parts of the white arcs with the bright spots pointing away from the Sun, the arrows mark the faint ,,parhelia’’ on the sides.
5. The colored nadir spot: sometimes the nadir spot was present, situated on one of the white arcs.  For this feature one must look at the plate from above in a steep angle.  
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The nadir spot is visible on the ,,vertical’’ arc
6. Looking through the ice plate: when looking at the Sun through an ice plate (holding it perpendicular to the Sun-observer axis), a regular six pointed star became visible ending in parhelia-like spots 25 degrees from the Sun (measured by Marko Riikonen using a starfield photo as reference). As I changed the angle and position of the plate, white and colored spots moved away from these parhelia. 
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The six pointed star with parhelia on the ends

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Changing the angle of the plate and looking through it to the side and below the Sun gave another set of white and colored spots
7. White star on the opposite side: holding the plate away from the Sun another six pointed star appeared with the reflection of the Sun at its center. In my opinion the arcs that make up this star are strongly related to the white arcs discussed at (3.). 
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The opposite-side star with the Sun’s reflection at its center
Links to my videos (montages created using the best parts of the original material):

2 comments:

  1. This is so cool! I'm gonna have to try this

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  2. Wow it's crazy what az-locked crystals can do! Look at those stars and curves. Amazing stuff!

    ReplyDelete