Thursday, 18 October 2018

New case of exotic 19d plate arcs

When I first noticed the display at 08-10 local time on 31 May 2018, it was already well-developed. The high cloud layer was very thin and it was visible only in the area close to the sun. At first the display consisted of an upper 23d plate arc and an upper quarter of a 23d halo. A little later I noticed arcs as light spots in the side area of the sun, that reminded me lower 24d plate arcs.

When I processed images, I noticed that these arcs are more like 18d plate arcs than lower 24d plate arcs. But during the observation I distinctly saw that the arcs were located at an elevation lower than the sun, while 18d plate arcs are always located at the same elevation as the sun. I applied stronger processing and revealed a gap between the arcs and a 18d halo. As a result, it became clear that the arcs are exotic 19d plate arcs that were first observed during the legendary Lascar display. In addition, a trace of exotic 28d halo was also revealed.

Sun elevation is about 36 degrees

Some analysis

The halos, known as Lascar halos, are caused by exotic pyramidal crystals with pyramidal faces of (2 0 2 3) Miller index. These exotic pyramids have a 39.1 apex angle while pyramids from regular pyramidal crystals have a 56.1 angle. To simulate the display, I used four different crystal populations. Not one of them have basal crystal faces. The first population is plate oriented pyramidal crystals with upper exotic and lower regular pyramidal faces. This population makes most visible features of the display (19d and 23d plate arcs). The second population consists of crystals with regular upper and lower pyramids, and it contributes to 18d and 23d halos. The population is poorly oriented, in order to  reproduce some features of 18d and 23d halos. The third population contains plate oriented regular pyramidal crystals consisting only of lower pyramidal faces. It needs only to enhance the upper 23d plate arc. Finally, the fourth population is added to reproduce the 28d halo. Its crystals is randomly oriented and consists of upper exotic pyramidal faces in triangular habit. That is, the crystals are almost regular tetrahedrons.

My attempt to simulate the display.
Software: HaloPoint 2.0 by Jukka Ruoskanen
The result shows quite good agreement with the observation, except for an exotic lower 3d plate arc. There are two possible reasons for it. The first is that a glow around the sun has much more intensity than halos presented here. It does not allow to reveal a 3d arc, unlike the Lascar display, whose observing place was located at an altitude more than of 4000 m above sea level. At this altitude the atmosphere has a low level of aerosols, and therefore the glow around the sun is very small, and the sky background is dark. My observation point was in Pskov Oblast, which has a flat topography with usual atmospheric conditions. The second reason is that exotic crystals may have triangular, but not hexagonal habit. The 3d arc disappears when triangular exotic crystals are applied.


 - Nicolas A. Lefaudeux, "Crystals of hexagonal ice with (2 0 2 3) Miller index faces explain exotic arcs in the Lascar halo display"
- Nicolas A. Lefaudeux (personal communication, 2018)


  1. This is crazy all these exotic new halos keep popping up.

  2. Thanks Nikita for another great post. Along with Jia Hao and our Chinese colleagues we are slowly building up a picture of the conditions necessary for the 28.

    One of the things that interested me in your post was mention of the theoretical 3d arc. This made me think how we could maximise our chances of capturing both the 3d and 5d arcs. Your point about altitude/aerosols is well made. However, I was wondering whether a dedicated Small Radius Halocam would be of any use? The glare from the sun is the obvious problem here; how do we block that sufficiently and highlight the faint small radius halos in close proximity? Would a heavily modified camera with internal filters removed (to maintain resolution) and replaced with astronomical narrowband filters help in this regard?

  3. This point was made by Nicolas. Initially, I thought that it is possible to reveal a 3d arc from my stack because the area of the arc was not covered by blocker and was not overexoposed (saturation of pixels in that area is only 16%).

    However brightness of the area a lot more than brightness of halos. In other words, light that is scattered by aerosols (glow), prevails over light that is refracted by crystals (3 arc). So if we could to block the scattered light with some kind of special filter, I think it is possible to catch the 3d arc. If I am mistaken, I hope Nicolas will correct me. I do not know, there are some kind of astronomical narrowband filters that are able to do it.

    1. I'm not sure whether there is a specific filter that would reduce the glare in this scenario. I have a feeling that the glare could only be controlled by a combination of a neutral density filter and/or exposure. The only downside to this approach would be that it would probably vastly underexpose the area in the vicinity of the 3d and 5d arcs. The other option would be to use a narrowband filter. If a H-alpha filter was used, I'm not sure whether any halos would be recorded. I think a viable alternative would be to try to match a specific filter to the particular wavelength of one component of a halo, say the red.

      I think this is an extremely specialised area of halo work. As I said earlier, a dedicated Small Radius Halocam is probably called for. A very accurate sun blocker (just covering the disk of the sun) and an equatorial mount might also be useful.

  4. I wonder how many more exotic halos are going to be found?

    1. An excellent question, Michael! I suppose we have to distinguish between those halos that have been predicted theoretically and then subsequently confirmed observationally or photographically, and those which have not been predicted and come as a complete and utter surprise. Considering the number of surprises we've had in the last few years, I would hazard a guess that there are more halos yet to be discovered. A number of years ago, Marko produced a list of all the then known halos. Off the top of my head, I think he listed about 90 distinct species. I'd be very interested to learn how many new ones have been added to the list since it was first produced.