Monday, 30 January 2017

Faint Kern arc, long Schulthess arcs and possible 46° contact arcs

On 12. November routine plate stuff prevailed over large areas around Rovaniemi. As I drove around, in one location my eye caught an uppervex Parry arc so I stopped to photograph. Visible was also circumzenith arc and even though it was far from dazzling, I aimed the lens at zenith with Kern in mind. After all, you have to account for the sun elevation upon judging how good the cza really is. Those bright at higher sun may lose much of their lustre at low sun. And the lower the sun, the better for Kern. In this case the sun was low: elevation at 1315 local time was 4.6 degrees.

As shown above, the attempt was not in vain: there really seems to be a full circle around zenith. True, the Kern is extremely faint and there are banded artefacts in the image, so one must be careful here. But the regular circle seems different from these artefacts.

The image above shows three versions of a stack of 21 photos taken during 1m 59s (sun has not been tracked in any part of the process). One is blue-minus-red image and two others are made with background removal technique with two different gaussian blur and median noise values. Nicolas Lefaudeux had instructed me on this method. It is also described on his site. The fourth image is a simulation made with HaloPoint.

Because of the Kern’s visibility display rather close to the noise threshold, it would probably be best to refrain from further analysis. But because I made the simulation anyway, let’s say that the Kern’s relatively uniform intensity seems to point at crystals towards triangular shape and I had to use quite extreme triangularity in simulation as depicted by the crystal figure below.

The display has also exceptionally long Schulthess arcs – they extend all the way to 46° halo. The Lowitz oriented population used to simulate them gives also 46° contact arcs. In the actual display 46° contact arcs may be visible too, but it is hard to say because the artefact bands are oriented in similar direction.

For reference, below is also a single image straight from camera. Enhancing it (not shown) seemed also give out Kern arc, but it was even closer to noise threshold than the stack. In any case, the diamond dust was so uniform stacking did not really bring that much benefit.

Wednesday, 25 January 2017

The Salzburg halo

On the 19th October 2013 Zbyněk Černoch was in Salzburg keeping an eye for halo events, since high cirrostratus clouds were rolling in. As they drifted towards the Sun, he first noticed bright parhelia. A few moments later, rather bright and colourful upper tangent arc with Parry arc and upper Lowitz appeared.

What stunned him later was a vivid arc similar to a parhelion, but located in the 18° region.

The arc lasted only a few minutes as the clouds were drifting away quickly. He haven't seen anything like this before.

After a few discussions the observation fell into oblivion. Until few days later when we thought we would like to figure it out once and for all.
It would seem that the arc itself is a fragment of 22° halo, but the halo itself was quite sharp and narrow in this observation.

After Zbyněk put together 3 photos to show that the 22° halo lies between the mystery arc and 22° parhelion, I decided to run some simulations.

I used pyramidal crystals with std of 15° in the c-axis and joined the simulation picture with the collage from Zbyněk.

My theory is that the mystery arc is actually an 18° plate arc. But there are some inconsistencies in the theory.

  1. The color vividness indicates that oriented crystals formed the arc. However 18° plate arc formed by oriented crystals has a typical convex shape which is absent here. 
  2. By increasing the std in the c-axis, halos from poorly oriented crystals are becoming more apparent, which is not the case here. 
  3. No other odd radius halos were observed. 
If it really is a case of an 18° plate arc, it is a really decent one.

Thursday, 19 January 2017

High-Cloud Tricker from Hungary

January 16 brought a whole-day high-cloud display to the greatest part of Hungary. The morning started with a spectacular sunpillar with bright upper-tangent arc, and the phenomenon became more and more complex during the day. Perhaps the most interesting feature of the display was caught by Ernő Berkó. He saw a Tricker-arc in Ludányhalászi, Hungary.

As he explained, he started reading emails from other Hungarian observers about the beautiful sunpillar of the morning, but in his region, there was thick fog with a night temperature of -17 °C. At 12:10 UT the fog disappeared, and he discovered the upper-tangent arc and the then symmetrical (about 60 degree-long) supralateral arc above it. Both were slightly coloured. While he was making preparations to take photos, the clouds thickened and covered the view. Later the conditions turned better again, the right side of the upper tangent arc strengthened, and he could take photos from 13:15 for 25 more minutes with the supra reaching lower and lower. He could see that it got brighter at the place where the parhelic circle crossed the upper tangent arc. It was only later that he realised the infralateral arc was also a bit visible and this caused the brightening. Ernő Berkó was trying to discover other halo forms but until 13:20 there were no other arcs, not even a 22 degree halo. It was only after this that the 22° halo started brightening here and there; the clouds were not uniform at all. A few minutes later he caught sight of the halos at the anthelic point. What he saw was mostly of the time a white oval brightening only, but the Tricker-arc also appeared, and sometimes a faint X-shape was also visible, suggesting Greenler arcs. To the naked eyes. the parhelic circle was invisible at this region of the sky. Later on the phenomenon weakened as the clouds thickened again. Ernő said he felt lucky that some days earlier he had shovelled a path in the snow to the back of his garden (where otherwise his telescope is), as this was the place where he could take images without many disturbing objects in view…and he caught the Tricker here, too.

It was Csaba Kozsa, who managed to take a picture of the infralateral arc. He was also reading messages in the buzzing Hungarian forum of halo observers, so in the early afternoon he rushed to his favourite observation point: Visegrád’s hill, Kis-Villám. This is the closest elevation to his home with a good view to South-Southwest, and the 270m hill made it possible to take a photo of the infra. He says that up till this day he had had very little knowledge about complex halo phenomena, but thanks to the several observations on Monday and the explanations of more experienced observers, he now knows where to look and what to pay more attention to.