Thursday, 12 March 2020

A divergent 46 contact arc

Shown is a display on the night of 10/11 December 2016 in Rovaniemi. I observed and photographed it on an open bog called Rikanaapa, a dark location 7 km east of the ski center, which is around where you most often get diamond dust in Rovaniemi. I think it was one of those colder nights, with temps on the lower side of -25 C.



As I stood there, watching the display and letting the camera snap away photos, I was wondering why the glitter that was the circumnadir arc looked so broad. Photos confirmed this impression. A comparison of long series of shots taken in fixed configuration showed that in the beginning there was briefly a normal looking circumnadir arc (the first photo in the collage above) but then it thickened by extending towards the lamp. Actually, photos reveal it quickly stretches all the way to the lamp in a manner similar to divergent 22 tangent arc.

I was flummoxed and asked Nicolas to take a look when he was visiting in Rovaniemi March next year. He told what should have been obvious: it's 46 contact arc. At the same time also some kind of subparhelic arcs (Schulthess arcs) appear, so it all fits.

But at least one question still hovers in the air: why the 46 contact arc is divergent, with not much at all the classical form to talk of? Only in the second image of the sequence there is what appears to be a classical instance of the 46 contact arc, with some shape and red inner edge. After that it turns into divergent form.

It is also noteworthy that the occurrence of the 46 contact arc coincides with the occurrence of divergent subparhelia. Something in the crystal properties seems to sometimes favour picking up the divergent rays of the spotlight beam. I have seen some really strong divergent subparhelia come and go in the  beam and think they appear when plate crystals get thick. But even then you have also strong classical subparhelia. However, having just divergent 46 contact arc without its classical instance is puzzling.

CORRECTION (13 March 2020): in the second image it is not the classical instance of 46 contact arc. It is way too high up. Anyway, the 46 contact arc hypotheses still probably holds up for the explanation of the stretching of circumnadir arc towards the lamp.
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3 comments:

  1. Michael Ellestad13 March 2020 at 05:10

    Very interesting indeed. I hope you have more halo stuff to post. This weekend I am going to go thru my photo archives and get all of my elliptical halo observations and put them into a separate folder then I will possibly make a post.

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  2. Jia Hao15 March 2020 at 12:43

    That brief moment in the animation when the divergent halos pop up is really cool! If my understanding of the divergent halo theory is correct, the responsible crystals in this case should be closer to the light source?

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  3. Marko16 March 2020 at 16:03

    I would think there was pretty uniform swarm between the lamp and camera in each of those 30s exposures because this was 6 km away from the guns. Right next to the snow guns there may be non-uniform distribution, such as is shown here:
    https://www.ursa.fi/blogi/wp-content/uploads/3Darte.jpg

    Here is a similar kind of case:
    http://www.thehalovault.org/2017/02/2d-slices-of-3d-tangent-arc.html

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