Wednesday 6 March 2024

Two below-sun Tape arcs in German Alps, 9 January 2024


by Lasse Nurminen:
 
One of the great things in the modern day observing is the possibility to explore the web for observations around the world. It gives just so much more to this hobby.
 
Once again I was browsing a German website´s forum (https://forum.meteoros.de/) about halos. There are several threads with links to halo displays visible in a network of cameras widely spread over the mountainous area in Central Europe (https://www.foto-webcam.eu/). In a post from Ruben Jakob (https://forum.meteoros.de/viewtopic.php?f=2&t=61705#p243845) one of the links at first looked like only a well formed infralateral arc, but something was disturbing me. Just as if there was a faint arc next to it, what could it be?

After a little thinking I thought it must be something related with the lower Tape arc/Parry infralateral. I took a couple of screenshots of the changing view with the available 10 minute steps and run them through a little Photoshop process to make the faint halos better visible. After that I had no other ideas but to let Mr. Riikonen to take a look at the situation in hands.
 

by Marko Riikonen:
 
Nurminen mailed me some photos of this 9 January 2024 display in Balderschwang, Germany. His hunch of something going on there was right. As shown by the title photo, two below-sun Tape arcs – one tangent to 46° infralateral arc and one to supralateral arc – are visible. The former is well established: although still a high rarity for any halo hunter to bag, over the years a representative collection of specimens have accumulated from ice fog displays. The latter is a much more furtive animal. It's been photographed in a couple of spotlight displays, but the only previous celestial light source record is the display that Herman Scheer photographed on 18 October 2010 in Sonnblick Observatory, Austria (https://www.ursa.fi/blogi/ice-crystal-halos/superb_diamond_dust_display_in_austria/). However, the 9.5 degrees sun elevation meant the two arcs were superposed, contributing about equally to the joint arc's intensity. And given in spotlight beam we have photos of only either lower arc, here is what raises the Balderschwang display on pedestal: it is the first record in which both arcs are visible individually at the same time.

 
The webcam took a photo every 10 minutes. The display started at 14:30 (title photo, sun 15.1°) and was seen until 16:10 (sun 4.4°) after which the sun got too low to shine anymore to the ice fog. At the end of the post the full sequence is shown. The most interesting stuff was in the first hour and above are three stages from this interval with (a bit wider angle) simulations using HaloPoint: 14:40 when the two arcs are distinct and nicely separated (arrows); 15:20 when they are already quite merged; and 15:30 when a practically perfect superposition is seen.
 
There are 8 arcs to this family, but normally we regard as Tape arcs only those that are solely made by Parry orientation. This reduces it to four arcs – two obliquely from the sun on 46° infralateral arc, two on supralateral arc – into which number also belong the two arcs in Balderschwang photos. As to the naming, the situation is unclear, at least on the level of individual arcs. Tape himself has used terms Parry infralateral and Parry supralateral (also some usages of 46° Parry by others can be found in the internet). But it appears that Tape only associated two of the four arcs with these names. If we look into the monolithic A General Setting for Halo Theory (1999) by Tape and Können, we see that while the other arc in the Balderschwang display would be called the Parry infralateral, the other one is "unnamed" (page 1581, left column, last paragraph). And given there have been really no serious suggestions stemming from the generic "Tape arc", the task of naming this quartet members appears to be still before us. As the arcs divide into above and below light source pairs, the names could play out like this:

upper Tape/Parry supralateral (arc)
upper Tape/Parry infralateral (arc)
lower Tape/Parry supralateral (arc)
lower Tape/Parry infralateral (arc)

However, because supra and infra are synonymous for upper and lower, there is a bit of the feel of those "logical monstrosities" that Tape writes about in the Nomenclature Woes chapter of his book Streetlight Halos (second part, page 170). Moreover, naming of any halos from double oriented crystals should be such that it takes into account also the corresponding arcs that arise from raypaths involving a reflection from horizontal crystal face and that's where the above suggestion again performs poorly by extending what are already quite long-winded names. What's the solution? Alphabet letters would seem to be one:

Tape A81✔️+33...-27
27

counter-Tape A'861
+33...-24
24
20...24
Tape B71✔️-33...+27
counter-Tape B'731
-33...+24

















counter-Tape A''831
-20...-27
never






counter-Tape B''761
+20...+27












Tape C15✔️+63...-3
3

counter-Tape C'164
+63...0
0
0
Tape D14✔️-63...+3
counter-Tape D'135
-63...0

















counter-Tape C''134
0...-3
0






counter-Tape D''165
0...+3












Tape E31
+32...0
0

counter-Tape E361
+32...0
0

Tape F61
-32...0
counter-Tape F631
-32...0












Tape G16
+58...+90
never

counter-Tape G163
+58...+90
never

Tape H13
-58...-90
counter-Tape H136
-58...-90


The arcs made exclusively by Parry crystals run from A to D. The two arcs in the Balderschwang display come out as Tape C and Tape B. Arcs E to H are better known as made by plate oriented crystals and include circumzenith arc and circumhorizon arc, but should these occur solely from Parry crystals I agree with Alexander Haussmann that they be recognized also as Tape arcs (Haussmann points this out, as pertaining to circumzenith arc, in the comments section of my post http://thehalovault.blogspot.com/2017/10/halos-on-6th-march-2017-in-rovaniemi.html). Perhaps surprisingly, these specific situations appear still undocumented.
 
The table is just a sample from one of alternative nomenclature suggestions for halos from double oriented crystals that I have been working on. While the lettered versions seem to me the best way to go with Tape arcs, the above sample is not necessarily my favourite. For example, letters could be assigned to each arc differently, such as running from the most common arc to the least common arc. This kind of ordering may not be so clear-cut for all halo families, though. If we would apply it to the Tape arcs in the above table, swapping letters B and C would probably get it right. I hope to get the nomenclature article done with and posted here some day.
 
None of the reflection arcs in the right side of the table corresponding to Tape A-D have been observed – yet. They don't come out well with regular hexagons. Triangular(ish) shape, which is common in ice fogs, is better as demonstrated by the HaloPoint simulations below, arrows marking the reflection arcs. These tentatively named "counter" arcs may get overwhelmed when they occur on infra- and supralateral arcs, but the good thing about snow gun generated ice fogs is that every now and then Parry orientation dominates: there are displays in which Tape arcs are seen without infra- and supralateral arcs (example: https://thehalovault.blogspot.com/2016/11/a-pure-breed-uppervex-hastings.html).
 
 
Of the two upper oblique Tape arcs we are all well familiar with the one that appears on supralateral arc, Tape A in the language of the table above. It is "the" Tape arc that accounts for the great majority of observations of the four arcs. The upper arc that is associated with infralateral arc, Tape D, has been observed only in spotlight displays. There is a reason for this: its light elevation range tops at 3 degrees above the horizon, rendering it improbably hard to see in solar and lunar displays. We have some representative photos of this arc (example: http://thehalovault.blogspot.com/2017/09/halos-on-night-of-1415-december-2016-in_30.html) but no particularly good co-occurrences with arc A. Looks like there are only two such displays. First was the halo complex that Marko Mikkilä, Jarmo Moilanen and I watched in Rovaniemi on the night of 23/24 November 2015 (https://www.haloblog.net/2016/10/25/major-spotlight-display-possible-4th-tape-arc-component/). The second is this winter's crop, a display that I photographed on the night of 10/11 November 2023 in Rovaniemi, shown below as two versions of a 11x15s stack (arrows mark Tape D) and a simulation. The two upper arcs in these displays are the corresponding arcs to the lower arcs in Balderschwang display, but in swapped positions due to light elevation difference. By the standard of Balderschwang, lightsaber wielders still have their work cut out to capture a better defined instance of the upper pair.
 


 


1 comment:

  1. What a nice display and work to reconnect with the HaloVault.
    thank you for sharing those impressive images.
    Nicolas R.

    ReplyDelete