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/
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.
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)
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:
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 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.
