Double the fun: Appearance of the 22° halo during a total solar eclipse

At the Arbeitskreis Meteore (AKM) spring meeting in March 2018, we discussed an observation made by Jörg Strunk during the “US eclipse” from August 21st, 2017: A 22° halo was visible in cirrus clouds around the sun up to around half a minute before the onset of totality. Similar observations have already been discussed in a paper by G. Können and C. Hinz from 2008. In this publication, it is mentioned that an initially very bright 22° halo could stay visible throughout the totality, created only by the light of the solar corona, and standing out against the twilight-like sky background.

The question I want to address here is: How would such a halo look – similar to the ones we know, being created by ~0.5° large, disk-like sources such as the sun or full moon? Or more diffuse due to the larger angular diameter of the corona?

For a “quick and dirty” simulation I took a radially symmetric fit for the corona brightness from here and combined it with another fit for the brightness of the solar disk from here, resulting in the combined brightness distribution depicted in the graph below (blue line, using λ = 500 nm for the photosphere formula). Simulations were carried out either with this full distribution (clearly dominated by the sun’s disk), or with the the photosphere fully obstructed, i.e. corresponding to an eclipse in which the apparent size of the moon matches exactly the sun’s disk (green line):


The calculations themselves are carried out in two steps: At first, I let a deep simulation (300 million rays) of an ordinary 22° circular halo run in HaloPoint 2.0, but using a point source instead of the usual sun disk. Next, each color channel of the simulation is convoluted with a matrix resembling the source’s intensity distribution. For this purpose, the brightness function was cut off at 7 solar radii (1.9° from the central point of the disk, assuming a radius of 0.27°). This approach is of course only justified as long as projection distortions can be neglected, i.e. in the vicinity of the projection center, otherwise a more complicated calculation involving spherical coordinates is required. Here, the field of view from the center to each edge amounts to about 29.0°, and the simulations are presented in Lambert’s equal area (azimuthal) projection. Under these conditions, the distortion error remains indeed small. The angular resolution is about 0.06°/pixel, as determined by the HaloPoint program.

The intensity distributions for the various light sources are depicted below: a) point-like, as assumed for the simulation, b) the non-eclipsed sun, dominated by the photosphere disk, and c) the corona with the photosphere blocked by the moon. The ratio of the integrated intensities between b) and c) amounts to about 900000. The resulting 22° halos are shown in subfigures d)-f), normalized each to the brightest pixel, and with zoom views of the left rim provided in g)-i). The integrated halo intensities scale with the same factor of 900000 as does the illumination.


The most prominent feature is the red double rim in f) and i), clearly a consequence of the ring-like source. But, even if the sky background illumination during the total phase permits a halo observation, it is not guaranteed that the double rim becomes visible, as diffraction is not accounted for in the halo simulation. Diffraction blurring decreases with increasing crystal size, which implies that the crystals have to be larger than a certain minimal value to allow finer halo features to be observed. For a rough estimation, it is possible to rely on the diffraction pattern of a single slit. The main peak has an angular full width at half maximum (FWHM) of about λ/b, with b denoting the slit width. For λ = 600 nm, and requiring that the FWHM should be smaller than 0.5° (i.e. roughly the distance between the two rims), this means that b has to be larger than 70 µm. This value corresponds to the width of one prismatic face of a hexagonal crystal, projected under the angle of incidence (about 41°) for minimal deflection. The corner-corner size of the hexagon equals then 2.6⋅b, i.e. the minimal crystal diameter amounts to about 180 µm.

Finally, it should be remarked that a double rim halo can also result from an annular eclipse. The chances for detection should be even better than for a corona halo, as the background contrast would not be much worse than for the non-eclipsed sun. In this situation, the azimuthal homogeneity of the source will also be much better. For the corona, this is only a rather crude approximation and under realistic circumstances this implies that the splitting of the corona halo might become prominent only at certain positions along its circumference.

The Fichtelberg halo display from December 18th, 2017

Over the past years, the Fichtelberg – Keilberg/Klínovec twin peak region in the German / Czech ore mountains has proven to be an unexpectedly active place for diamond dust halos. As shown in a recent study by Claudia Hinz et al., this high halo activity may have already been present there for decades or even longer, resulting in local myths but sadly few scientific reports in the halo literature up to several years ago.

Another exceptional display was observed on the top of the Fichtelberg (1215 m) on December 18th, 2017, by Gerd Franze, the head of the local meteorological station. He took about 400 photographs from about 12.20 to 13.20 CET (at sun elevations from 16.0° to 14.3°). During the course of the display, the temperature increased from –3.6 °C to its peak value of –1.9 °C at 13:10, followed by a decline down to –5.0 °C over the subsequent hour. Wind was noticed only at very low speeds of about 2-4 m/s coming from between southern and southwestern directions. Fog from the bohemian basin was drifting over the mountain top the whole day. No snow guns were running, as there already was enough natural snow for skiing.


a) view towards the sun, b) view towards the anthelion, c) and d) corresponding simulations using the parameters below


Simulation parameters for HaloPoint 2.0

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Moilanen arc observations from Davos

Moilanen arc in Davos. Photo: Bertram Radelow

This report describes halo displays including Moilanen arcs (MAs) which occurred in Davos from 2005 to 2017, caused by snow gun induced diamond dust. Four of them are discussed in more detail. The MAs are presented in relation to the degree of ripening of the diamond dust crystals, and with respect to the other halo types in the displays.

Davos is a world-renowned winter sport resort. Because of the decreasing amount of natural snow over the past decades, numerous snow guns have been installed. In the winter season 2017/18, more than 150 of them are running.

Snow guns produce artificial snow made from clumps of small ice spheres without any discernible hexagonal crystal structures. However, a part of the water spray is converted into microscopically small ice spheres which drift off. These will serve as condensation seeds for larger crystals, often in column shape, but also plates do occur.

Remarkably, MAs are preferably generated during the first stage of crystal growth. Full-ripe crystals produce only dull MAs or none at all.

Author: Bertram Radelow, Davos, Switzerland

Diamond dust halos on December 2nd, 2017 at the Fichtelgebirge

Concave and convexe Parry arc. Photos: Ruben Jakob

At the Fichtelgebirge, a low mountain range in northeastern Bavaria, there were similar halo phenomena as in the ore mountains circa 100km away (see article). In the morning, the high fog staunched and dissolved while passing the mountains. From afar, a very interesting cloud edge was seen at Mount Schneeberg (1051m above sea level). Shortly ahead Schneeberg, the temperature fell below -8 °C and rose to -5 °C in the fog. In the valley between the two mountains diamond dust was non-existent. I kept searching and trying to get out of the fog. Just in the village Grassemann (about 700m above sea level) there was a right parhelion and shortly afterwards a Supralateral arc visible above the fog. After 20 minutes, the fog suddenly came up to me and conjured a variety of halos in the sky. When I saw the concave and the convex Parry arc as well as the Moilanen arc distinctly, I was speechless. After that, the halo activity waned slowly. At 10 o’clock there were no more halos seen. The high fog broadened increasingly and the sun couldn’t shine through.

All in all, the following halo types were visible:
– 22°-halo
– Parhelia
– Upper tangent arc
– Upper and lower sun pillar
– Circumzenithal arc
– 46°-halo and Supralateral arc
– Fragment of parhelic circle
– Upper concave and convex Parry arcs
– Upper Tape arcs
– Heliac arc
– Subhelic arc
– Moilanen arc

Afterwards I drove to the top of Mount Ochsenkopf (1024m above sea level). There was a second halo show starting at 11.10 CET. However, expect of the right infralateral arc with the Tape arc, there were no further halo types.

Author: Ruben Jacob, Burgkunstadt, Germany

Diamond dust halos on December 2nd, 2017 at the rim of the Ore Mountains

Due to the inflow of polar air in the night of december 1st to 2nd there was fog formation in the valleys of the ore mountains. At the top of Fichtelberg (1214m above sea level) Claudia Hinz could observe the lower tangent arc and in parts also the submoon repeatedly in the lower sea of clouds. About 03.45 CET single shreds of clouds rose from the valley and forms an extensive halo phenomenon for 5 minutes as it is rare at the moon certainly. There was the 22°-halo, both parhelia, bright upper and lower tangent arcs, the complete light pillar, 46°-halo, supra- and infralateral arc, parhelic circle, submoon and both subparhelia. The evaluation of the photos revealed the Parry arc, the Lowitz arc on the left side and the upper and lower Tape arcs.

During the day Wolfgang Hinz took over the further observation at the rim of the ore mountains. At the sun the halo types changed quite fast, too. Several times the 16 visual observed halo types formed phenomena including 19 halos. Pictures showed two more halo types. Some halos lasted just for minutes, whereas the 22°-halo and the parhelia were there from 9.30 CET to 13.00 CET. But they appered earlier and continued towards the later afternoon. The Halos were observed on top of Fichtelberg and its surrounding area, at the border to Bohemia and at the Czech ski area Neklid, as well as on the way home in the mountain village Tellerhäuser.

All in all, the following halo types were observed:
– 22°-halo with both parhelia
– Upper and lower tangent arc
– Upper and lower sun pillar
– Circumzenital arc
– 46°-Halo
– Parhelic circle
– The left Lowitz arc shortly
– Left 120°-parhelia
– Supra- and infralateral arcs
– Parry arc
– Subsun
– Tape arc’s
– Trickers’s anthelic arc
– Heliac arc
– Moilanen arc (faint)
– Subhelic arc (Photo)
– Anthelion (Photo)

Author: Claudia and Wolfgang Hinz, Schwarzenberg, Germany

Moving Ripples in a sundog

On March 26, 2017, I could observe this phenomenon for the second time. The first time I observed it in Munich about 20 years ago, when within a few minutes two boundles of “moving ripples” crossed a left-hand sundog. At that time I did not know what I was seeing. I learned it thereafter, also the name of the phenomenon, that it has been observed several times until then and that it may be related to acustic waves. Later, the video of the “extermination” of a sundog by a rocket launch became well known. But I did not see this phenomen again till March 26, 2017.

It is just a “must” for me to photograph with my pocket camera every halo I see mostly only to get the time mark of its beginning and/or end for the record. On that day I was several times on my balcony to check for halos. The sky had only contrail-cirrus (now officially termed Ci-homomutatus), but no halos. But once I discovered a faint sundog it may have been the only halo-active contrail-cirrus group of that day. I observed the sundog coming and going with the respective cirrus couds resp. the standing sundog against the moving clouds. The sundog was faint the whole time and all but remarcable. But then I surprisingly realized that there were some odd dark strokes crossing part of the sundog diagonally. Remembering the moving ripples, I immediately zoomed in. I could record this phenomenon in some pictures. It did last only about 30 seconds, but visually the dark stripes were much more evident than the photographs suggest.

The exceptionality of this observation was that the otherwise “moving” ripples were in fact “standing”: They moved with the cloud through the sundog. This can be seen very nicely on the photographs: the ripples seem to be a fixed structure of the cloud. But they were only visible in the area of the sundog. Outside this area the ripple pattern did not show up: the dark stripes were not there! Clearly recognizable is also the fact that each ripple began weak and increased its intensity towards a maximum in the centre of the ripple area, and to vanish at the other border of the ripple area.

Remarcable was also the fact that the ripples showed up only in a part of the cloud resp. the sundog area. For me it remains a mystery why (only) a small part of the cloud was “trapped” in these “acustic waves”…

Author: Christoph Gerber, Heidelberg (BW), Germany

Link to the topic: Collection of all known observations

Halos in pyramidal ice crystals in Ohio

On June 17th, 2017 Michael Ellestad have recorded in Ohio, USA a nice pyramidal crystal halo complex. began in the morning and ended at midday and near evening the halos returned. In all he observed 9, 18, 23, 24deg and faint 35deg halos with upper and lower 9deg tangent plate arcs, 18deg lateral plate arcs, 23deg upper plate arc, upper and lower 24deg plate arcs and maybe the very rare pyramidal helic arc.

The cause was the warm front of a small low pressure area over Lake Michigan. In such small depressions crystals often have only a short lifetime, but have optimal optical properties during this time. Since these small-scale low-pressure areas occur very frequently in the 5 Great Lakes of North America, this could be the reason why pyramidal crystals and haloes occur several times a year, more frequently than in most other parts of the world. But also the jetstream, which runs mstly over Ohio, could have a positive influence on the crystals, as was investigated by Rainer Schmidt in this article.

 

Oblique sun pillar at the Mt. Zugspitze

On November, 23rd, 2016, I observed in Altocumulus virga a sun pillar from Mt. Zugspitze which exhibited a certain amount of inclination with respect to the otherwise common vertical direction. At first it appeared rather diffuse, but later on the distinct tilt became clearly visible.

That morning was relatively warm with temperatures around –3°C on the 2963 meter high summit, and a squally foehny wind gusted with peaks up to 80 km/h. Warm air was sucked from the Mediterranean sea by a severe southern air current. I suspect that this wind led to the inclination of the sun pillar by systematically tilting the ice crystal axes into a preferential direction.

There are only a few similar observations that can be found in the literature. On January 1st, 1969, K. Lenggenhager documented a tilted and split lower sun pillar in diamond dust on the Mt. Säntis (2502m). He explained the phenomenon by air currents which were forced to ascend a ridge, and the crystal axes being turned by various amounts on different levels of altitude (see graphic from [1]).

Similar conditions might have prevailed in my observation. The air masses of the Mediterranean sea were forced to ascend the Alps, and therefore they might have tipped the crystal axes increasingly with rising altitude.

Sun pillar in original and with unsharp mask

Another oblique sun pillar was described by Edgar W. Wooland after an observation in Boulder, Colorado [2] on January 10th, 1918, and I myself could also already observe oblique and displaced halos [3]. Unfortunately, there seem to be no further documented cases.

Any appropriate notes on the subject are highly welcome.

References
[1] K. Lenggenhager: “Seitlich verschobene, umschriebene Halostücke, schräg ovaler Halo und schräge Lichtsäulen”, Archiv für Meteorologie, Geophysik und Bioklimatologie, June 1977, Volume 26, Issue 2, pp 275–282
[2] Edgar W. Woolard: “The Boulder Halo Of January 10, 1918”
[3] Claudia Hinz: “Double Halos”

Author: Claudia Hinz

Diamond dust halos in Jena, Germany

On January 22nd 2017 I had the opportunity to witness a halo phenomenon in my home town for the first time.

The observation took place in Jena-Maua Germany (50°51’59.4″N 11°36’02.0″E) from 8:45-10:45 CET within about one kilometer. The maximum activity was observed between 10:15 and 10:45 CET.

We had a high-pressure weather situation with more and more lifting and dispelling fog (starting with 50m AGL) in the ‘Saale’-valley. Measured temperatures were about -10 to -6 degrees.

After recognizing the lower sun pillar besides the left Subparhelia in front of the fog boundary (seen from 300m height) I drove closer to the fog and found myself standing inside diamond dust (height 150m).

Between 9:45 and 10:45 the following types of halos have been witnessed: 22° halo, left and right parhelia, upper and lower tangent arcs, upper and lower sun pillar, Circumzenithal Arc, parhelic circle, Anthelion, left and right 120° parhelia, Supralateral arc, Parry arc, Subsun, left and right subparhelia, Tricker’s anthelic arc, Tapes arcs, Heliac arc and subhelic arc.

Uncertainties exist concerning the following observations: Lowitz arcs and Moilanen arc.

To sum up the best possibility of seeing this phenomenon was inside or near Jena-Maua – a small district of the city Jena which has some industry chimneys (compare the last photographs with the smoke trail). It seems legit to suppose that industrial fine particules conduced sublimation/condensation nucleus for the diamond dust development.

Author: Marco Rank, Jena, Thuringia, Germany

Trickers and Wegeners Arcs in Jordan

Observer: Michael Heiß, Greifswald – Germany
Website of the phenomena: www.meteoros.de

The following observation was made along the motorway leading from Wadi Rum to Akaba in Jordan on December 4, 2016. At a position of 29.641553North and 35.196915 East, halo activity reached its maximum between 9:30 and 9:45 a.m. EET(East European Time). The temperature was at 14°C. After it had been clear, cloudiness now increased (Cirrus clouds).

Some fragments of the 22°-halo and both sundogs had already been visible in scattered cirrus clouds the evening before and through the morning hours. The cirrus clouds increased during the morning, becoming scattered over the whole sky by 9:30 EET. During a stop by the roadside, the whole scale of haloes could be observed. The sun was surrounded by a bright 22°-halo, both sundogs, upper tangent arc and a complete and colourful Parry arc. A faint circumzenithal arc was also visible. While the cirrus clouds spread more and more over the sky, some fragments of the parhelic circle became visible, merging to form a complete parhelic circle which was so bright that it got a brownish-red upper rim. The two characteristic bulges in the areas of the 120°degree-sundogs were also clearly visible. The highlight of the phenomenon, however, was Tricker´s anthelic arc which appeared for about 5 minutes as an accentuated “V” turned upside down opposite the sun beneath the parhelic circle. At the point where the “V” tapered, the anthelion could also be detected.

The cirrus clouds rapidly thickened, which caused most haloes to fade away. Only the 22°-halo persisted for several hours before disappearing in the afternoon.
Wegeners antihelic arc appears if the USM method is used.

All pictures are taken with a full-frame camera (Canon 6D) at a focal length of 24 millimetres.