Rare subhorizon halo

Last updated on September 16, 2023

Rare Subhorizon Halo: A Phenomenon at the Edge of Invisibility

Scientific discoveries often emerge from observations that push the boundaries of visibility. In the field of atmospheric optics, one such discovery was made by Tomas Trzicky during a flight from Helsinki to Prague on July 31, '09. Trzicky captured an exceedingly rare sub 120° parhelion, a sighting that might be the first-ever photograph of this phenomenon under natural lighting conditions.

Unveiling the Subhorizon Halo

Trzicky's unique sighting occurred as he looked downwards below the horizon. All the halos he observed were subhorizon arcs, with the long arc representing the subparhelic circle. To the left of this circle, other arcs intersected at the subanthelic (antisolar) point. While all these sights were already rare, the true rarity lay in the white spot on the right—a sub 120° parhelion.

Confirming the Phenomenon

To ensure the authenticity of the sub 120° parhelion sighting, Trzicky provided a series of montage images taken within a 17-second interval. These images were captured using a lens with a constant known zoom setting. By aligning the images based on the sharp end of the bright part of the subparhelic circle, Trzicky estimated the angular distance from the subanthelic point to the right-hand bright spot. Remarkably, this distance fell within 5° of the expected sub 120° parhelion position. A second set of images taken 13 seconds apart yielded consistent results, further confirming the rarity of this sighting.

Unraveling the Science Behind Sub 120° Parhelion

The above-horizon 120° parhelion is relatively uncommon due to its colorless nature, making it less conspicuous against a cloud background compared to its 22° counterpart. The formation of the 120° parhelion involves complex ray paths through plate crystals with large hexagonal faces nearly horizontal. However, an additional internal reflection from the lower crystal face is required to create the elusive sub 120° parhelion.

Several factors contribute to the generation of the sub 120° parhelion:

• Plate thickness: Thicker plates facilitate the wayward ray path necessary for the sub 120° parhelion.
• Crystal shape: Crystals with at least one set of alternate long and short side faces enhance the formation of the sub 120° parhelion.

Insights from Intensity Variation

The intensity variation along the parhelic circle offers valuable insights into the sub 120° parhelion phenomenon. HaloSim ray tracing simulations demonstrate that plate crystals with alternate side face lengths of 4:1 yield a fairly uniform intensity along the bright part of the subparhelic circle, consistent with Trzicky's images. Conversely, simulations using thick regular hexagonal crystals produce uneven subparhelic intensities, accompanied by pronounced Liljequist parhelia brightenings and additional unobserved intensity near the sub 120° parhelion itself. However, it is important to note that these simulations provide only preliminary indications due to the various parameters involved, including crystal tilts.

Possible Contributions to the Phenomenon

While the exact origin of the diffuse arcs crossing the subanthelion remains subject to further investigation, simulations suggest that singly oriented column crystals, as well as Parry columns, might have contributed to both the arcs in Trzicky's images and his subparhelic circle.

Capturing a Rare Phenomenon

In conclusion, Trzicky's photographs of the sub 120° parhelion are not only visually striking but also likely represent a groundbreaking first. Previous images of this phenomenon were captured at night using artificial illumination to highlight crystals formed by snow machines on ski slopes. Trzicky's images, on the other hand, depict the subparhelion under the natural illumination of the sun, with crystals naturally formed in the clouds.

Exceedingly rare sub 120° parhelion sighting by Tomas Trzicky (atmospheric optics site) during a flight from Helsinki to Prague on July 31, '09. The subparhelion is arrowed at right. This is possibly the first time it has been photographed under natural lighting conditions. �Tomas Trzicky, shown with permission..

Scientific discoveries often necessarily involve observations at the ragged edge of invisibility. These were indeed through a glass darkly. Through the unwanted impediment of a badly scratched and cleaned airplane window!

Tomas is looking downwards below the horizon. All the halos are subhorizon arcs. The long arc is the subparhelic circle. At left, other arcs cross it at the subanthelic (antisolar) point. All these are rare sights but the extreme rarity is the white spot at right – a sub 120° parhelion.

Are we sure? The montage images are from a lens at a constant known zoom setting and taken within 17 seconds of each other. They are aligned on the reasonably sharp end of the bright part of the subparhelic circle. The estimated angular distance from the subanthelic point to the right hand bright spot is within a 5° of the required sub 120° parhelion position. The second montage at page bottom is from two more images 13 seconds apart and gives the same result.

The above horizon 120° parhelion is relatively rare partly because unlike its 22° counterpart it is colourless and therefore are less easy to see against a cloud background. Rays forming it (left) follow a complicated path through plate crystals drifting with their large hexagonal faces nearly horizontal. Rays that undergo an additional internal reflection from the lower crystal face form the elusive sub 120° parhelion.

The wayward ray path is best achieved if the plates are thicker than is usual. It is also helped if the crystals are not regular hexagons but have at least one set of alternate long and short side faces.

The intensity variation along the parhelic circle gives some clues to what is happening. The top HaloSim ray tracing used plate crystals with alternate side face lengths of 4:1. The intensity along the bright part of the subparhelic circle is fairly uniform as it is in the images.

The lower ray tracing used thick regular hexagons and they produced an uneven subparhelic intensity with pronounced Liljequist parhelia brightenings and further unobserved intensity near the sub 120° parhelion itself. But there are many parameters including the crystal tilts and these few ray tracings are only indications.

The diffuse arcs crossing the subanthelion were produced in the simulation by adding singly oriented column crystals. Parry columns might also have contributed to those in Tomas’s images and to his subparhelic circle.

Whatever the finer details of their origin, the sub 120° parhelion images are impressive and probably a first. Photos at night using a lamp to illuminate crystals nucleated by ski-slope snow machines have caught the subparhelion before but not as here with the sun shining down on crystals naturally formed in the clouds.

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