The phenomenon known as Lowitz arcs, named after Tobias Lowitz, a professor of chemistry at St Petersburg, has intrigued atmospheric optics enthusiasts since its discovery in 1790. These faint arcs extend from the sundogs of a bright halo display, but sightings have been sporadic, making them a rarity in the atmospheric optics world. Interestingly, the absence of Lowitz arcs over Antarctica, a place renowned for excellent halo displays and rare halos, has further deepened the mystery surrounding their existence.
While Lowitz arcs are not commonly observed, their presence has been captured in stunning images taken in Antarctica. One such image, captured by Richard Corbett at the UK Halley Research Station on the Brunt Ice Shelf, showcases these elusive arcs against the backdrop of the polar landscape. It's important to note that these ice crystals were not formed by ski-resort snow machines, which are often responsible for generating rare halos. The natural occurrence of Lowitz arcs in Antarctica adds to their intrigue and raises questions about their formation.
Lowitz arcs are visible as faint arcs extending upwards and outwards from each parhelion, commonly known as sundogs. In some instances, two arcs can be observed from each parhelion, located outside the 22° halo. A notable feature is the variation in apparent sky brightness on either side of the outermost arc. This disparity in brightness adds to the complexity of understanding the formation and behavior of Lowitz arcs.
To comprehend the ray paths responsible for Lowitz arcs, researchers have identified three possible paths: 'upper' U rays, 'lower' L rays, and 'reflected' arcs UR and LR. The presence of an odd number of internal reflections within the Lowitz oriented plate crystals modifies these ray paths. However, the production of Lowitz arcs is inefficient, requiring approximately 70% of Lowitz oriented plates to generate these weak arcs. This inefficiency contributes to their rarity and remains a subject of ongoing research.
Simulating the formation of Lowitz arcs involves considering the crystal structure and orientation. In one such simulation using regular hexagonal plates, researchers found that a c/a ratio (thickness ratio) of 0.1 produced the 22° halo. Additionally, a Gaussian tilt distribution around zero with a standard deviation of 10° was applied to mimic limited rotation about the Lowitz axis AA. It's worth noting that the traditional Lowitz model assumes a uniform distribution of tilts.
Further improvement in matching the simulation to observed Lowitz displays may be possible through parameter adjustments and the inclusion of non-regular crystals. However, experience suggests that significant improvements in fit are rare. Despite extensive research, certain aspects of Lowitz displays still elude complete understanding, adding to the mystique surrounding these rare atmospheric phenomena.
While Antarctica provides a unique environment for observing atmospheric optics phenomena, it's important to consider other potential sources of halo crystal forming nuclei. Even in this remote region, anthropogenic influences cannot be entirely ruled out, and further investigations are required to determine their contribution, if any.
As researchers continue to delve into the mysteries of Lowitz arcs, questions persist regarding their formation, behavior, and prevalence. Unanswered queries include:
The enigmatic nature of Lowitz arcs continues to captivate scientists and enthusiasts alike. With ongoing research and advancements in atmospheric optics, we hope to unlock the secrets behind these rare and elusive atmospheric phenomena.
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Lowitz in Antarctica ~ Images by Richard Corbett at the UK Halley Research Station on the Brunt Ice Shelf at approximately 75°S ©Richard Corbett, shown with permission.
Tobias Lowitz, professor of chemistry at St Petersburg started something in 1790 when he sketched faint arcs extending from the sundogs of a bright halo display. The arcs were not reliably sighted thereafter and their absence over Antarctica - a place for excellent halo displays and rare halos - strengthened doubts as to their existence. Lowitz arcs remain a rarity and these Antarctic images especially so. One thing we can definitely rule out. The ice crystals here were not nucleated by ski-resort snow machines, the modern source of many rare halos!.
The Lowitz arcs are visible in the upper unenhanced image as faint arcs reaching upwards and outwards from each parhelion (sundog). The lower image (levels shifted and unsharp masked) reveals two arcs from each parhelion and outside the 22° halo. Note the differences in apparent sky brightness each side of the outermost arc.
22° Lowitz arcs have three possible ray paths. Here we see 'upper' U and 'lower' L rays but modified because there are an odd number of internal reflections within the Lowitz oriented plate crystals. The odd number of internal reflections produce the so called 'reflected' arcs UR and LR.
The HaloSim ray tracing computation at left is an approximate fit to the display. 20% of crystals that were randomly oriented columns gave the 22° halo. 10% of plates produced the parhelic circle and parhelia. 70% of Lowitz oriented plates were needed to give the weak reflected Lowitz arcs. 70%? - Lowitz arc production is inefficient - one reason for their rarity.
The simulation used regular hexagonal plates with a c/a ratio (thickness ratio) of 0.1. They had limited rotation about the Lowitz axis AA with a Gaussian tilt distribution around zero of standard deviation 10°. The traditional Lowitz model has a uniform distribution of tilts.
The match might be improved by further tweaking of parameters and perhaps using non regular crystals. However, this was not done because experience shows that the fit rarely gets that much better. There are aspects of Lowitz displays that are not understood.
. But even in the Antarctic there might be other anthropogenic sources of halo crystal forming nuclei.
.. The differences in slope of the right and left hand Lowitz arcs results from the sun being off-centre in the original uncropped image.
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"Lowitz in Antarctica". Atmospheric Optics. Accessed on March 1, 2024. https://atoptics.co.uk/blog/lowitz-in-antarctica/.
"Lowitz in Antarctica". Atmospheric Optics, https://atoptics.co.uk/blog/lowitz-in-antarctica/. Accessed 1 March, 2024
Lowitz in Antarctica. Atmospheric Optics. Retrieved from https://atoptics.co.uk/blog/lowitz-in-antarctica/.