**Liljequist Parhelion**
**Definition**
The Liljequist parhelion is a rare atmospheric optical phenomenon characterized by a bright spot or halo appearing on the parhelic circle, typically located about 150° to 160° from the Sun. It is a type of halo caused by the refraction and reflection of sunlight through hexagonal ice crystals in the upper atmosphere.
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## Liljequist Parhelion
The Liljequist parhelion is an uncommon and intriguing atmospheric optical phenomenon that belongs to the family of halos and parhelia (commonly known as sundogs). It manifests as a bright, often bluish or white spot on the parhelic circle, a white horizontal circle that appears at the same altitude as the Sun. The Liljequist parhelion is typically observed at an angular distance of approximately 150° to 160° from the Sun, making it a distinctive and relatively rare feature in the sky.
Named after the Finnish meteorologist Gösta Hjalmar Liljequist, who first described the phenomenon in the mid-20th century, the Liljequist parhelion is caused by the complex interaction of sunlight with ice crystals suspended in the atmosphere. These ice crystals, usually in the form of hexagonal plates or columns, refract and reflect sunlight in specific ways that produce a variety of halo phenomena, including the Liljequist parhelion.
### Historical Background
The study of atmospheric halos has a long history, with many phenomena documented since antiquity. However, the Liljequist parhelion was not clearly identified until the 20th century. Gösta Hjalmar Liljequist, a Finnish meteorologist and glaciologist, made significant contributions to the understanding of optical phenomena in polar regions. His observations and analyses led to the identification and explanation of this particular parhelion.
The Liljequist parhelion is less commonly observed than other halo phenomena such as the common sundogs (parhelia) at 22°, the 46° halo, or the circumzenithal arc. Its rarity is due to the specific orientation and shape of the ice crystals required to produce it, as well as the atmospheric conditions necessary for its visibility.
### Formation and Optical Mechanism
The Liljequist parhelion forms as a result of sunlight interacting with hexagonal ice crystals suspended in cirrus or cirrostratus clouds in the upper troposphere. The phenomenon is part of the broader category of halos, which are optical effects caused by the refraction, reflection, and dispersion of light through ice crystals.
#### Ice Crystal Geometry
The ice crystals responsible for the Liljequist parhelion are typically hexagonal plates or columns. These crystals are oriented horizontally as they fall through the atmosphere, allowing sunlight to enter and exit through specific crystal faces. The precise geometry and orientation of the crystals determine the angles at which light is refracted and reflected, producing various halo phenomena.
#### Light Path and Refraction
The Liljequist parhelion is produced by a complex light path involving multiple internal reflections and refractions within the ice crystals. Unlike the common sundogs, which are formed by light refracted through 22° prisms of hexagonal crystals, the Liljequist parhelion involves light rays that undergo two internal reflections inside the crystal before exiting.
This double internal reflection causes the light to emerge at an angle of approximately 150° to 160° from the Sun, creating a bright spot on the parhelic circle at this angular distance. The parhelic circle itself is a white, horizontal halo that encircles the sky at the solar elevation, formed by reflection of sunlight from vertical faces of ice crystals.
#### Coloration
The Liljequist parhelion often appears white or slightly bluish. The coloration is less vivid than that of common sundogs, which display bright red and blue fringes due to dispersion. The subdued color of the Liljequist parhelion is attributed to the nature of the light path and the multiple internal reflections, which reduce chromatic dispersion.
### Observation and Visibility
The Liljequist parhelion is a rare and subtle halo phenomenon, making it difficult to observe and document. It is most commonly seen in polar and subpolar regions, where the atmospheric conditions favor the formation of the necessary ice crystals. However, it can occasionally be observed at lower latitudes under suitable conditions.
#### Conditions for Observation
– **Ice Crystal Presence:** The presence of hexagonal ice crystals in cirrus or cirrostratus clouds is essential.
– **Solar Elevation:** The phenomenon is best observed when the Sun is relatively low on the horizon, typically below 30° elevation.
– **Atmospheric Clarity:** Clear skies with minimal atmospheric pollution or haze improve visibility.
– **Observer Location:** Polar and high-latitude regions provide the most frequent opportunities due to prevalent ice crystal clouds.
#### Appearance and Location in the Sky
The Liljequist parhelion appears as a bright spot on the parhelic circle, which is a white horizontal halo at the solar elevation. It is located approximately 150° to 160° from the Sun, meaning it appears on the opposite side of the sky relative to the Sun but offset by about 20° to 30° from the antisolar point.
Because the parhelic circle is a full 360° circle around the observer at the solar elevation, the Liljequist parhelion is one of several bright spots or features that can appear along this circle, including the common sundogs at 22°, the 120° parhelia, and other rare halo phenomena.
### Related Halo Phenomena
The Liljequist parhelion is part of a complex system of halo phenomena caused by ice crystals. Understanding its relationship to other halos helps contextualize its formation and appearance.
#### Parhelia (Sundogs)
Common sundogs are bright spots appearing at 22° to the left and right of the Sun, caused by refraction through 22° prisms in hexagonal ice crystals. They are often colorful, with red on the side nearest the Sun and blue on the outer edge.
#### 120° Parhelia
These are bright spots located approximately 120° from the Sun on the parhelic circle. They are caused by multiple internal reflections in ice crystals and are more common than the Liljequist parhelion.
#### Parhelic Circle
The parhelic circle is a white, horizontal halo at the solar elevation, formed by reflection of sunlight from vertical faces of ice crystals. It serves as the baseline for locating various parhelia, including the Liljequist parhelion.
#### Other Rare Halos
Other rare halos include the 46° halo, circumzenithal arc, and various low-probability phenomena such as the subparhelic circle and the Liljequist arc, which is related but distinct from the Liljequist parhelion.
### Scientific Significance
The study of the Liljequist parhelion and related halo phenomena contributes to the broader understanding of atmospheric optics, cloud microphysics, and the properties of ice crystals in the atmosphere. These phenomena provide natural laboratories for studying light behavior and the physical characteristics of ice crystals.
#### Atmospheric Research
Observations of halos help meteorologists and atmospheric scientists infer the presence, shape, and orientation of ice crystals in cirrus clouds. This information is valuable for climate studies, weather prediction, and understanding radiative transfer in the atmosphere.
#### Optical Physics
The Liljequist parhelion exemplifies complex light paths involving multiple internal reflections and refractions, offering insights into the principles of optics and the interaction of light with anisotropic media.
### Documentation and Photographic Records
Due to its rarity and subtlety, photographic documentation of the Liljequist parhelion is limited. Advances in digital photography and increased interest in atmospheric optics have led to more frequent and detailed observations in recent decades.
Photographers and halo enthusiasts often use wide-angle lenses and high-resolution cameras to capture the parhelic circle and its associated phenomena. Careful analysis of images helps confirm the presence of the Liljequist parhelion and distinguish it from other bright spots on the parhelic circle.
### Summary
The Liljequist parhelion is a rare and fascinating atmospheric optical phenomenon appearing as a bright spot on the parhelic circle approximately 150° to 160° from the Sun. Caused by complex refraction and multiple internal reflections of sunlight through hexagonal ice crystals, it is most commonly observed in polar regions under specific atmospheric conditions. Its study enhances understanding of atmospheric optics and the microphysical properties of ice crystals.
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**Meta Description**
The Liljequist parhelion is a rare atmospheric halo appearing as a bright spot on the parhelic circle, caused by sunlight interacting with hexagonal ice crystals. It is typically observed at 150° to 160° from the Sun and is most common in polar regions.