A Liljequist parhelion is a rare halo, an optical phenomenon in the form of a brightened spot on the parhelic circle approximately 150âÂÂ160ð from the sun; i.e., between the position of the 120ð parhelion and the anthelion.
While the sun touches the horizon, a Liljequist parhelion is located approximately 160ð from the sun and is about 10ð long. As the sun rises up to 30ð the phenomenon gradually moves towards 150ð, and as the sun reaches over 30ð the optical effect vanishes. The parhelia are caused by light rays passing through oriented plate crystals. .
The phenomenon was first observed by Gösta Hjalmar Liljequist in 1951 at Maudheim, Antarctica during the NorwegianâÂÂBritishâÂÂSwedish Antarctic Expedition in 1949âÂÂ1952. It was then simulated by Dr. Eberhard Tränkle (1937âÂÂ1997) and Robert Greenler in 1987 and theoretically explained by Walter Tape in 1994.
A theoretical and experimental investigation of the Liljequist parhelion caused by perfect hexagonal plate crystals showed that the azimuthal position of maximum intensity occurs at
,
where the refractive index to use for the angle of total internal reflection is Bravais' index for inclined rays, i.e. for a solar elevation . For ice at zero solar elevation this angle is . The dispersion of ice causes a variation of this angle, leading to a blueish/cyan coloring close to this azimuthal coordinate. The halo ends towards the anthelion at an angle
.