Liljequist parhelia (EE28)

Liljequist parhelia lie on the parhelic circle at about 150° to 160° in azimuth from the sun, close to the anthelion. They appear as elongated white patches. Their inner edge facing the sun is sharp and can be slightly bluish. Towards the anthelion they gradually fade. They are named after the Swedish meteorologist Gösta Hjalmar Liljequist, who observed them in Antarctica in 1951. Liljequist parhelia are among the rare halos. They appear mainly when the sun is low, usually together with a bright parhelic circle and the 120° parhelia. As the sun rises they become broader and fainter. Below the horizon, the subparhelic circle carries a counterpart, the Liljequist subparhelia (EE29). They can only be seen from mountains or from an aircraft.

Liljequist parhelia form in floating plate crystals that are oriented horizontally. Light enters the crystal through a side face, is reflected several times at side faces inside the crystal and leaves it through a side face. Two light paths contribute about equally to the halo. In the first, the light is reflected twice and leaves through a different side face. In the second, it is reflected three times and leaves through the face it entered. The graphic shows both light paths in a top view of the plate. The sharp inner edge at about 153° comes from the second path, where two of the reflections become total internal reflections. For blue light this limit lies about 2° closer to the sun than for red light. The edge can therefore look bluish, while the rest of the halo is white. The light paths were explained by Walter Tape in 1994. Eberhard Tränkle and Robert Greenler had already simulated the halo in 1986.