Middle Lowitz Arc
Andreas Livbom saw this halo display at Luleĺ, Northern Sweden. More of his pictures are below. ?The ‘straight from the ‘phone’ image at left has a hint of a halo arc just outside the bright 22 degree halo and below the sundog. Could it be a rare middle Lowitz arc? Yes! It’s clear in the colour subtraction enhancement at right. The arc shows up too on other pictures taken minutes apart. ?The puzzle is that there are no upper and lower Lowitz arcs branching from the sundog - these are usually far more prominent. Why are they absent?
Lowitz famously drew a lower Lowitz arc in a St Petersburg halo display in 1790. Their very existence was thereafter controversial. There were no unequivocal published photographs until just thirteen years ago. The arcs remain rare sightings, the ‘middle’ arc particularly so.?There are three 22° Lowitz arcs from three possible ray paths through hexagonal prisms - shown at left. The prism's rotational axis is peculiar. It passes across the prism and intersects two side face junctions. Lowitz crystals can be plates or 'rocking' Parry oriented columns.?How to explain the absence of the upper and lower arcs in Andreas' display? ?One way is to posit crystals with small or non-existent top and bottom side faces relative to the Lowitz axis. One is shown at far left. In extremis the crystal becomes rhomboidal. These variants are physically permitted and in nature plate crystals are rarely regular hexagons. Do this and the upper and lower arc rays are then seriously hindered or even eliminated. These crystals explained the Vaala, Finland display of 1994.?The HaloSim ray tracing at left used the crystals and the only Lowitz arc visible is the middle one. Its position matches the arc in Andreas' images.