|New Halo seen by Marko Riikonen (site) on November 6,'08 at Rovaniemi, Finland. Images ©Marko Riikonen, some images were published previously in Tähdet ja avaruus magazine, Finland.
Marko sighted the new halo in the daytime part of a marathon halo observing session. During the previous night he had lit diamond dust crystals with a halogen lamp to generate spectacular and rare displays (1,2). The next morning the sun took over from the lamp.
"The daytime display had been going on from sunrise (it was just a continuation of the night crystal storm), but it was only at 11.30 am that I noticed the new halo." Like the night's display, the halos were generated by ice crystals nucleated by ski-slope snow machines. "The [crystal] formation was peculiar in that it was actually foggy all over Rovaniemi, but around the snow guns at about 1-2 km distance the supercooled water fog was nucleated to ice crystals by the small ice particles that the guns generate."
Marko's main camera had problems and these images were taken with a back-up pocket camera. The upper image is unsharp masked to strengthen the halos. The near noon sun was 7.5° high. The gull-wings of an upper tangent arc dominate the scene. Above it there is a faint sunvex Parry arc. But the surprise was the sharp V-shaped arc whose lower tip was just 7-7.5° above the sun. The arc resembled the unexplained Moilanen arc which indeed was also sometimes visible as in the lower images.
The new arc generated considerable excitement and discussion among halo experts. Passage of rays through an ice wedge of 23° compared with that of 34° for the Moilanen arc generates an arc in the correct position. Pyramidal crystals with very unphysical angles might have the required dihedral but simulations showed that in any case they produced an arc that was too wide.
The halo was explained by Walter Tape as a 'reflected' lower sunvex Parry arc. A ray enters the lower sloping face of a Parry oriented column crystal. It is then internally reflected from the upper horizontal face (just once or for a total odd number of internal reflections) before leaving via the opposite upper sloping face (ray path 5-3-8).
The weakness of the 'normal' upper sunvex Parry arc and the non visibility of the upper suncave Parry was explained by Marko by invoking Parry oriented columns of flattened habit. The interfacial angles remain the same and, although unusual, they are physically fully realistic. Those used at left in the HaloSim ray tracing simulation had sloping side faces less than10% of the length of the horizontal upper and lower faces.
As the sun gets lower the arc climbs higher. At solar elevations ~5° it is in the same position as the Moilanen arc. More low sun observations on both arcs are desirable together with the advice 'always carry not just one but two cameras'!