Halogen Lamp Anthelic Halos produced by Marko Riikonen (site) November 5, 2008 at Rovaniemi, Finland. Image ©Marko Riikonen, shown with permission.
   
The superb display of rare halos was 1-2 km downwind of ski-slope snow machines. The machines provide nuclei on which high quality ice crystals (not snow crystals) slowly form and the resulting halo displays are proving very valuable for halo research.

The images, like the previous ‘sunward’ one, were taken at night with a 100m distant halogen lamp replacing the sun. In cold weather we can sometimes see halos formed by bright street lights but if distortion is to be minimized the lamp must be far away so that its rays appear reasonably parallel to the ice crystal swarm.

The camera is pointing away from the lamp. Two points help define this part of the sky. Exactly opposite the sun, or lamp, and marked by the camera shadow is the antisolar or subanthelic point. Above that and at the same altitude as the sun or lamp is the anthelic point.

Sweeping around the sky parallel to the horizon is the parhelic circle. Notice how the intensity alters along the circle in both the photograph and the HaloSim ray tracing simulation. The blue spots in the simulation and perhaps also showing faintly in the image are a real effect produced by a difference between blue and red light in angle of cessation of total internal reflection.

Rising from the anthelic point in a huge ‘Y’ are the rare diffuse arcs. Their formation is via complicated ray paths involving multiple internal reflections in horizontal column crystals. Other rare column halos are the Tricker and Wegener arcs.

Touching the Tricker arcs above the anthelic point is a subhelic arc made by horizontal columns and Parry oriented crystals. Parry crystals alone generated the subanthelic (antisolar) and helic arcs, the latter are also visible in the view facing the lamp.

In natural displays many of these arcs are rarely seen outside of Antarctica. Compare the lamp view with the exquisite South Pole display also photographed by Marko Riikonen.