Halogen Lamp Halos generated by Marko Riikonen (site) November 5, 2008 at Rovaniemi, Finland. Image ©Marko Riikonen, shown with permission.
   
Ice crystals of high optical quality (rather than snow crystals) can grow downwind of ski-slope snow machines to give outstanding halo displays. They have a different magic to the unexpected, unpredictable and ephemeral beauty of natural displays but are excellent for halo research. However, they still depend on the whims of sunlight and their rare and exquisite quality arcs would be better studied against a high contrast dark background instead of the daylight sky.  

Here, halo researcher Marko Riikonen has replaced the sun with a 100W halogen lamp shining at night. The lamp was 100m distant to get reasonably parallel light. The closer the halo forming crystals are to the lamp the more the halos are distorted and divergent light halos are much more complicated to simulate than those from parallel light.

The HaloSim ray tracing simulation, for parallel light, identifies the halos in the stack of five individual images.

Rarities are helic (heliac) arcs formed by external reflection from crystal side faces and the Parry supralateral (Tape) arcs. Both halos are almost never seen in natural displays outside of Antarctica . The rare and unexplained Moilanen arc is resolved into the glints of nearby crystals. Crystal samples taken during displays like this should eventually tell us more about this and other arcs.