Text: Alaska Science Forum April 16, 1976 Sun Dogs and Light Shafts Article #36 by Kolf Jayaweera This column is provided as a public service by the Geophysical Institute, University of Alaska Fairbanks, in cooperation with the UAF research community. T. Neil Davis is a seismologist with the institute. "Diamond Dust" or tiny ice crystals in the winter air over Fairbanks cause many curious optical phenomena in the sky. They sometimes cause a multicolored halo around the sun or moon. Within the solar halo ring, on either side of the sun, one often sees two bright spots called "sun dogs" or "mock suns". At night one frequently sees pillars of light extending vertically upward from street lights or other bright sources. These forests of glowing vertical shafts can be easily mistaken for displays of the rayed aurora. These optical effects and others like them are caused by the reflection and refraction (bending) of light by the small ice crystals. The ice crystals arc flat and they fall through the air with their flat sides horizontal. Light reflecting from the flat faces of the ice crystals causes vertical light shafts. Reflection alone does not alter the color of light, so the vertical pillars are the color of the light source, usually white. On the other hand, sun dogs and related phenomena are multicolored because they are caused by refraction of the light through the ice crystals. Refraction breaks sunlight up into the colors of the rainbow. Sun dogs and halos always are at an angle of 22° from the sun. That particular angle, curiously enough, occurs because the refracting ice crystals have hexagonal (six-sided) faces.

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