Identification of Odd-Radius Halo Arcs and of 44 degrees /46 degrees Parhelia by Their Inner-Edge Polarization.
Birefringence of ice causes the inner edges of refraction halos to be polarized. The direction of this polarization relates directly to the projection of the crystal main axis onto the sky. This implies that the inner-edge polarization can serve as an observational diagnostic for determining the actual nature of a halo arc if two competing explanations exist. The direction and the visibility of the inner-edge polarization of arcs and circular halos arising from usual ice crystals and from ice crystals with pyramidal ends are calculated. It is found that the observation of inner-edge polarization can be decisive for the identification of a spot that might be either a 44 degrees parhelion or a 46 degrees parhelion, of an arc that might be either a 22 degrees sunvex Parry arc or a 20 degrees Parroid arc arising from plate-oriented pyramidal crystals, and of an arc that might be either a 22 degrees suncave Parry arc or a 23 degrees Parroid arc from plate-oriented pyramidal crystals. (With a Parroid arc, a halo is that which arises from an ice wedge made up of two faces of a crystal that rotates about a vertically oriented spin axis, and the edge of the wedge is perpendicular to this spin axis.) Polarization properties of other rare arcs are discussed. Practical hints are given for observing visually the inner-edge polarization of halos.
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