Identification of odd-radius arcs and of 44o/46o 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 observation 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 parhelion or a 46 parhelion, of an arc that might be either a 22 sunvex Parry arc or a 20 Parroid arc arising from plate-oriented pyramidal crystals, and of an arc that might be either a 22 sunvace Parry arc or a 23 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 crystals that rotates about a vertically oriented spin axis, and the edge of the wegde 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.