Abstract

Birefringence of ice causes inner edges of refraction halos to be polarized. The direction of this polarization relates directly with the projection of the crystal optic axis (C-axis) onto the sky. Hence, the inner-edge polarisation can serve as an observational diagnostic for determining the actual nature of halo arcs that resembles each other closely. It can be e.g. 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 that arises from plate-oriented pyramidal crystals, and of an arc that might be either a 22° suncave Parry arc or a 23° Parroid arc from plate-oriented pyramidal crystals (with Parriod arcs I mean arcs that arise from a wedge in a doubly oriented pyramidal crystal for which the refracting edge is horizontal). Polarization properties of other rare arcs will be discussed. Practical hints are given for observing the inner edge polarization of halos, arcs and spots.

© 1997 Optical Society of America