Polarization structures in parhelic circles and in 120° parhelia

Abstract

Parhelic circles due to plate-oriented crystals (hence, with main axes vertical) and 120 degrees parhelia change in position when viewed through a rotating polarizer. The parhelic circle moves vertically; its largest shift is found at an azimuthal distance between 90 degrees and 120 degrees from the Sun. The 120 degrees parhelia move both vertically and horizontally. The magnitudes of the shifts are between 0.1 degrees and 0.3 degrees , depending on solar elevation. The mechanism is polarization-sensitive internal reflection by prism faces of the ice crystals. We outline the theory and present three visual and one instrumental observation of the displacements of these halos in polarized light.