Parhelic circles due to plate-oriented crystals (main axes vertical) and 120° 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° and 120° from the sun. The 120° parhelia move both vertically and horizontally. The magnitudes of the shifts are between 0.1° and 0.3°, 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.
GP Können, J Tinbergen. Polarized structures in parhelic circles and 120 degree parhelia
published, Appl. Optics, 1998, 37