Abstract

As can easily be seen with a polarizer, halo scattering by birefringent crystals gives rise to a strong peak in the polarization, concentrated in a narrow scattering angular range. This peak can easily be observed in ice crystal halos by the naked eye, equipped with a polarizer. It becomes immediately evident that the inner edge of a parhelion is completely polarized. This narrow polarization peak provides a sensitive diagnostic for detecting birefringent crystals in their often complicated atmosphere and it may be worthwhile to scan their polarization with high angular resolution to look for their halos. However, as the halo angles for bright halos are considerably less than 90°, one has to be at the far side of the planet. For Earth-bound observers, this is only possible for the inner planets, of which only Venus has an atmosphere.

© 1990 Optical Society of America