Circular polarization of twilight.

Review of observations of circular polarization of twilight performed with a polarimeter which uses an electronically switched Pockels cell operated as a reversible quarter-wave plate to convert circular into linear polarization. The latter was then analyzed by a Wollaston prism followed by two gallium-arsenide photomultipliers. The discovery of a definite natural circular polarization at twilight does suggest that, with increased observation precision, measurements of the small daylight component are possible. These could give useful information about particles in the atmosphere and be valuable in studies of meteorology and air pollution.