Elliptical polarization of Saturn Kilometric Radiation observedfrom high latitudes

摘要

The high-inclination orbits of the Cassini spacecraft from autumn 2006 until spring2007 allowed the Cassini/RPWS (Radio and Plasma Wave Science) instrument to observeSaturn Kilometric Radiation (SKR) from latitudes up to 60° for the first time. This hasrevealed a surprising new property of SKR: above --,30° in observational latitude, asignificant amount of SKR is strongly elliptically polarized, in marked contrast to previousobservations from low latitudes, which showed only circular polarization. There aretransitional latitudes where the elliptical polarization occurs in "patches" in the time-frequency spectrograms next to regions of still completely circularly polarized SKR. From—45° to 60° in northern latitude, it is found that most of the SKR is elliptically polarizedthroughout its entire frequency range with an average degree of —0.7 in linearpolarization. We demonstrate the ellipticity of SKR by using the concept of "apparentpolarization" in case of two-antenna measurements, but also show three-antennameasurements from which the polarization can be unambiguously determined. Possiblereasons for the variation of SKR polarization with the observer's latitude will bediscussed.