Limb polarization of Uranus and Neptune II. Spectropolarimetric observations

Aims. We have detected a strong limb polarization for Uranus and Neptune. With spectropolarimetric observations we characterize the spectral dependence of this limb polarization and explore the diagnostic potential for investigating the distribution and properties of the scattering particles. Methods. We present disk resolved spectropolarimetry of Uranus and Neptune covering the wavelength range from 5300 angstrom to 9300 angstrom and compare the spectropolarimetric signal for different limb sections and the center of the planetary disk. As an additional benefit we obtained center to limb disk profiles for the intensity and polarization for various wavelengths. Results. Our data show for both planets a strong linear limb polarization oriented perpendicular to the limb. The polarization at the center of the planetary disks is essentially zero. Overall the limb polarization increases towards shorter wavelengths, and it is further enhanced in the methane absorption bands when compared to adjacent inter band regions. Qualitatively, the polarization signal is very similar for Uranus and Neptune. For Uranus the peak limb polarization of the methane absorption bands reaches a maximum of 3% whereas in the nearby higher albedo regions it peaks at about 2%. The measured polarization for Neptune is significantly lower because the signal is averaged down by the limited spatial resolution of our Neptune data. The disk profiles for Uranus (center to limb profiles) show for the intensity a strong asymmetry between the bright South pole regions and the darker northern latitudes. The asymmetry is particularly strong in the methane bands. We measure a significant limb brightening in the methane bands at the Uranian eastern and western limbs. The intensity asymmetry between North and South and the limb brightening display a tight anti-correlation with the albedo of the selected wavelength interval. From the polarization profiles of Uranus we determine slit integrated polarization values in order to derive intrinsic limb polarization parameters which are corrected for the limited spatial resolution of our data. These parameters are compared with previous imaging polarimetry. Conclusions. The polarimetric measurements of Uranus and Neptune are compatible with expectations for the limb polarization of scattering atmospheres containing substantial layers of particles which scatter radiation similar to Rayleigh scattering. It seems that the limb polarization contains interesting diagnostic information, in particular on the scattering properties of the aerosol particles in the haze layers at high altitudes.