A new method for density and temperature retrieval from Cassini/UVIS solar occultations at Saturn

Abstract

Ultraviolet solar occultations by Saturn probe the extent, temperature and density of the thermosphere. The Cassini Ultraviolet Imaging Spectrograph (UVIS) observed 24 solar occultations by Saturn between 2007 and 2013 in the extreme ultraviolet (EUV). We have developed a new methodology to retrieve temperature and density profiles from the solar occultations, providing calibration-independent

profiles. To demonstrate this methodology here, we present a case study on 17 Nov 2007 at a latitude of -47.5 °.Our motivation was to retrieve a robust H profile for comparison with Lyman $\alpha$ dayglow observations of this location and time, given the uncertainty in absolute brightness calibration at Lyman $\alpha$ wavelengths. We will apply this method to the remaining solar occultations in future work. With improved removal of background contamination compared to previous analyses, we retrieve H and H₂ densities as deep as the methane homopause level, simultaneously fitting the full available altitude and wavelength range in the EUV. This demonstrates the potential of solar occultations to constrain the density and temperature profiles in Saturn’s upper atmosphere and extend the coverage of other observations. We closely reproduce the observed transmission from 560 Å to 1150 Å, with substantial absorption by

including the fine structure of the H₂ band absorption cross sections above 804 Å. We find a revised exospheric temperature of $487\pm 22$  K, a reduction from the $526\pm 11$  K previously retrieved, due to wider spectral coverage. The atomic H column density of $2.82\pm 0.2\times 10^{16}$  cm⁻² above the methane homopause is consistent with column densities required to generate the Lyman-$\alpha$ emissions from Saturn’s disk, also observed with Cassini/UVIS. The atomic H column density and temperatures are also consistent with the Voyager Ultraviolet Spectrometer (UVS) solar occultations, but the column density exceeds photochemical model predictions by a factor of two. This may be driven by circulation or otherwise enhanced mixing near the homopause level.