Characteristics of Saturn’s FUV airglow from limb-viewing spectra obtained with Cassini-UVIS

Abstract

This study reports the analysis of far ultraviolet (FUV) limb spectra of the airglow of Saturn in the 1150-1850 Å spectral window, obtained with the Ultraviolet Imaging Spectrograph (UVIS) onboard Cassini, spanning altitudes from -1200 to 4000 km. The FUV limb emission consists of three main contributions: (1) H Ly-α peaking at 1100 km with a brightness of 0.8 kilo-Rayleighs (kR), (2) reflected sunlight longward of 1550 Å which maximizes at -950 km with 16.5 kR and (3) H ₂ bands in the 1150-1650 Å bandwidth, peaking at 1050 km reaching a maximum of 3.9 kR. A vertical profile of the local H ₂ volume emission rate has been derived using the hydrocarbon density profiles from a model of the Saturn equatorial atmosphere. It is well matched by a Chapman function, characterized by a maximum value of 3.5 photons cm ^-3 s ^-1 in the 800-1650 Å UV bandwidth, peaking at 1020 km. Comparisons between the observed spectra and a first-order synthetic airglow H ₂ model in the 1150-1650 Å bandwidth show that the spectral shape of the H ₂ bands is accounted for by solar fluorescence and photoelectron excitation. The best fits are obtained with a combination of H ₂ fluorescence lines and 20 eV electron impact spectra, the latter contributing ∼68% of the total H ₂ airglow emission.