Ultraviolet Spectroscopy of Saturn: Determination of Auroral characteristics with FUSE, STIS and UVIS Spectra

Ultraviolet (UV) lines from Saturn's aurorae are emitted following inelastic collisions between energetic electrons and H[SUB]2[/SUB] molecules. De-excitation by radiative process lead to Far Ultraviolet (FUV) and Extreme Ultraviolet (EUV) emissions in the 750-1750 Å spectral window. Low resolution spectra ( 12 Å) obtained with HST/STIS in the 900-1700 Å window are employed to derive the absorption of the auroral photons by hydrocarbons (mainly methane) and depth of the auroral energy deposition. Determination of H[SUB]2[/SUB] temperature and overlying H[SUB]2[/SUB] column is achieved by analyzing spectra obtained with the FUSE instrument ( 0.2 Å resolution with the LWRS aperture in the 900-1200 Å spectral window). Below 1100 Å, transitions connecting to the v" = 0 and 1 levels of ground-state H[SUB]2[/SUB] are partially or totally absorbed by ambient H[SUB]2[/SUB] (self-absorption). Comparisons between observed and synthetic spectra allow to derive the gaz temperature and the overlying H[SUB]2[/SUB] column, which is an indicator of the depth of the aurora, hence of the energy of the precipitated electrons. Auroral spectra of Saturn at 12 Å resolution were also obtained with Cassini's Ultraviolet Imaging Spectrograph (UVIS), both in the EUV and FUV spectral bands. While the spectral resolution of UVIS is too low to estimate temperatures, the coverage of the entire UV domain allows to determine the energy of precipitated particles through the quantification of the methane and molecular hydrogen columns. Results from observations with these three instruments are presented and compared in order to characterize the auroral atmosphere and auroral electrons precipitated from Saturn's magnetosphere