Characteristics of Saturn's polar atmosphere and auroral electrons derived from HST/STIS, FUSE and Cassini/UVIS spectra

Abstract Ultraviolet (UV) spectra of Saturn's aurora obtained with the Hubble Space Telescope Imaging Spectrograph (STIS), the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Far Ultraviolet Spectroscopic Explorer (FUSE) have been analyzed. Comparisons between the observed spectra and synthetic models of electron-excited H2 have been used to determine various auroral characteristics. Far ultraviolet (FUV: 1200–1700 A) STIS and UVIS spectra exhibit, below 1400 A, weak absorption due to methane, with a vertical column ranging between 1.4 × 10 15 and 1.2 × 10 16 cm − 2 . Using the low-latitude Moses et al. [Moses, J.I., Bezard, B., Lellouch, E., Feuchtgruber, H., Gladstone, G.R., Allen, M., 2000. Icarus, 143, 244–298] atmospheric model of Saturn and an electron energy–H2 column relationship, these methane columns are converted into the mean energy of the primary precipitating electrons, estimated to lie in the range 10–18 keV. This result is confirmed by the study of self-absorption with UVIS and FUSE extreme ultraviolet (EUV: 900–1200 A) spectra. Below 1200 A, it is seen that transitions connecting to the v ″ 2 vibrational levels of the H2 electronic ground state are partially self-absorbed by H2 molecules overlying the auroral emission. Because of its low spectral resolution (∼5.5 A), the UVIS EUV spectrum we analyzed does not allow us to unequivocally determine reasonable ranges of temperatures and H2 columns. On the other hand, the high spectral resolution (∼0.2 A) of the FUSE LiF1a and LiF2a EUV spectra we examined resolve the H2 rotational lines and makes it possible to determine the H2 temperature. The modeled spectrum best fitting the FUSE LiF1a observation reveals a temperature of 500 K and self-absorption by a H2 vertical column of 3 × 10 19 cm − 2 . When converted to energy of precipitating electrons, this H2 column corresponds to primary electrons of ∼10 keV. The model that best fits the LiF2a spectrum is characterized by a temperature of 400 K and is not self-absorbed, making this segment ideal to determine the H2 temperature at the altitude of the auroral emission. The latter value is in agreement with temperatures obtained from H 3 + infrared polar spectra. Self-absorption is detectable in the LiF2a segment for H2 columns exceeding 6 × 10 19 cm − 2 , which sets the maximum mean energy determined from the FUSE observations to ∼15 keV. The total electron energy range of 10–18 keV deduced from FUV and EUV observations places the auroral emission peak between the 0.1 and 0.3 μbar pressure levels. These values should be seen as an upper limit, since most of the Voyager UVS spectra of Saturn's aurora examined by Sandel et al. [Sandel, B.R., Shemansky, D.E., Broadfoot, A.L., Holberg, J.B., Smith, G.R., 1982. Science 215, 548] do not exhibit methane absorption. The auroral H2 emission is thus likely located above but close to the methane homopause. The H2 auroral brightness in the 800–1700 A bandwidth varies from 2.9 kR to 139 kR, comparable to values derived from FUV Faint Object Camera (FOC) and STIS images.