The altitude of Saturn’s aurora: implications for the auroral electron energy and location of the polar homopause

The Space Telescope Imaging Spectrograph (STIS) and the Advanced Camera for Surveys (ACS) on
board the Hubble Space Telescope (HST) have been used to study the characteristics of Saturn’s FUV
aurora, which is dominated by the Lyman and Werner bands of H2 and the H Lyman-α line. Images have
been analyzed to determine the altitude of the emission above the limb and infer the characteristic
energy of the auroral electrons. Our study shows that the peak of the emission is located 1000-1300 km
above the limb, indicating that the precipitated electrons are mostly in the 0.1-1 keV energy range,
that is significantly less than on Jupiter. Another source of information on the electron energy are the
FUV spectra obtained with Voyager UVS (Sandel et al., 1982) and the HST-STIS (Gérard et al., 2004) and
Cassini-UVIS spectrographs which only occasionally show a signature of absorption by hydrocarbons.
Consequently, the emission is located above but close to the methane homopause. Finally, EUV spectra
collected with the FUSE satellite (Gustin et al., 2008) provide information on the H2 column overlying the
aurora through analysis of self-absorption and rotational temperature of the emitting layer from the
intensity distribution among the H2 lines. If Moses et al. (2000)’s low latitude model is used to
convert altitudes into pressure levels, H2 and hydrocarbon columns and temperature, discrepancies
appear between the observed temperature, ultraviolet colour ratio and geometric altitude of the emission.
One possibility is that the temperature near the homopause sharply increases at a pressure level higher
than in the equatorial regions.