VIMS observations of Saturn’s infrared aurorae

The H3+ infrared aurorae of Saturn were analysed using the Cassini Visual and Infrared Mapping Spectrometer (VIMS) to produce two studies of the emission above the limb. The first, following Stallard et al. (2012a), was a case study of the peak altitude of the southern auroral emission above the limb. The results showed that the measured peak emission altitude is dominated by the alignment of the auroral curtain with the limb, and that accounting for this alignment issue results in a peak intensity altitude of 1215 ± 119 km above the 1 bar level. In the second study, a new projection technique mapped the H3+ limb emissions with respect to latitude, altitude and local time. This technique enabled a statistical analysis of the average latitude-altitude structure of the auroral intensity, temperature and density using 511 observations from the years 2005-2010. For the northern auroral oval, the peak emission altitude was found to be 1333 ± 152 km above the 1 bar level, the average temperature for the peak emission layer was 549 −12+34 K and the nadir column density was 1.6−0.9+3.6 × 1015 m-2. Likewise for the southern auroral oval, the peak emission altitude was 1225 ± 193 km, the average temperature was 585 −29+6 K and the nadir column density was 6.2−0.3+0.4 × 1015 m-2. The peak emission latitude was found to be 74º ± 1º for both hemispheres, though the southern auroral emission was found to have a wider latitudinal distribution than the north. The north-south asymmetry in the magnetic field strength was used to explain the hemispheric differences in these properties. Pressure scale height analysis of H3+ revealed that in both hemispheres, the H3+ altitudinal distribution is controlled by its production and loss mechanisms and not gravity.