The response of gas giant ionospheres to space environment forcing

At high spatial and spectral resolution, the 10-metre Keck and 3- metre NASA IRTF ground-based telescopes were used to observe Saturn and Jupiter, respectively. Pole-to-pole profiles of H[superscript +, subscript 3] emission were recorded along the planets’ respective noon meridians. The low latitude ionospheric H[superscript +, subscript 3] emission of these planets was thought to be broadly uniformly decreasing towards the equator, with the transition from bright emission at the poles produced by particle precipitation, to the weaker background glow elsewhere produced by sunlight. Instead, however, a pattern of intensity variability was detected at both Jupiter and Saturn. This pattern was found to be symmetric about the magnetic equator at Saturn, with peaks in H[superscript +, subscript 3] intensity magnetically mapping to gaps in Saturn’s rings. The transport of water ions from the gaps in Saturn’s rings to the planetary ionosphere, delivered via magnetic field lines, was used to explain this, as water ions cause an increased H[superscript +, subscript 3] density and therefore emission. In the same dataset, the temperature of Saturn’s H[superscript +, subscript 3] aurorae remained effectively constant, whilst the H[superscript +, subscript 3] column density and total emission varied greatly. The southern auroral oval was found to be significantly warmer than its northern counterpart, having average temperatures of 583 and 527 K, respectively. This asymmetry was attributed to an inverse relationship between ionospheric Joule and ion drag heating with magnetic field strength. Jupiter’s low latitude ionosphere also appears to vary significantly in H[superscript +, subscript 3] emission, but this time in longitude. This may be due to an inversely proportional relationship between magnetic field strength and particle precipitation. In summary, the planetary ionospheres of Saturn and Jupiter have been found to be globally subjected to space environment forcing. Whilst such forcing was well established for the auroral regions, we have here discovered that particle precipitation can dominate the low latitude ionospheres of the gas giants.