The far-ultraviolet aurorae observed on Ganymede by the Juno spacecraft: the discovery of meso-scale structures

Abstract

NASA’s Juno spacecraft, launched in 2011 and in orbit around Jupiter since 2016, conducted two close flybys of Ganymede, the Jupiter’s largest moon and the only known natural satellite with an intrinsic magnetic field. During the first one, on 7 June 2021, the spacecraft flew within 1049 km of Ganymede’s surface, allowing observations from inside the moon’s magnetosphere. These encounters yielded valuable data on Ganymede’s interior, surface, and auroral emissions. This thesis investigates Ganymede’s far ultraviolet aurorae using data collected by Juno’s ultraviolet spectrograph (UVS) during the above-mentioned close encounter with the moon, with a focus on the presence of small-scale features. In order to analyse Ganymede's aurora, we developed a Python toolkit that implements NASA's Spacecraft Planet Instrument C-matrix Events system (SPICE) libraries, and processes the ultraviolet data retrieved from NASA's Planetary Data System (PDS). This toolkit reconstructs Juno’s trajectory, attitude, and pointing geometry with high precision, enabling the generation of 2D and 3D maps of the aurora. Special attention was given to the emissions at 130.4 nm and 135.6 nm, corresponding to the OI1304 and OI1356 oxygen lines, which trace the presence of neutral oxygen in Ganymede’s atmosphere. The maps revealed localised auroral ovals in both hemispheres, aligned with the expected Open-Closed Field Line Boundary (OCFB) - a boundary shaped by the interaction between Ganymede’s intrinsic magnetic field and Jupiter’s magnetosphere, separating magnetic field lines that close on Ganymede from those connected to Jupiter. The auroral emissions exhibit a poleward shift upstream and an equatorward shift downstream, which is caused by the interaction of Ganymede's magnetosphere with Jupiter's magnetosphere. The major result of this thesis is the discovery of meso-scale structures along Ganymede's aurora, which were not expected among the scientific objectives of the mission. Although the details of the underlying physical processes are still unknown, we suggest that such meso-scale features resemble the auroral "beads" that commonly precede the auroral substorms on Earth. These results emphasise the scientific value of the Juno dataset. Additionally, the northern aurora was found to be brighter than the southern: this suggests an asymmetric electron precipitation consistent with Ganymede’s location south of Jupiter’s plasma disk during the flyby. The brightness profile along the auroral oval shows variations in shape and intensity that appear dependent on the solar illumination of the atmosphere. Lastly, the ratio of the OI1356-to-OI1304 line intensity allowed us to determine the importance of the electron precipitation over other atmospheric processes in the auroral regions of the moon. Expected to arrive at Jupiter in 2031, the ESA's Jupiter Icy Moons Explorer (Juice) mission offers a unique opportunity to advance our understanding of Ganymede’s aurora. Equipped with an ultraviolet spectrograph similar to Juno’s, Juice will enable detailed, long-term monitoring of auroral variability, providing key insights into the dynamic coupling between Ganymede’s and Jupiter’s magnetospheres.