Le mardi 23 juin 2026, Linus Alexander HEAD présentera l'examen en vue de l’obtention du grade académique de Docteur en Sciences (Collège de doctorat en Sciences spatiales) sous la direction de Denis GRODENT et Bertrand BONFOND.

Cette épreuve consistera en la défense publique d’une dissertation intitulée :

« Variability of Jupiter's aurora, moons, and magnetosphere investigated using advanced data analysis ».

Le Jury sera composé de :

Mme V. VAN GROOTEL (Présidente), Mmes et MM. B. BONFOND (Co-promoteur), C. CASTAGNOLI (National Institute for Astrophysics Rome), D. GRODENT (Promoteur), S. ROBERT (Institut Royal d'Aéronomie Spatiale de Belgique), L. SORET (Secrétaire).

Summary

After more than 70 years of study, Jupiter’s aurorae, moons, and magnetosphere have become fertile ground for investigation by large-scale, automated data- and image-analysis methods. This thesis comprises four related investigations of Jupiter’s aurora, magnetosphere, and its moon Io, in which advanced image-analysis techniques are applied to large datasets to draw novel conclusions about their origins and properties. 

The first study explores the neutral sodium cloud(s) around Io and their interaction with the plasma torus and Jupiter’s wider space environment. Despite the presence of an unprecedentedly bright jet in late 2014, the brightness of the Io plasma torus and Jupiter’s extended sodium nebula were unaffected. Conversely, a period of simultaneous enhancement of the plasma torus and sodium nebula in 2015 was not accompanied by a conjugate enhancement in the jet. This indicates that the jet does not contribute toward populating these two structures, at least not in a straight-forward way. 

The second study concerns the variability of the size and brightness of the jovian UV main auroral emission. This analysis shows that the main emission is both brighter and more contracted from its average position at dusk than at dawn. Additionally, the main emission is shown to have undergone a global contraction and brightening when the magnetosphere was compressed. The fact that the main emission is brighter at dusk than at dawn, and that it globally brightens during magnetospheric compression, contrasts with the predictions and observations of field-aligned currents in the magnetosphere, indicating that the generation of the main emission may predominantly occur via other mechanisms. 

The third study investigates the “bridge” features sometimes seen in the dusk-side UV polar aurora. Juno crossings of these bridges reveal that existing theories on the generation of the polar aurora involving large-amplitude electrostatic waves are consistent with the measured properties of bridges. They are also characterised by the presence of majority-upward-travelling electrons, which indicates that they may be instances of the Zone-II main emission that have become separated from the rest of the Zone-I aurora. This interpretation is strengthened by the presence of bridge-like signatures (electrostatic waves, significant upward electron flux) for the main emission when bridges are not visually present in the aurora, and the lack of these features in the main emission when separate bridges are present. 

The fourth and final study deals with signatures of plasma injection in the UV outer emission. The electron distributions during Juno crossings of injection signatures, as well as the inverse proportionality identified between surface magnetic field strength and auroral brightness, indicate that pitch-angle scattering in the equatorial plasma sheet is likely the dominant mechanism to produce injection signatures. Evidence is provided that there exist two classes of injection signature, dawn-storm and non-dawn-storm signatures; this latter class had been previously presented in the literature in only a limited and tentative fashion. Finally, injection-signature brightness is correlated with both the dissipative Alfvénic flux and field-aligned current density measured by Juno. Since both Alfvénic and current-based generation mechanisms are not supported by other properties of injection signatures, it is suggested that Alfvénic flux and field-aligned currents are secondary products of the plasma injection and do not meaningfully contribute towards auroral injection signatures. 

In all, this thesis highlights the usefulness of advanced and automated image-analysis techniques in the study of Jupiter’s aurora, its moons, and its magnetosphere. The results of this thesis also contribute to the growing body of evidence that precipitating electrons contributing to the UV main auroral emission are dominated by stochastic, broadband acceleration by Alfvén waves, rather than (or perhaps alongside) acceleration from quasi-static field-aligned potentials. 

Lien Orbi