PhD candidate: Marie Devinat
PhD supervisors: Nicolas André (ISAE-Supaero, IRAP), Michel Blanc (IRAP, Shandong University)
Start of the thesis: 2023
Thematic group: PEPS
Date of the defense: Monday, September 28, 2026, 2PM
Reviewers: Licia Ray (Lancaster University), Matteo Faganello (Physique des Interactions Ioniques et Moléculaires)
Examiners: Vincent Génot (IRAP), Solène Lejosne (DGA Paris), Nicholas Achilleos (University College London)
Location of the defense: Salle Coriolis, OMP, Toulouse
Abstract
Jupiter and Saturn bear strong magnetic fields which interact with the supersonic flow of plasma escaping from the Sun (the Solar Wind) and create large magnetic cavities inside of it, named magnetospheres. Those cavities host circumplanetary disks which originate from the volcanic activity of the active moons present in the inner regions (Io at 6 Jovian radii from Jupiter, Enceladus at 4 Kronian radii from Saturn). The neutral tori created by the moons undergo ionization processes, resulting in a mixed neutral-plasma torus in the inner regions. Coupling between charged particles and the strong magnetic field of the planets drives plasma into co-rotation at the planetary period, about 10 h, shorter than the Keplerian orbital period at those radii. The plasma is thus continuously dragged outwards by the dominant centrifugal force and forms a centrifugated magnetodisk extending to tens of Kronian and hundreds of Jovian radii in the equatorial plane. Both disks are characterized by a weak gas/plasma coupling and a strong coupling between magnetospheric plasma and magnetic field.
In the innermost regions of the system, transport is thought to be driven by a Rayleigh-Taylor-like instability named the flux tube interchange instability, under which a plasma-loaded flux tube located inward and a plasma depleted flux tube located outward exchange places under the action of the centrifugal force. Depleted flux tubes have been observed in the magnetospheres and interpreted as a result of this instability. Though amply documented at Saturn, their plasma properties could not be studied at Jupiter due to limited plasma diagnostics from Galileo. Based on the high-resolution plasma observation of the Juno mission, I provide a first analysis of flux tube plasma properties around Jupiter, confirm their similarity with the Kronian structures and infer the distance they have travelled.
I then develop a first formalism to describe the integrated fluxes of mass, energy and angular momentum in the case of axisymmetrical, instability-driven plasma transport. The magnetospheric system is separated between the thermosphere/ionosphere, a partially ionized medium where plasma dynamics is set through neutral/ion collisions and ion/magnetic field coupling, and the magnetosphere, a close-to-fully-ionized medium where plasma and magnetic field are strongly coupled and conservation laws apply. Both regions interact through the high-latitude field line region where field-aligned particle acceleration takes place. Using a quasi-linear treatment of the fundamental laws, I combine previous descriptions of the transport of mass, energy, angular momentum and (partial) coupling between ionospheres and magnetosphere. Our formalism accounts for plasma sources embedded inside the plasma disk, partial coupling with the high atmosphere, finite vertical distribution of the disk and slow time variations in the system. It is limited to the inner magnetospheric regions, where the disk is nearly axisymmetrical and field-aligned coupling is instantaneous.
In addition to this internally-driven plasma transport, multiple pieces of evidence indicate the presence of a large-scale electric field in the magnetospheres of Jupiter and Saturn, which could drive a plasma cycle reminiscent to Earth convection. Based on the first theoretical description detailed previously, I model the penetration of an external electric field inside the Jovian system. I find that theoretical models developed in the Earth magnetosphere could apply to the Jovian case, adding a contribution from the fast rotation of the planet and the disk. Our model qualitatively reproduces available estimates of the convection electric field in the Jovian magnetosphere. However, more observational constraints are required to better qualify the validity of our model and investigate the nature of the source electric field. Such observational constraints could come from field-aligned current or equatorial plasma drifts.