Dottorato in ASTRONOMIA

Titolo della tesi: The electromagnetic coupling between Jupiter and its moons during the Juno mission: discoveries and analysis of the Jovian Infrared Auroral Mapper (JIRAM) observations

The present thesis investigates the electromagnetic interaction between the Galilean moons and Jupiter, which is usually called satellite-ionosphere coupling. This interaction can occur in planetary systems with a magnetosphere that is large enough to include the orbit of the moon(s). The magnetic field of the Jupiter is rooted in the ionosphere and entangles the plasma (that is: a gas made by ions and electrons) of the whole magnetosphere: as a consequence, a system of plasma waves and currents transfers the Jovian rotation to magnetosphere, which tends to rigidly corotate with Jupiter. This plasma is ultimately supplied by the constant volcanic activity on Io, the innermost of the Galilean moons. Indeed, the Ionian surface is coated by frozen SO_2 originating from Io's volcanoes: this sublimates due to the solar radiation and contributes, together with the gases ejected by the volcanoes, to Io's atmosphere, which is subsequently lost into the Jovian magnetosphere and ionized by collisions with the magnetospheric particles. A dense plasma cloud, called Io Plasma Torus, is thus produced around Io's orbit, and the plasma diffuses from here towards the rest of the magnetosphere. Due to the fast rotation of Jupiter (one sidereal Jovian day lasts about 10 hours), the Galilean moons are thus constantly overtaken by the magnetospheric plasma, and hence they represent an obstacle to the plasma flow. This produces a local perturbation around the satellites, whose details depends on the plasma environment, the local magnetic field and the characteristic of the moons. At Jupiter, the typical conditions near Io, Europa and Ganymede - the three innermost Galilean moons - allow the propagation of field-aligned plasma modes known as Alfven waves. These modes transmit the perturbation from the satellites to the planetary ionosphere, which takes between a few minutes to half an hour, depending on the considered moon and its longitude in Jupiter's frame. Near the Jovian ionosphere, the Alfven waves partially transfer their energy to the electrons, which are accelerated into the Jovian ionosphere. Here, the electron precipitation trigger a chain of reactions with the atmospheric particles and generates an auroral emission, which, in the case of the satellite-ionosphere coupling, is called satellite auroral footprint (or just footprint). Since 2016, the Juno mission has been flying around Jupiter in highly eccentric, polar orbits. Thanks to the spacecraft orbit geometry, the high spatial resolution of its Jovian InfraRed Auroral Mapper (JIRAM) has been delivering images of the polar regions of Jupiter with an unprecedented level of spatial detail. Among its observations, JIRAM reports several detection of the footprint of Io, Europa and Ganymede. The observations performed by JIRAM have revealed new details in the structure of the footprint emissions: a chain of regularly-spaced patches of emission is consistently found along the track of the footprint of Io, Europa and Ganymede. The features of this new structure - named sub-dots - can not be fully explained by current models of the satellite-ionosphere coupling. Instead, we suggest that the high intensity of the electron precipitation associated with the footprint can trigger an ionospheric feedback, which can shape the morphology of the auroral emission. This feedback process has only been applied to the terrestrial magnetosphere so far, but the order of magnitude adaptation from Earth to Jupiter shown in Chapter 5 agrees with JIRAM observations. In the last chapter of this thesis, the position of the Io footprint is used to determine the state of the Io Plasma Torus. Indeed, the Alfven waves speed - and hence their travel time - depends on both the magnetic field magnitude and the plasma density. Therefore, the position of the footprint can be compared with the position predicted by a model of Jupiter's magnetic field and of the Io Plasma Torus, to determine the plasma content of the latter. This represents a new method to constrain the conditions of the Io torus, whose monitoring is fundamental to understand the interplay between Io's atmosphere, the torus and the Jovian magnetosphere. We shows that the position of the Io footprint can indeed be used to quantitatively detect episodes of variability in the Io torus. As Juno-JIRAM has been gathering observation for about 6 years, this represents a unique source of information on the plasma source of the Jovian magnetosphere. Therefore, we derived the torus conditions using all the available observations of JIRAM. To further improve the quality of the results, the torus states derived from JIRAM are used to simulate the radio occultations of the Io torus performed by Juno during each spacecraft closest approach to Jupiter. We conclude that the Io Plasma Torus was in a denser and hotter state compared to Voyager 1, on average. Nevertheless, large density and temperature fluctuations have often been observed to occur over a few week or months. Similar variations have already been documented in literature, but not satisfactorily explained yet. It is even still debated if such variability is caused either by variations in the plasma supply from Io, or by external factors. At present, no theoretical model is available to compare with the variability reported in this thesis. The Juno dataset obtained by combining the Io footprint position and the radio occultations represents an important survey of the plasma conditions around Io, hence it can be used to constrain future models of the Io Plasma Torus.