Jonas Rabia

The precipitation of charged particles onto Mercury's surface is a key process in the Hermean system, as it is one of the main loss mechanisms for magnetospheric particles as well as results in surface sputtering, an important source of charged and neutral particles in the Hermean magnetosphere. These processes strongly depend on the properties of the precipitating particles, in particular their energy, which controls not only how they interact with the surface but also where precipitation occurs.In this study, we use single-particle tracing numerical simulations to investigate the effect of the global magnetospheric electric field on precipitating particle properties. Ion measurements made by the Miniature Ion Precipitation Analyzer (MIPA) instrument onboard BepiColombo during its second Mercury flyby are used as inputs for our model.Using these observed ion distributions, we show that the inclusion of an electric field in the simulations generates a dawn-dusk asymmetry in both the energies of the precipitated ions and the precipitating flux. Furthermore, we demonstrated that the presence of an electric field allows for increased dayside ion precipitation, created by low-energy ions transported towards the dayside by the E x B drift. This mechanism produces low-latitude and dayside precipitation regardless of upstream conditions, e.g. solar wind dynamic pressure or interplanetary magnetic field (IMF) orientation, which contrasts with results from hybrid simulations.The charged particles traced in this work will eventually interact with Mercury's surface, inducing e.g. the release of Energetic Neutral Atom (ENA) stripped from the surface which could be monitored by the ENA instrument onboard BepiColombo. This study therefore illustrates that the combined analysis of ion and ENA measurements together with particle tracing simulations provides a powerful framework to better understand the plasma circulation in Mercury's magnetosphere.

Mercury's lack of atmosphere allows magnetospheric and solar wind plasma direct access to the surface through precipitation, altering the surface and plasma composition. In this way, its space plasma environment interactions resemble that of the Moon. Lunar measurements show that such exposed surfaces are subject to electrostatic charging, where photoelectrons generated on the dayside creating a positive potential of a few V, while electron transport to the nightside induces a negative surface potential up to -100s V. Given Mercury's similar surface properties and long diurnal cycle, we expect a similar pattern in surface potential; indeed, the nightside potential is likely even more negative at Mercury than at the Moon due to high energy electron precipitation.Lunar Prospector electron reflectrometry data indicates that this surface potential can be estimated remotely from pitch angle distributions of reflected particles, as the potential adds an energy-dependent term to the particle loss cone [1]. For positively charged ions and a negative surface potential, the size of the upward loss cone becomes inversely proportional to ion energy and can include all pitch angles < 90 degrees for sufficiently negative potentials. Thus, this effect has important implications for the ability of Mercury's magnetic field to trap particles on the nightside. We examine nightside pitch angle distributions from the Miniature Ion Precipitation Analyzer (MIPA) in the SERENA package from BepiColombo's second Mercury flyby for evidence of this loss cone energy dependence and find a noticeably visible effect for upward-going ions. Lunar surface potential analysis shows a strong dependence on electron temperature [2], so we use Mercury Electron Analyzer (MEA) electron temperature measurements from MFB2 [3] as a basis for fitting the loss cone and find good agreement with the MIPA data. This case study shows that estimation of surface potential from ion measurements is indeed possible, and promises interesting results to come from the BepiColombo orbital phase.[1] Halekas, J. S., G. T. Delory, R. P. Lin, T. J. Stubbs, and W. M. Farrell. “Lunar Prospector Observations of the Electrostatic Potential of the Lunar Surface and Its Response to Incident Currents.” JGR: Space Physics 113, no. A9 (2008). https://doi.org/10.1029/2008JA013194.[2] Stubbs, T. J., W. M. Farrell, J. S. Halekas, et al. “Dependence of Lunar Surface Charging on Solar Wind Plasma Conditions and Solar Irradiation.” P&SS 90 (January 2014): 10–27. https://doi.org/10.1016/j.pss.2013.07.008.[3] Rojo, M., N. André, S. Aizawa, et al. “Structure and Dynamics of the Hermean Magnetosphere Revealed by Electron Observations from the Mercury Electron Analyzer after the First Three Mercury Flybys of BepiColombo.” A&A 687 (July 2024): A243. https://doi.org/10.1051/0004-6361/202449450.

Caractérisation des signatures à haute latitude des interactions lunes-magnétosphère autour de Jupiter : analyse multi-instrument des données de la mission Juno: Les lunes galiléennes Io, Europe, Ganymède, et Callisto orbitent à l’intérieur de la magnétosphère de Jupiter, dans un milieu constitué d’un plasma dense. Au voisinage des lunes, ce plasma, entraîné en quasi-corotation par le champ magnétique de Jupiter, s’écoule à des vitesses significativement supérieures aux vitesses orbitales des lunes. En conséquence, les lunes galiléennes perturbent l’écoulement du plasma. L'interaction entre le flux de plasma et les lunes, connue sous le nom d'interaction lune-magnétosphère, donne lieu à de multiples processus physiques, notamment la génération d'ondes d'Alfvén qui se propagent le long des lignes de champ magnétique et s'éloignent des lunes. Ces ondes d'Alfvén peuvent accélérer des particules chargées vers Jupiter qui précipitent dans son atmosphère, induisant ainsi des émissions aurorales caractéristiques observées à la fois dans les longueurs d’ondes ultraviolettes et infrarouges. Ces émissions aurorales multi-longueur d'onde liées aux lunes, sans contrepartie sur Terre, sont connues sous le nom d'empreintes aurorales.La mission Juno, en orbite autour de Jupiter depuis Juillet 2016, évolue sur une orbite polaire de forte excentricité autour de la planète géante. En effectuant des survols rapprochés des pôles de Jupiter à chaque orbite, la mission permet de caractériser simultanément les émissions aurorales et les particules chargées qui les génèrent avec des résolutions temporelles et spatiales sans précédent.Toutefois, les mécanismes physiques responsables des empreintes aurorales d'Europe, de Ganymède, et de Callisto restent peu documentés. En effet, leurs interactions avec le flux de plasma sont de plus faibles intensités que celles d'Io et sont sujettes à des variabilités spatiales et temporelles plus importantes, rendant plus complexe l'identification de leurs signatures dans les données. Dans ce travail, nous proposons de caractériser les processus physiques et les propriétés des particules chargées responsables des empreintes aurorales d'Europe, de Ganymède, et de Callisto et de les comparer aux observations précédemment effectuées à Io. Pour cela, nous utilisons une approche multi-instrumentale basée sur les données de la mission Juno, combinée à une modélisation numérique du champ magnétique de Jupiter. Les mécanismes physiques responsables des propriétés des particules chargées observées par Juno sont étudiés à l'aide de modélisations numériques et comparés à des observations de la mission Cassini autour de Saturne.Ces travaux permettent de mieux comprendre les processus physiques impliqués dans les interactions plasma entre une lune et sa planète hôte, que ce soit dans la magnétosphère de Jupiter, de Saturne ou dans d’autres systèmes planétaires.

The moon Io, the most volcanically active body in the Solar System, functions as the “energy converter” of the Jovian environment: tidal heating by Jupiter and its Galilean satellites powers intense outgassing that sustains both the neutral and plasma tori and feeds the Jovian magnetodisk via centrifugal processes. Spacecraft traversals and remote observations, from ground‐based telescopes to JAXA’s Hisaki, have revealed the Io Plasma Torus’s (IPT) intricate spatial architecture and variability across timescales from hours to decades. Yet, fully characterizing the drivers of IPT structure and magnetospheric dynamics remains an open challenge. In this respect, NASA’s Juno mission, in its highly inclined, low‑perijove trajectory, provides in situ measurements of magnetic perturbations, plasma composition and abundance, through both plasma wave and radio occultation experiments, while refining tidal‐dissipation models through gravity science. Complementarily, a future network of longitudinally distributed Earth‑based observatories will deliver continuous, global monitoring of auroral emissions and torus brightness, capturing rapid fluctuations that single‐station views cannot. The synergy of Juno’s detailed local sampling and round‑the‑clock remote surveillance promises a holistic view of source–sink interactions driving Jupiter’s magnetosphere and the Io torus.

The electrodynamic interaction between Saturn's magnetosphere and Enceladus accelerates electrons along magnetic field lines. These electrons propagate inside magnetic flux tubes connecting the moon to the giant planet, generating distinctive auroral hiss and auroral footprint signatures, both previously observed by the Cassini spacecraft. In this study, we analyze low‐energy electron measurements made during multiple mid‐latitude crossings of magnetic flux tubes connected to Enceladus' wake. We show that the properties of the observed electrons are consistent with those of electrons inducing Enceladus' auroral hiss, and discuss the physical processes responsible for their pitch‐angle distributions and acceleration. Field‐aligned electron beams have very different properties from those triggering the Enceladus ultraviolet footprint, with a much lower characteristic energy and energy flux. Observations of electron beams resulting from the moon‐magnetosphere interactions up to 30° downstream of the moon reveal that the coupling system between Enceladus and Saturn is significantly more extended than previously anticipated.

Energetic particle injections provide a means for fast inward radial transport and adiabatic acceleration of electrons in the magnetosphere of Jupiter. Furthermore, enhanced whistler‐mode wave activity during injections can further accelerate electrons to MeV energy. All in all, injections may contribute to the creation of the most extreme radiation belts of the solar system. In this article, we present the first survey of the >1 and >11 MeV electron fluxes observed by the Galileo mission during M = 11–20 when the flux of MeV electrons is increased by a factor of more than 3 during injection events. The Galileo observations therefore indicate that magnetospheric injections may play a dominant role for the radial transport and acceleration of MeV electrons at Jupiter, at least transiently.

Jupiter's moon Callisto orbits in a highly variable magnetospheric environment depending on its position relative to the Jovian current sheet. The Juno and Galileo missions have visited the Jovian magnetosphere and crossed Callisto's orbit several times in a variety of configurations, providing an opportunity to better characterize Callisto's orbital environment. The aim of this work is to characterize the variability of Jupiter's magnetospheric environment properties at Callisto's orbit. After identifying the time intervals during which both missions crossed the moon's orbit, Juno's charged particle data from the Jupiter Energetic Particle Detector Instrument and the Jovian Auroral Distributions Experiment are combined to build composite spectra of the electron differential fluxes and derive the electron density and pressure. Based on these observations, we provide empirical models of the energy spectrum of the electron flux for different positions. The electron densities and pressures estimated from our composite energy spectra vary from 0.2 cm ⁻³ and 0.1 nPa, respectively, at the center of the current sheet to 0.009 cm ⁻³ and 0.004 nPa, respectively, outside. We compare these observations with those obtained by the Energetic Particles Detector onboard Galileo, which reveal an additional variation of electron fluxes with local time, possibly due to a variation of the current sheet thickness. A similar comparison of magnetic field observations with the latest magnetic field models shows a good agreement. These results can be used for studying the moon‐magnetosphere interactions in preparation for the arrival of the Jupiter Icy Moons Explorer, which will perform multiple flybys of Callisto.

The vast Jovian magnetosphere is filled with plasma originating from Io's volcanic activity in the inner magnetosphere. This plasma is structured as a sheet in near-corotation with Jupiter's magnetic field and confined to the centrifugal equator. The Galilean moons act as physical obstacles to this plasma flow, generating Alfvén wings. The propagation and reflection of Alfvén waves between Jupiter's northern and southern polar ionospheres create several auroral spots, whose brightness and morphology are controlled by the moon's position within the plasma sheet. These initial Alfvén waves progressively lose energy as they undergo multiple partial reflections between Jovian hemispheres, characterized by the brightness decrease of the auroral footprint tail downstream of the main spot in ultraviolet and infrared spectra. Previous observing campaigns using the Hubble Space Telescope have characterized this brightness decrease, measuring an ultraviolet e-folding distance of 21,000 km, correponding to about 40˚ of longitude (Bonfond, et al. 2009, The Io UV footprint: Location, inter-spot distances and tail vertical extent, J. Geophys. Res., 114, A07224, doi:10.1029/2009JA014312.). Juno's unique vantage point in the Jovian system now allows for measurements of the entire Io footprint structure at almost every perijoves, using the Juno-UVS instrument. We extend previous work characterizing the Io footprint by analyzing the Juno-UVS dataset collected during the Juno prime mission.

The encounter between the Jovian co‐rotating plasma and Ganymede gives rise to electromagnetic waves that propagate along the magnetic field lines and accelerate particles by resonant or non‐resonant wave‐particle interaction. They ultimately precipitate into Jupiter's atmosphere and trigger auroral emissions. In this study, we use Juno/JADE, Juno/UVS data, and magnetic field line tracing to characterize the properties of electrons accelerated by the Ganymede‐magnetosphere interaction in the far‐field region. We show that the precipitating energy flux exhibits an exponential decay as a function of downtail distance from the moon, with an e‐folding value of 29°, consistent with previous UV observations from the Hubble Space Telescope (HST). We characterize the electron energy distributions and show that two distributions exist. Electrons creating the Main Alfvén Wing (MAW) spot and the auroral tail always have broadband distribution and a mean characteristic energy of 2.2 keV while in the region connected to the Transhemispheric Electron Beam (TEB) spot the electrons are distributed non‐monotonically, with a higher characteristic energy above 10 keV. Based on the observation of bidirectional electron beams, we suggest that Juno was located within the acceleration region during the 11 observations reported. We thus estimate that the acceleration region is extended, at least, between an altitude of 0.5 and 1.3 Jupiter radius above the 1‐bar surface. Finally, we estimate the size of the interaction region in the Ganymede orbital plane using far‐field measurements. These observations provide important insights for the study of particle acceleration processes involved in moon‐magnetosphere interactions.