Nicolas Andre

Saturn's Kilometric Radiation (SKR) is a powerful non‐thermal radio auroral emission produced by the Cyclotron Maser Instability (CMI), which generates waves close to the local cyclotron frequency in a highly magnetized plasma from mildly relativistic electrons. The source of free energy for SKR was identified by Cassini to be the monoenergetic shell (horseshoe) distributions of 6–12 keV. Since then, Juno identified loss cone and shell distributions as the main and secondary CMI sources of free energy at Jupiter. We reanalyze Cassini measurements during SKR source crossings. We identify similar empty loss cones between 1.6 and 8.9 keV on 6 occasions alongside shell distributions. We show that loss cone distributions are unstable by computing the CMI growth rate. The amplified waves are predicted to have frequencies between 1.003 and 1.017 with a beaming angle close to . Finally, we compare our findings with those obtained at Jupiter.

Saturn's Kilometric Radiation (SKR) is a powerful non-thermal radio auroral emission produced by the Cyclotron Maser Instability (CMI), which generates waves close to the local cyclotron frequency in a highly magnetized plasma from mildly relativistic electrons. The source of free energy for SKR was identified by Cassini to be the monoenergetic shell (horseshoe) distributions of 6-12 keV. Since then, Juno identified loss cone and shell distributions as the main and secondary CMI sources of free energy at Jupiter. We reanalyze Cassini measurements during SKR source crossings. We identify similar empty loss cones between 1.6 and 8.9 keV on 6 occasions alongside shell distributions. We show that loss cone distributions are unstable by computing the CMI growth rate. The amplified waves are predicted to have frequencies between 1.003fce and 1.017fce with a beaming angle close to 80°. Finally, we compare our findings with those obtained at Jupiter.

Electron radiation belts, reservoirs of stably trapped energetic electrons hazardous to space technology, are a prevalent feature of most magnetized planetary environments. However, their existence in the compact magnetosphere of Mercury has been a subject of debate for decades. The presence of energetic electrons was first identified by Mariner 10’s observations in the 1970s but was subsequently questioned by MESSENGER’s limited energetic particle measurements 40 years later. Here we integrate MESSENGER’s more sensitive indirect energetic electron measurements with new analysis techniques, particle simulations and theory to discover a structured radiation belt with a variable morphology subject to solar wind activity. This radiation belt can persist for several Earth days and, thus, seems to be quasi-permanent under weak solar wind forcing. However, it becomes unstable under strong driving as electron drift orbits bifurcate about the magnetic equator within the dayside magnetosphere and particles are rapidly lost. Our results establish that even compact magnetospheres can host radiation belts and, furthermore, that Mercury is a natural laboratory for radiation-belt physics usually inaccessible at Earth: one of an extreme, rapidly driven state.

We present large‐scale simulations of energetic neutral atom (ENA) emissions at Uranus from a spacecraft viewpoint. Models of magnetic field, extended hydrogen exosphere, moon‐sourced neutral tori, and proton radiation belt are implemented into a simulator to evaluate the production of ENAs for = 1–15. Our ENA maps are first computed with a resolution of 0.25 × 0.25 degrees to cover a field of view of 90 × 120 degrees, before undergoing post‐processing to account for the resolution and response of a typical ENA imager. Our key data products consist of spatiotemporal distribution movies along the Voyager 2 trajectory within ∼52 from Uranus. Our simulator considers the possibility that the proton belt does not populate the region 4.1, peaks at = 3 or = 1.5. Our results suggest a strong probability of detecting ENAs resulting from charge exchange between the proton belt and the exosphere with a Cassini‐like ENA imager. The exosphere‐sourced ENA production rate for our worst‐case scenario, in which no protons populate the inner region ( 4.1), remains comparable to those typically measured at Saturn by the Cassini ENA detector during quiet magnetospheric activities in 2004–2017. Although the detectability of ENAs from icy moons environment cannot be confirmed due to significant uncertainty regarding neutral density levels, our results reveal that an ENA imager would have provided at least key constraints on the distribution and intensity of Uranus's proton belt and a first glance at its dynamical behavior during a compressed phase of Uranus's magnetosphere.

We report in situ evidence for Enceladus' Alfvén wing system and its coupling with Saturn's ionosphere, based on multi‐instrument observations from the Cassini spacecraft. Analysis of 36 events, including 13 from non‐flyby paths, confirms the existence of a Main Alfvén Wing (MAW) current system generated at Enceladus, and associated Reflected Alfvén Wings (RAWs) occurring both at Saturn's ionosphere and on the density gradient of Enceladus' plasma torus, extending longitudinally to at least (2,000 moon radii) downstream of the moon. Additionally, the observations reveal the systematic existence of a filamentation process of these large‐scale Alfvénic perturbations (MAW and RAWs) during their propagation at any distance from their source. These findings demonstrate a more extensive electrodynamic coupling than previously reported for Enceladus and more generally for any moon‐magnetosphere interaction. Moreover, the observation of energetic electron depletions and water‐group ion signatures at longitudes even further from the moon supports the interpretation of an extended and persistent interaction region. These results highlight Enceladus' role in shaping Saturn's magnetospheric environment and underscore the importance of future missions to exhaustively analyze this type of complex interaction between a moon and a planet.

Interplanetary coronal mass ejections (ICMEs) cause “Forbush decreases” (FDs), which are local decreases in background galactic cosmic rays (GCRs). Even though FDs can be observed with simple particle instruments, their amplitude and shape provide physical profiles of passing ICMEs. However, in some cases, previous statistical studies of the heliocentric distance dependence of FD changes associated with ICME propagation have found no strong correlation. We need the criteria for evaluating the relationship between ICME structure and FDs, necessary for the FD’s statistical analysis. This study investigates the effect of the evolution and interactions of ICMEs on FD profiles in the inner solar system using multipoint comparisons. We focus on multipoint ICME observations by Solar Orbiter, BepiColombo, and near-Earth spacecraft from 2022 March 10 to 16, when these spacecraft were ideally located for studying the radial and longitudinal evolution of ICMEs and accompanying FDs. We compared GCR variations with the multiple in situ data and ICME model, clarifying the correspondence between the evolution of each ICME structure in the radial and azimuthal directions and the depth and gradients of the FD. The radial comparison revealed decreases in FD intensities and gradients associated with the expansion of the ICME. The longitudinal difference found in FD intensity indicates longitudinal variations of the ICME’s shielding effect. These results suggest that accurate multipoint FD comparisons require determining the relationship between the observer’s position and the inner structure of the passing ICMEs.

Many current models of plasma transport in the Jovian magnetodisk/plasmasheet consider it to be azimuthally symmetric over radial distances extending from the outer edge of the Io torus to about 50 Jovian radii. But there are also many pieces of evidence pointing to a local time asymmetry in this system at such radial distances, and in the upper atmosphere to which it is coupled. Many of these observed asymmetries have been interpreted as the result of a large-scale dawn-to-dusk electric field generated across the magnetospheric cavity by its interaction with the Solar Wind that would be superimposed to the dominant corotation electric field. But no consistent model of this electric field, of its generation and of its mapping to different magnetospheric radial distances and ionospheric altitudes exists yet. We attempt to fill this gap by developing a simple semi-analytical model of electric fields, plasma convection, and current flows in the Jovian ionosphere and magnetosphere derived from the Earth case, which describes their variations with ionospheric colatitude and magnetospheric radial distance. Comparison to existing estimates of asymmetries and currents support the idea that the dawn-dusk electrostatic potential existing across the polar cap inside the main auroral emissions does penetrate to lower latitudes and down to the Io torus location, and is only partly attenuated by the shielding effect of trapped particles in the magnetodisk.

The SP@M (Solar Particles @ Mars) instrument, part of the M-MATISSE mission payload, is designed to measure solar energetic particles (electrons ranging from 30 keV up to 1 MeV and ions from 30 keV up to 10 MeV) using a compact detection strategy. Instead of conventional detector stacks, SP@M relies on a single 1.5 mm-thick silicon detector, where particle identification is achieved through pulse-shape analysis. This technique exploits the dependence of signal rise time on the depth of energy deposition and charge-collection dynamics. A dedicated modeling framework combining Monte Carlo simulations and charge-transport tools was first developed to pre-dimension the prototypes. It provided first-order predictions of rise time distributions for electrons, protons and alpha particles under over-depletion regime. Experimental characterizations were then performed with ¹³³Ba and ²⁰⁷Bi conversion electrons, and a tri-alpha source (²³⁹Pu, ²⁴¹Am, ²⁴⁴Cm), using controlled irradiation on both the junction and ohmic sides of the detector. The results show that the irradiation side has a critical impact: ohmic-side irradiation systematically yields longer and more dispersed rise times, enhancing species separation. For deposited energies above 600 keV, electrons and alpha particles are clearly discriminated, while below this threshold the overlap prevents reliable separation at 4.0 ns sampling. These investigations demonstrate the feasibility of PSA for charged-particle discrimination in thick silicon detectors and highlight both its potential and limitations.